1 Electrified Reefs: Enhancing Growth in a Temperate Solitary
Electrified Reefs: Enhancing Growth in a Temperate Solitary Coral Species?
A Thesis Presented to The Faculty of the Environmental Program The Colorado College
In Partial Fulfillment of the Requirement for the Degree Bachelor of Arts
By
Rebekah Barnett
May 2015
Approved By:
Miroslav Kummel
Marc Snyder
1 TABLE OF CONTENTS
I. ABSTRACT 3
II. INTRODUCTION 4 i. Claims About the Effectiveness of Biorock® Technology 11 ii. Independent Research on Biorock® 12 iii. Proposed Mechanisms for Biorock®’s Beneficial Impact 16 iv. Our Research On Biorock® 17
III. METHODS 18 i. Tank Description 18 ii. Electrical System 18 iii. Biology 19 iv. Growth Measurements 20 v. Algae Measurements 20 vi. Data Analysis 21
IV. RESULTS 21 i. Coral Growth 21 ii. Coral Growth Based on Size 25 iii. Algae Growth 29 iv. Algae Growth Based on Algae Class 30
V. DISCUSSION 32
VI. WORKS CITED 48
2 ABSTRACT:
Coral reefs provide humans with important ecosystem services including food, pharmaceuticals, and water filtration (Moberg and Folke 1999). These ecosystem services, however, are at risk from ocean acidification, coral bleaching, and other destructive anthropogenic activities. Since these ecosystem services are costly to replace and their natural ability to restore themselves has been compromised, active restoration is necessary. A promising tool for active restoration is the Biorock® method. The Biorock® method consists of running an electrode through a large metal cathode and smaller anode.
The electrical current causes the precipitation of calcium carbonate onto the cathode. The cathode works as an artificial reef framework by providing a natural substrate onto which corals can grow. Claims about the effectiveness of this technology in terms of increased coral growth, reproduction, health, and diversity are spectacular (Goreau and Hilbertz
2005), but independent research is divided on the question of its effectiveness.
In an attempt to help clarify the technology’s effectiveness, we conducted laboratory experiments to test the claims of growth enhancement for the electrolytic technology by exposing the temperate solitary coral Balanophyllia elegans to powered and unpowered treatments. The study focused on coral growth, but careful observations were made on other aspects of the community including presence and thickness of algal cover. Our results are not consistent with claims made about the benefits of electrolysis on coral growth. Growth between treatments was significantly different but, while claims suggest a 3 to 5-fold increase in growth (Goreau 2014), corals exposed to an electrical current grew less than control corals. The results suggest that the electric current may actually depress growth.
3 A species-specific tolerance to electrical currents may help explain our results as well as the variation seen in other studies. In addition to variation caused by species tolerance and current level, there also seem to be overall trends of the technology being effective for warm-water corals, but ineffective or even detrimental to cold-water corals.
The variation in effectiveness between warm and cold-water species is surprising given the proposed physiological mechanisms behind the benefit and suggests that photosynthesis may play an important role in determining whether or not the technology is effective. Our results suggest an ability of the electrical current to depress the growth of algae, which can positively impact photosynthesis and the corals’ ability to calcify.
Algae inhibition can therefore play an important role in determining whether or not the technology is effective, but is still part of a complex interplay between current density, ion availability, algae and photosynthesis. Further study is needed to clarify these interactions and their role in determining the effectiveness of the Biorock® method.
INTRODUCTION:
Coral reefs are imperiled on multiple fronts by ocean acidification caused by rising atmospheric carbon dioxide levels, rising sea surface temperatures, and other anthropogenic impacts including overfishing, pollution, and human introduced pathogens and invasive species (Goreau and Hilbertz 2005; Moberg and Folke 1999). At the same time, these ecosystems provide human society with important services that would be costly or impossible to replace. It is therefore crucial that we consider all alternatives of restoration of coral reefs, especially those that directly counter the effects of increased temperature and acidification.
4 Coral reefs are one of the most biologically diverse ecosystems on the planet.
The coral reef framework itself supports this biodiversity through its complex physical shape. The complex, three-dimensional framework creates high habitat heterogeneity and the possibility for niche diversification, resulting in high species diversity. For example, a third of all marine fish species are found on coral reefs. This high biodiversity results in a wide array of biological resources that humans use for everything from pharmaceutical products, jewelry, ornamentation, to food (Moberg and Folke 1999).
Coral reefs serve as important fisheries and despite their relatively small geographic area of 0.1-0.5% of ocean floor cover (Moberg and Folke 1999), they constitute approximately 9% of annual global fish catch (Smith 1978). This does not include their production of mussels, crustaceans, and other organisms consumed for food (Moberg and
Folke 1999). Consumers, local and global, rely on these organisms as an important component of their diet, especially as a source of protein. Fishermen rely on these organisms for their livelihood. In addition, coral reefs serve as spawning, breeding, and nursery grounds for many marine species, including species from sea-grass beds and mangroves (Moberg and Folke 1999). By providing these functions to non-reef species, coral reefs help to replenish fish stocks and maintain the health of other marine ecosystems on which humans rely.
Although biological services are the most easily recognized, coral reefs also provide chemical, physical, and recreational services to humans. Water filtration enhances coastal water quality and helps to process wastes released by humans (Moberg and Folke 1999; Goreau and Hilbertz 2005). Coral reefs therefore serve as de facto sewage treatment plants that can help alleviate pollution problems that negatively impact
5 marine life. Coral reefs serve as physical structures that protect and buildup shorelines by dissipating wave and current energy (Moberg and Folke 1999). This service is not only important during storm events as protection from destructive storm surges, but helps protect coastlines from continual erosion that can result in a significant loss of coastal land over time. Lastly, coral reefs serve as hot spots for tourism due to recreational opportunities for scuba diving and snorkeling as well as their sedimentary contribution to the white tropical beaches attractive to tourists (Moberg and Folke 1999). Tourism supports local economies and provides a viable livelihood to locals, and many countries rely on tourism for the economic health of their nation.
The aforementioned services (and numerous others), however, are at risk. The physical reef framework is the basis of the production of these services, but the reef is a structure dependent on reef building processes that equal or exceed erosive processes.
The building process, calcification, is not fully understood. Calcification occurs in the lowest layer of tissue, the calicoblastic epithelium. Calcium and carbonate ions must be transported from the water column and into the ECF for calcification to occur. The transport of carbon to the site of calcification is poorly known (Gattuso et al. 1999).
Calcium ions are transported either through paracellular pathways (passive transport) or through the transcellular pathway (active transport) (Al-Horani et al. 2002; Tanbutte et al.
1996). Studies show that transportation may combine both of these pathways with paracellular transport occurring along the concentration gradient from the external environmental into the gastrovacular compartment (Tanbutte et al. 1996). Active transportation then moves the calcium ions to the calicoblastic epithelium where they combine with carbonate. Once the concentration is supersaturated, calcium carbonate
6 precipitates (Tanbutte et al. 1996; Al-Horani et al. 2002). The active transport of ions makes calcification an energy intensive process. Calcification is heavily dependent on the corals’ symbiotic relationship with zooxanethellae as photosynthesis enhances calcification (Muller-Parker and D’Elia 1997).
Calcification allows the coral reef to grow and maintain itself in the face of erosion. Human activities, however, are rapidly perturbing both calcification and erosion processes. This perturbation may cause erosion to exceed construction, resulting in a net loss of the reef framework. Sedimentation can reduce light availability as can phytoplankton growth caused by nutrient inputs from runoff (Moberg and Folke 1999;
Hoegh-Guldberg 1999). Since calcification is photosynthetically enhanced, reduced light results in reduced calcification (Muller-Parker and D’Elia 1997). Dissolution of atmospheric carbon dioxide into the ocean is changing the ocean’s pH into a range that is less favorable for corals. When carbon dioxide dissolves, it reacts with water and forms carbonic acid. Carbonic acid then dissociates, forming bicarbonate and hydrogen ions.
The hydrogen ions react with carbonate, forming bicarbonate, but reducing the availability of carbonate, an ion necessary for calcification (Doney et al. 2009). The chemical reactions discussed above can be seen below (Doney et al. 2009).
Ultimately, the decreased availability of carbonate decreases calcification, which can have one of two effects. It can either decrease skeletal growth, or skeletal growth rates may be maintained by reducing skeletal density. Reduced density can promote erosion because herbivores preferentially remove carbonate from lower density substrates. In addition, decreased density makes the reef more susceptible to storm
7 damage, which can work to destroy the reef framework (Hoegh-Guldberg et al. 2007).
Ocean acidification, therefore, not only reduces the growth of the reef framework, but can promote its erosion through decreased skeletal density.
In addition, bleaching events can reduce calcification by negatively impacting the coral-zooxanthellae symbiosis as well as by promoting erosion of the reef structure. The majority of corals are tolerant of temperatures from 18°C to 30°C (Muller-Parker and
D’Elia 1997; Hoegh-Guldberg 1999), but corals are more frequently exposed to temperatures above this range as ocean temperatures continue to climb. Exposure to extreme temperatures can cause coral bleaching, an event where corals’ symbiotic algae, zooxanthellae, leave the tissue of their host. Bleaching can ultimately result in coral mortality (Hoegh-Guldberg 1999). Loss of the zooxanthellae results in a loss of the photosynthetic process upon which calcification depends. The loss of zooxanthellae causes reduced calcification, which in turn causes growth to decline or stop (Baker et al.
2008). Bleaching events have also been linked to increased bioerosion on the reef (Baker et al. 2008), resulting in a loss of the reef framework.
Interaction between decreased light, increased acidity, and bleaching events may disrupt the equilibrium between reef building and eroding processes. Less calcification and increased erosion will result in a net loss of the coral reef framework. Loss of this framework will impact the survival of organisms that have evolved to rely on the complexity of the reef for their unique habitat and ultimately decrease the ability of coral reefs to provide ecosystem services.
While human-induced impacts on calcification and erosion are severe, coral reefs also face many acute anthropogenic stresses. Fishing pressure has severely reduced fish
8 stocks and decreased ecological complexity and biodiversity. In a process known as
“fishing down the food chain,” humans have focused selectively on top predators until they disappear before moving onto the next largest species, and the process repeats itself
(Goreau and Hilbertz 2005). The resulting decrease in ecological complexity and biodiversity reduces the resiliency of the ecosystem to other stresses. Introduction of invasive species can have the same impact (Goreau and Hilbertz 2005). Invasive species outcompete and replace native ones. The disappearance of natural species can cause the food chain to collapse while the invasive species proliferate, resulting in lowered ecological complexity, biodiversity, and resiliency. Humans and invasive species also introduce foreign pathogens, which can devastate native species (Goreau and Hilbertz
2005). The release of chemicals into the ocean can be toxic to some marine organisms whereas fertilizer, sewage, and animal waste run-off can stimulate the rapid proliferation of species such as algae. These algal blooms result in eutrophication and the subsequent
“smothering” of corals (Howarth et al. 2000; Goreau and Thacker 1994). Algae growth can also be stimulated by the removal of herbivore species, causing overgrowth of corals by algae (Moberg and Folke 1999; McCook et al. 2001). Lastly, degradation and loss of coral reefs disrupt the spawning, breeding, and nursery functions they provide, which endangers the continued biological diversity and health of other marine ecosystems.
Clearly, coral reefs and the services they provide are at risk. The cost to replace these services is immense, making protection of these valuable ecosystems imperative.
The chronic stress, however, that anthropogenic activities put on coral reef ecosystems reduces their resiliency. The loss of resiliency reduces their ability to “bounce back” and restore themselves naturally, especially given habitat degradation, which has left them
9 unable to support the organisms that they once did (Goreau and Hilbertz 2005). This means that active restoration is necessary if they are to be preserved. The historical method of coral reef restoration is transplantation, which involves reattaching coral fragments to the reef matrix (Goreau and Hilbertz 2005). This restoration strategy, however, does not address the root cause of the problem, which is degraded environmental conditions and chronic anthropogenic stress. Transplanted corals are equally susceptible to those stresses, making the success of restoration limited.
Transplanted corals tend to face high mortality rates, decreased growth, and reduced fecundity (Abelson 2006). At the same time, while local stresses can be mitigated, their reduction would require large-scale system change of human activities, which seems unlikely to happen in a time frame short enough to allow for reef restoration before it is too late. Thus, a restoration method is needed that helps increase coral resistance in the face of environmental stress.
One such method that has the potential to increase growth and survival even in the face of environmental stress is the Biorock® method. Originally developed in the 1970’s by Wolf Hilbertz as a way to grow construction materials using dissolved ions in seawater, Biorock® consists of calcium carbonate precipitated onto submerged metal cathodes through electrolysis generated by an electrical current. In the electrolytic process, water is broken down at the anode to form oxygen gas and hydrogen ions while it is broken down at the cathode to form hydrogen gas and hydroxyl ions. The resulting environment at the anode is acidic and at the cathode is alkaline. The hydrogen ions produced at the anode react with limestone sediment while the hydroxyl ions produced at the cathode react with calcium ions and bicarbonate to form calcium carbonate. Thus,
10 dissolution of calcium carbonate occurs at the anode and precipitation of calcium carbonate occurs at the cathode (Goreau 2012). The calcium carbonate-covered cathodes serve as an artificial reef framework onto which corals and other organisms can grow.
Claims About the Effectiveness of Biorock® Technology:
Biorock® has been implemented in dozens of countries by collaborations of non- profit organizations, NGO’s, governments, and other interested parties with spectacular claims as to its effectiveness. It claims to be the only restoration method that results in coral survival in the face of environmental stresses normally fatal while simultaneously increasing growth rates, reproduction, diversity, and overall health (Goreau and Hilbertz
2005). On average, claims are made of growth rates of corals on Biorock® being 3-5 times faster than controls in the same habitat even when conditions were severely stressful (Goreau 2014). Claims are also made that corals on Biorock® structures have
16-50 times higher survival from high temperature bleaching stress (Goreau and Hilbertz
2005). In addition, there are claims that corals appear to be surviving in areas that have become too stressful for natural reefs. In Jamaica, nutrient input caused natural reefs to be overgrown by algae and die, but corals on Biorock® grew extremely well in those same areas, with claims of growth up to three to five times faster than the maximum rate known for the species (Goreau and Hilbertz 2012). In Mexico, staghorn and elkhorn coral disappeared from many areas and are now claimed to be growing well on Biorock® structures. In Thailand, claims are made that corals are growing quickly in areas experiencing heavy sedimentation while in the Maldives, 95-99% of “natural” corals died
11 in a bleaching event, but only 20-50% of corals on Biorock® structures died (Goreau and
Hilbertz 2005).
The benefits, however, are not limited to the corals. Biorock® structures have become home to dense schools of diverse fish, which include fish species that have mostly disappeared from surrounding reefs. Other organisms appear to be more resistant to environmental stresses, grow more quickly, exhibit denser settlement, and higher survival rates than their counterparts on natural reefs even under stressful conditions
(Goreau 2012; Goreau 2014). The structures also act to absorb wave energy and not only prevent further beach erosion, but create a sedimentary environment in which sand can be deposited and the beach can grow. It is claimed that a beach in the Maldives has grown
15 meters since the structure was implemented (Goreau and Hilbertz 2005). The technique is also cost and energy efficient. One kilogram of calcium carbonate can be produced using just 1kWh. Power has been supplied with renewable energy sources, in particular solar, in order to eliminate greenhouse gas emissions that are contributing to the problem in the first place (Goreau and Hilbertz 2005). In terms of coastal protection, it is much more cost efficient than building seawalls. Typically it costs approximately
$15 million to build a one-kilometer seawall, but one kilometer of Biorock® structures only cost $50,000 to $100,000 (Goreau and Hilbertz 2005). The cost difference is immense, making it a more economically viable option for coastal protection. Based upon the claimed biological benefits and cost and energy efficiency of Biorock®, it is a viable method for reef restoration that deserves thorough exploration.
Independent Research on Biorock®:
12 Few independent studies, however, have been done to corroborate the claims made by the technology’s proponents. One study was performed in the Philippines using the coral species Porites cylindrical Dana. Corals were exposed to three separate treatments. One group was exposed to electrolysis and electrochemical deposition of
CaCO3 on a cathode structure (treated), one group was also on a cathode but received no electricity (untreated), and the third group was a random sample of naturally growing corals (natural.) The three treatments allow comparison of artificial reefs with and without the electrolytic technology as well as comparison of natural and electrically stimulated corals. Longitudinal growth, girth growth, and survival were assessed at the end of a six-month period. Natural corals exhibited the highest mean percentage longitudinal growth and the untreated corals showed the lowest mean percentage growth.
The treated corals also exhibited higher girth growth rates than the untreated corals.
Girth growth rates are not discussed for natural corals. Treated corals had lower mortality rates than untreated corals while natural corals showed the lowest mortality.
Most mortality in the untreated corals was due to algal overgrowth (Sabater and Yap
2002). The results show greater growth for electrically stimulated corals on artificial reefs as compared to their non-electrified counterparts for both longitudinal and girth growth as well as lower mortality. However, natural corals exhibit the least mortality and highest longitudinal growth. The evidence suggests that while electrified artificial reefs may be more beneficial to corals than non-electrified artificial reefs, growth and survival of electrified corals do not exceed those of natural corals as has been claimed. At the same time, the stress of transplantation may result in the differences between natural and
13 treated corals. These differences may change drastically over time once the stress is gone and the differences may show a completely different pattern.
Long-term effects for the same study were also assessed. Electricity was provided for 6-months, the period discussed above, and was discontinued after for another 6-month period, allowing for comparison between a mineral accretion phase and a post accretion phase. For longitudinal growth, significant differences in growth between treatments only occurred during the period of mineral accretion. Treated corals still show significantly higher girth growth than untreated corals during the post mineral accretion phase, but the difference in growth is less. Treated corals showed significantly higher survival than untreated corals for the duration of the experiment. Natural corals, however, had the highest survival (Sabater and Yap 2004). These results suggest a greater impact on growth while mineral accretion is occurring. Survival benefits for treated corals compared to untreated corals continue even in the post mineral accretion phase. Over all, Sabater and Yap’s studies show a benefit for treated corals versus untreated corals, but the benefit is greatest during the mineral accretion period. After the mineral accretion stops, the benefit diminishes or disappears.
Another study was performed in Ras Mohammed National Park in the Red Sea in
Egypt to assess the growth and survival rates of 15 species of scleractinian corals transplanted onto electrified reefs. The study site experienced continual sedimentation, which adversely affected coral cover, and was chosen in order to assess the electrolytic technology under environmentally stressful conditions. Transplanted nubbins were assessed regularly for mortality and axial growth. Some species of the experimental corals exhibited higher coral growth rates than documented rates for the same species in
14 the Red Sea. In addition, several species showed up to 95-97% survival rate
(Schuhmacher et al. 2000) While the results seem promising, there are no control corals present in the study itself. It is therefore difficult to know whether the higher growth rates and high survival rates were due to environmental conditions at the time of the study or due to the electrolytic technology itself.
A separate study conducted in Japan compared the effect of different magnitudes of electrical current on growth. Corals were exposed to four different current densities:
0mA/m2, 20-50 mA/m2, 50-100mA/m2, and 100mA/m2. Three species of coral were transplanted onto the metal structures. Growth was evaluated seasonally for two-years.
Only the data for one species, Pocillopora damicornis, is evaluated. Mean growth for
20-40 mA/m2 equaled that of the control null current while mean growth for 50-
100mA/m2 exceeded that of the control. Mean growth was lower for corals exposed to
100mA/m2 than mean growth for the control (Kihara et al. 2013). These results suggest a benefit of an electrical current density between 50-100mA/m2 compared to zero current, but a detrimental effect for currents exceeding 100mA/m2. Electrical currents may be beneficial only for certain current density ranges, which may help explain inconsistencies in results across studies. It is unclear how growth rates in the study compare to growth rates for naturally growing corals in the same area. Overall, the study shows a benefit only if lower current levels are used.
While most work on the electrolysis method has been performed on tropical species, Stromberg et al. (2010) examined responses of Lophelia pertusa, a cold-water coral that is an important ecosystem engineer in the Northeast Atlantic. Lophelia pertusa was exposed to unpowered conditions, a mineral accreted surface with no
15 current, galvanic elements, and three applied currents. After a 6-month period, corals were assessed for growth. The study found no significant differences in growth between treatments. However, the lowest applied current treatment group showed a slightly higher mean growth than the controls albeit not significant. All other treatments had mean growth rates lower than the controls. It is important to note that all growth rates were below or within the range reported for the species (Stromberg et al. 2010). The results corroborate Kihara et al.’s (2013) conclusion that lower current densities may benefit the corals. It also supports Sabater and Yap’s (2002) conclusion that growth on the corals is lower than that of naturally growing corals. Lastly a single study has evaluated growth in a non-coral species, the native eastern oyster, exposed to no current and four levels of electrical current for four months. The study found no significant difference in growth between treatments (Piazza et al. 2009), suggesting that benefits may be limited to corals despite claims that all organisms benefit. However, further study of non-coral species is necessary before conclusions can be made. Overall,
Stromberg et al. and Piazza et al. do not show significant growth benefits for cold-water and non-coral species.
Proposed Mechanisms for Biorock®‘s Beneficial Impact:
There are three primary mechanisms hypothesized to result in a benefit from the electrical current. These are increased pH at the cathode, increased availability of ions, and stimulation of ATP production. Electrolysis creates an alkaline environment on the cathode. This is hypothesized to benefit the corals by creating the alkaline conditions that they rely upon for calcification. This eliminates the need for them to expend energy
16 to create those alkaline conditions and leaves more energy available for growth (Goreau
1997). However, the hydroxyl ions that create the alkaline conditions are almost immediately neutralized by mineral deposition and little pH change is measurable
(Goreau 2012), suggesting a limitation to this hypothesis. Another hypothesis is that the benefit is due to increased ion concentration at the surface of the cathode. Calcification relies on the passive diffusion of ions into the coelenteron (gastrovacular cavity), so increased ion concentration near the surface of the cathode is hypothesized to enhance calcification by enhancing diffusion (Sabater and Yap 2004). Lastly, it is hypothesized that the current may stimulate the production of ATP, which is required for the active transportation of calcium ions to the site of calcification (Stromberg et al. 2010; Goreau
2014). By stimulating the production of more ATP, the electrical current provides additional energy for the energy-demanding processes of calcification, allowing it to grow more quickly. It is important to note that all of these are hypotheses about why corals on electrified structures show growth benefits, but the hypotheses have only been applied theoretically to the results and not specifically tested.
Our Research on Biorock®:
Based on the uncertainty regarding Biorock®’s benefits, further research is necessary to clarify its effectiveness. We conducted laboratory experiments to test the claims of growth enhancement for the electrolytic technology by exposing the temperate solitary coral Balanophyllia elegans to powered and unpowered treatments. The study focused on coral growth, but careful observations were made on other aspects of the community including presence and thickness of algal cover.
17
METHODS:
Tank Description:
We used three circular outdoor tanks at UC Santa Cruz’s Long Marine Lab. One tank was an unpowered control (tank 5) and two tanks were replicated powered experimental tanks (tanks 4 and 6.) For the purposes of this paper, Tank 5 will be referred to as the control tank, Tank 4 will be referred to as Experimental Tank 1, and
Tank 6 will be referred to as Experimental Tank 2. The tanks were plastic and free of substrate. Tanks were 2 meters in diameter and held an estimated 2940 liters. Tanks were
0.85 meters deep. Filtered seawater flowed into the tanks at 4 liters per minute, resulting in a 12-hour turnover time. Each tank contained a cathode and anode placed 2 meters apart, with the same orientation of the cathode and anode in all tanks. The anode and cathode in the control were not connected to a power source.
Electrical System:
The anode was composed of MMO coated titanium mesh. The surface area of the anode was 0.0018 meters squared. The cathode was a square sheet (0.6m X 0.6m) of welded stainless steel ¼” square mesh, with a surface area of 0.294m2. We used a power supply with an output of 12V with 13.5ohm resistors, so that each cathode received between 600-615mA. Each tank had a current density of 2.04A/meter squared. The battery was connected to an outdoor outlet to ensure a continuous power supply. The current level we used falls within the range of current levels used in previous studies of
20mA to 4.16A (Sabater and Yap 2002; Borell et al. 2002; Sabater and Yap 2004; Borell et al. 2010; Stromberg et al. 2010; Kihara et al. 2013.
18
Biology:
We used juveniles from a temperate solitary coral, Balanophyllia elegans that were spawned from mature corals in the UCSC Long Marine Laboratory. The adults were originally collected from the Monterey Peninsula and had been maintained for approximately fifteen years in the UCSC marine laboratory. The juveniles were growing on plastic tiles (25x25 to 40x40mm) to which the mature corals had been attached. We selected 21 tiles and removed other organisms with epoxy, leaving juveniles about 0.6-
3.5mm diameter. After ranking tiles by the number of juveniles on them, we randomly assigned successive sets of 3 tiles to the 3 tanks and a total range of 94-100 healthy juveniles and 112-113 total juveniles. Trays were composed of two pieces of styrene plastic fluorescent light fixture diffuser grating with dimensions 43.3cm X 12.9cm X
3.2cm. Wire hinges allowed the trays to be opened for access to the corals.
We hung the Balanophyllia from the surface of the cathode in plastic cages. We covered the tanks in agricultural shade cloth in order to reduce direct sunlight exposure.
After 7 days it became clear that this shading was inadequate because the coralline algae specimens also present in the tank began to show signs of stress. Semi-circular wooden boards were used to entirely shade the half of the tank containing the biology from direct sunlight. We started the electrical system on July 7. After three days for the system to equilibrate and achieve the desired power level, given a lack of guidelines on appropriate power level and cathode size, we settled the power level at one producing a slow but steady stream of hydrogen bubbles at the cathode. We added the Balanophyllia to the tanks on July 10. Electrical and temperature measurements were taken daily excluding weekends. Water samples were taken periodically of the cathodic surface, the anodes,
19 tank water, and inlet water, and analyzed at the Monterey Bay Aquarium Research
Institute for dissolved inorganic carbon using a Li-Cor 6262 LI-6262 CO2/H20 analyzer.
We fed the corals fresh brine shrimp Artemia maritima nauplii on Monday,
Wednesday, and Friday. We removed the tiles from the tanks and trays, placed corals from each tank into separate plastic feeding trays, and gave them equivalent amounts of the brine shrimp. After one hour, we returned them to the tanks. The experiment ran for fourteen days before corals were analyzed for growth.
Growth Measurements:
We took high-resolution photographs of the corals at the start of the experiment.
We rephotographed them at the end and used the initial and final photographs to determine growth. Coral diameter was measured in both photographs. The measurements of diameter were converted to absolute measurements by measuring a known length in the photo and dividing the measurement by the known length to give a factor. The diameter measurement was divided by this factor in order to calculate the absolute diameter. Initial diameter was subtracted from final diameter to calculate the absolute growth of each individual.
Algae Measurements:
We used the initial and final high-resolution photographs of the corals to measure algae on each tile. To measure the algae, we created a 0-2 scale. A 0 meant no algae, a 1 meant a thin to moderate layer of algae, and a 2 meant a thick layer of algae. We laid a grid over each tile and took a measurement (0-2) at each intersection. Each tile had a total of 30 measurements. For each tile, we calculated the proportion of each algae level
20 before and after for each tile and each tank. We also added up the 30 measurements to get an overall score for each tile both before and after.
Data Analysis:
We ran our statistics using SPSS software. For coral growth, we took the natural log of the final size divided by the initial size (Ln(Final/Initial) and plotted it against the initial size of the coral in order to see the influence of initial size on growth for the three tanks. We plotted the average and maximum growth for each tile against the position of that tile in each tank in order to explore spatial differences in growth. We explored spatial differences in growth within each tile by mapping the growths of individuals on a few tiles from each tank. We divided the corals from each tank into three size classes based on initial size. The three classes were 1-1.5mm, 1.5-2mm, and 2-2.5mm. We compared growth between tanks for each of the three size classes. We used one-way and
Welch ANOVA tests to determine whether differences in growth were significant between tanks.
For the algae, we used one-way and Welch ANOVA tests to determine whether there were significant differences in algae between tanks. We compared the number of each class before and after and the proportion of each class before and after between the tanks for each class of algae (0-2.) We also compared the overall score before and after as well as the difference in the before and after scores for each tank.
RESULTS:
Coral Growth:
21 Average growth in Experimental Tank 1 was 0.145mm. Experimental Tank 2 had an average growth of 0.199mm. Average growth in the Control Tank was 0.216mm.
The Control Tank exhibited the highest growth and Experimental Tank 1 the lowest growth with Experimental Tank 2 in the middle. Growth between tanks was significantly different. Variance was unequal between the tanks, so a Welch ANOVA was performed.
The ANOVA test showed a p-value of 0.001 (statistic=7.611, df1=2, df2=182.436.)
Further examination showed that Experimental Tank 1 and the control were significantly different (Bonferroni p-value 0.001) while the control and Experimental Tank 2 were not significantly different (Bonferroni p-value 0.197). Experimental Tanks 1 and 2 were also not significantly different (Bonferroni p-value 0.259). Looking at mean growth values for the tanks, this means that Experimental Tank 1 grew significantly less than the control. While Experimental Tank 2 grew less than the control, it was not a significant difference. This is the same for Experimental Tank 1 and Experimental Tank 2. While
Experimental Tank 1 grew less, the difference was not significant.
Table 1. Figure 1 shows the average growth for the three tanks.
Tank Average Growth (mm) Control 0.216 Experimental Tank 1 0.145 Experimental Tank 2 0.199
Table 2. Figure 2 shows the results of the Welch ANOVA comparing the tanks.
Tanks Compared Statistics All Tanks Statistic=7.611, df1=2, df2=182.436, p-value=0.001 Experimental Tank 1 & Control p=0.001 Bonferroni post-hoc Experimental Tank 2 & Control p=0.197 Bonferroni post-hoc Experimental Tank 1 & 2 p=0.259 Bonferroni post-hoc
22 The plots of maximum and average growth versus tile position showed no obvious pattern between tanks. When facing the cathode, position 1 is the tile on the far left of the cathode while position 7 is on the far right. The tile in position 1 was growing well in the Control Tank, but poorly in Experimental Tanks 1 and 2. Position 2 tiles grew well in Experimental Tank 2, poorly in Experimental Tank 1, and mediocrely in the control. The tile in position 3 grew very poorly in Experimental Tank 2 and mediocrely in Experimental Tank 1. The tile in position 3 in the Control Tank was lost partway through the experiment. Position 4 tile grew very well in Experimental Tank 2, moderately well in the control, and poorly in Experimental Tank 1. The tile in position 5 grew moderately well in the Control Tank, but poorly in Experimental Tanks 1 and 2.
Position 6 tile grew very well in Experimental Tanks 1 and 2, but only moderately well in the control. Position 7 grew poorly in all three tanks. The graphs of growth versus tile position can be seen in Figures 1-3. Spatial mapping within tiles also showed no pattern in growth based on the position of the corals.
Figure 1. Figure 1 shows average and maximum growth versus tile position for Experimental Tank 1.
Growth & Tile Position for Experimental Tank 1
0.4 1
0.3 0.8 0.6 0.2 0.4 Avg Growth 0.1 0.2 Max Growth Average Growth (mm) 0 0 (mm) Maximum Growth Tile Position 0 2 4 6 8
23 Figure 2. Figure 2 shows average and maximum growth versus tile position for Experimental Tank 2.
Growth & Tile Position for Experimental Tank 2
0.4 1 0.35 0.8 0.3 0.25 0.6 0.2 0.15 0.4 Avg Growth 0.1 Average Growth (mm) 0.2 Max Growth
0.05 (mm) Maximum Growth 0 0 Tile Position 0 2 4 6 8
Figure 3. Figure 3 shows average and maximum growth versus tile position for Experimental Tank 1.
Growth & Tile Position for Control Tank
0.4 1 0.35 0.8 0.3 0.25 0.6 0.2 0.15 0.4 Avg 0.1 Average Growth (mm) 0.2 Max
0.05 (mm) Maximum Growth 0 0 Tile Position 0 1 2 3 4 5 6 7
24 Coral Growth Based on Size:
Corals in each tank were separated into size class based on their initial size. The first size class was 1-1.5mm, the second size class was 1.5-2mm, and the third size class was 2-2.5mm. For size class 1-1.5mm, mean growth in millimeters for Experimental
Tank 1 was 0.139, for Experimental Tank 2 was 0.260, and for the Control Tank was
0.214. Variance was equal for initial size for the 1-1.5mm size class, so a one-way
ANOVA was used. Initial size between tanks was not significantly different (df1=2, df2=109, F=0.686, p-value=0.506). Variance was unequal for growth, so a Welch
ANOVA was used. Growth was just above being significantly different (statistic=3.165 df1=2, df2=42.803, p-value=0.052.) Growth between Experimental Tank 1 and the control was not significantly different (Bonferroni p-value 0.188). Growth between
Experimental Tank 2 and the control was not significantly different (Bonferroni p-value
1.00). Growth was also not significant between Experimental Tanks 1 and 2 (Bonferroni p-value .062).
For size class 1.5-2mm, mean growth in millimeters for Experimental Tank 1 was
0.156, for Experimental Tank 2 was 0.204, and for the Control Tank was 0.239.
Variance was equal for both initial size and growth, so a one-way ANOVA was used.
Initial size and growth are not significantly different between tanks (initial size: df1=2, df2=126, F=0.207, p-value=0.813; growth: df1=2, df2=126, F=1.284, p-value=0.281.)
Growth between Experimental Tanks 1 and 2 is not significantly different with a
Bonferroni p-value of 1.00, growth between the control and Experimental Tank 1 is also not significantly different with a Bonferroni p-value of 0.335, and the control and
Experimental Tank 2 are not significantly different with a Bonferroni p-value of 1.00.
25 For size class 2-2.5mm, mean growth in millimeters was 0.098 for Experimental
Tank 1, 0.115 for Experimental Tank 2, and 0.445 for the Control Tank. Variance was unequal for initial size, so a Welch ANOVA was performed. Initial size was not significantly different between tanks with a p-value of 0.962 (statistic=0.038, df1=2, df2=14.204). Variance was equal for growth, so a one-way ANOVA was used. Growth was significantly different between tanks with a p-value of <0.0001 (df1=2, df2=44,
F=12.862.) Growth between Experimental Tanks 1 and 2 were not significantly different with a Bonferroni p-value of 1.00. Growth was significantly different between
Experimental Tank 1 and the Control Tank and between Experimental Tank 2 and the
Control Tank, both with Bonferroni p-values of <0.0001. Based on mean growth, the control grew significantly more than Experimental Tanks 1 and 2. Mean growth and
ANOVA results for all three size classes can be seen in Tables 3 and 4.
Table 3. Table 3 shows the mean growth of corals in each of the three size classes for the three tanks.
Tank Mean Growth (mm) Mean Growth (mm) Mean Growth (mm) Size Class 1-1.5mm Size Class 1.5-2mm Size Class 2-2.5mm Control 0.214 0.239 0.445 Experimental Tank 1 0.139 0.156 0.098 Experimental Tank 2 0.260 0.204 0.115
Table 4. Table 4 shows the results of the Welch ANOVA comparing the growth of the 1- 1.5mm size class and the one-way ANOVA results comparing growth of the 1.5-2mm and 2-2.5mm size classes. The p-values comparing between specific tanks (e.g., Experimental 1 & Control) are Bonferroni post-hoc values.
Tanks Compared Statistics 1-1.5mm Statistics 1.5-2mm Statistics 2-2.5mm All Tanks Statistic=3.165 df1=2, df2=126, df1=2, df2=44, df1=2,df2=42.803, F=1.284, p=0.281 F=12.862, p=<.0001 p=0.052 Experimental 1 & Control p=0.188 p=0.335 p=<0.0001 Experimental 2 & Control p=1.00 p=1.00 p=<0.0001 Experimental 1 & 2 p=0.062 p=1.00 p=1.00
26
The plot of the ln(final size/initial size) against the initial size showed a slight downward linear trend for all three tanks. The smallest corals had the highest growth and the largest corals had the lowest growth. Experimental Tank 2 shows the fastest decrease as initial size increases with a slope of -0.1758 (R2 0.2). Experimental Tank 1 had the second fastest decrease with a slope of -.0825 (R2 0.1). The Control Tank exhibited the slowest decrease with a slope of -.0431(R2 0.02). Both experimental tanks are decreasing more quickly compared to the unpowered tank. Experimental Tank 2 is decreasing approximately four times as quickly as the control. Experimental Tank 1 is decreasing about twice as quickly as the control. This means that there is a larger difference in growth between smaller and larger corals in the powered than the unpowered tanks. The graphs of ln(final size/initial size) can be seen in Figures 4-6.
Figure 4. Figure 4 shows the graph of the ln(final size/initial size) versus initial size for Experimental Tank 2.
Experimental Tank 1
0.5
0.4
0.3
0.2 Series1
Ln(Final/Initial) Linear (Series1) 0.1
0 0 0.5 Initial 1 1.5 Size 2 (mm) 2.5 3 -‐0.1 y = -‐0.0825x + 0.2193
27 Figure 5. Figure 5 shows the graph of the ln(final size/initial size) versus initial size for Experimental Tank 2.
Experimental Tank 2
0.8
0.6
0.4 LN Linear (LN) Ln(Final/Initial) 0.2 Linear (LN) 0 0 1 Initial 2 Size (mm) 3 4 y = -‐0.1758x + 0.4244 -‐0.2 R² = 0.20339
Figure 6. Figure 6 shows the graph of the ln(final size/initial size) versus initial size for Experimental Tank 2.
Control Tank
0.8 0.7 0.6 0.5
0.4 0.3 LN
Ln(Final/Initial) 0.2 Linear (LN) 0.1 0 0 0.5 1 1.5 2 2.5 3 3.5 -‐0.1 Initial Size (mm) y = -‐0.0431x + 0.2051 R² = 0.02204 -‐0.2
28 Algae Growth:
The mean initial overall score for the algae was 15.00 for Experimental Tank 1,
14.85 for Experimental Tank 2, and 16.16 for the Control Tank. The results of the one- way ANOVA test do not show a significant difference in the mean initial scores between tanks (df1=2, df2=17, F=0.145, p-value=0.866). The mean final overall score was 20.28 for Experimental Tank 1, 21.42 for Experimental Tank 2, and 36.00 for the Control Tank.
The results of the one-way ANOVA test do show a significant difference in final overall score between tanks (df1=2, df2=17, F=5.472, p-value=0.015.) The mean difference
(final score-initial score) for Experimental Tank 1 was 5.28, 6.57 for Experimental Tank
2, and 19.83 for the control. The one-way ANOVA shows a significant difference in the difference of the overall score between tanks (df1=2, df2=17, F=6.308, p-value=0.009).
Further examination shows that for the final score Experimental Tank 1 and the control are significantly different (Bonferroni p-value 0.024). Experimental Tank 2 and the control are also significantly different (Bonferroni p-value 0.038). Experimental Tank 1 and Experimental Tank 2, however, are not significantly different (Bonferroni p-value
1.00). Based on the mean final scores, the experimental tanks have significantly less algae than the control. The difference between final and initial score shows the same pattern. Experimental Tanks 1 and 2 are significantly different from the control
(Bonferroni p-values 0.014 and 0.026), but are not significantly different from each other
(Bonferroni p-value 1.00.) Based on the mean differences, Experimental Tanks 1 and 2 have a significantly lower difference than the Control Tank. The mean scores and
ANOVA results can be seen in Tables 5 and 6.
29 Table 5. Table 5 shows the mean initial score, the mean final score, and the mean difference in score for overall algae in the three tanks.
Tank Mean Initial Score Mean Final Score Mean Difference Control 16.16 36.00 19.83 Experimental 1 15.00 20.28 5.28 Experimental 2 14.85 21.42 6.57
Table 6. Table 6 shows the results of the one-way ANOVA comparing the mean initial algae score, the mean final algae score, and the mean difference in score for the three tanks. The p-values comparing between specific tanks (e.g., Experimental 1 & Control) are Bonferroni post-hoc values.
Tanks Compared Initial Score Statistics Final Score Statistics Difference Statistics All Tanks df1=2, df2=17, df1=2, df2=17, df1=2, df2=17, F=0.145, p=0.866 F=5.472, p=0.015 F=6.308, p=0.009 Experimental 1 & p=1.00 p=0.024 p=0.014 Control Experimental 2 & p=1.00 p=0.038 p=0.026 Control Experimental 1 & p=1.00 p=1.00 p=1.00 2
Algae Growth Based on Algae Class:
The mean initial proportion for Class 0 algae is 0.581 for Experimental Tank 1,
0.519 for Experimental Tank 2, and 0.556 for the Control Tank. Variance was unequal so a Welch ANOVA was performed. Initial proportions were not significantly different with a p-value of 0.693 (statistic=0.382, df1=2, df2=9.218.) Mean final proportion for
Experimental Tank 1 is 0.486, for Experimental Tank 2 is 0.412, and for the Control
Tank is 0.244. Variance was equal so a one-way ANOVA was performed. Final proportions between tanks were not significantly different with a p-value of 0.064
(df1=2, df2=17, F=3.240.) Class 0 algae decreased by 9.5% in Experimental Tank 1, by
10.7% in Experimental Tank 2, and by 31.2% in the Control Tank. Although the proportions are not significantly different, Class 0 algae decreased approximately 3 times as much in the control as compared to the experimental tanks.
30 The mean initial proportion for Class I algae is 0.339 for Experimental Tank 1,
0.467 for Experimental Tank 2, and 0.350 for the Control Tank. The mean final proportion is 0.353 for Experimental Tank 1, 0.429 for Experimental Tank 2, and 0.300 for the Control Tank. The results of the one-way ANOVA for the proportion of Class I
(thin to moderate) algae do not show a significant difference between the tanks either initially (df1=2, df2=17, F=1.339, p=0.288) or after (df1=2, df2=17, F=0.958, p=0.403).
Class I algae increased by 1.4% in Experimental Tank 1, decreased by 3.8% in
Experimental Tank 2, and decreased by 5% in the Control Tank. Although proportions are not significantly different, Class I algae is only increasing in Experimental Tank 1, but decreasing in the Control Tank and Experimental Tank 2. Class I algae decreased slightly more in the control than Experimental Tank 2.
The mean initial proportion for Class II (thick) algae is 0.08 for Experimental
Tank 1, 0.01 for Experimental Tank 2, and 0.09 for the Control Tank. Due to unequal variances, a Welch ANOVA was performed. The results of the ANOVA do not show a significant difference between tanks for the initial proportion of class II algae
(statistic=2.755, df1=2, df2=9.014, p-value .0116). The mean final proportion for Class
II algae is 0.16 for Experimental Tank 1, 0.14 for Experimental Tank 2, and 0.45 for the control. Variances were equal so a one-way ANOVA was performed. The one-way
ANOVA shows a significant difference between tanks for the final proportion of Class II algae (df1=2, df2=17, F=5.561, p=0.014). Further examination shows that Experimental
Tanks 1 and 2 are significantly different from the Control Tank (Tukey p-values 0.03 and
0.02), but are not significantly different from each other (Tukey p-value 0.980). Based on the mean final proportions, the proportion of class II algae is significantly higher in
31 the control tank than in Experimental Tanks 1 and 2. Class II algae increased by 36% in the control, but only by 8% in Experimental Tank 1 and 13% in Experimental Tank 2.
The mean initial and mean final proportion for all three algae classes can be seen in
Tables 7 and 8. The ANOVA results for the final proportion of each algae class can be seen in Table 9.
Table 7. Table 7 shows the mean initial proportion for each of the three algae classes.
Tank Initial Class 0 Initial Class I Initial Class II Control 0.556 0.350 0.09 Experimental 1 0.581 0.339 0.08 Experimental 2 0.519 0.467 0.01
Table 8. Table 8 shows the mean final proportion for each of the three algae classes.
Tank Final Class 0 Final Class I Final Class II Control 0.244 0.300 0.45 Experimental 1 0.486 0.353 0.16 Experimental 2 0.412 0.429 0.14
Table 9. Table 9 shows the results of the one-way ANOVA for the final proportion for each of the three algae classes. It should be noted that the initial proportion showed no significant difference between tanks for any of the size classes. The p-values comparing between specific tanks (e.g., Experimental 1 & Control) are Tukey post-hoc values.
Tanks Compared Class 0 Statistics Class I Statistics Class II Statistics All Tanks df1=2, df2=17, df1=2, df2=17, df1=2, df2=17, F=3.240, p=0.064 F=0.958, p=0.403 F=5.561, p=0.014 Experimental 1 & p=0.056 p=0.844 p=0.03 Control Experimental 2 & p=0.220 p=0.379 p=0.02 Control Experimental 1 &2 p=0.711 p=0.682 p=0.980
DISCUSSION:
Our results show either no impact or a detrimental impact on growth for corals exposed to the electrical current. These results are inconsistent with our hypothesis as
32 well as the claims set forth by the technology’s proponents (Goreau 2014). While these results may be partially explained by species-specific differences in tolerance to the electrical current, it does not fully explain the overall pattern seen in our study and the larger literature. Research on the subject exhibits a broad trend of the technology being effective for warm-water corals but ineffective for cold-water corals, which suggests an influence of photosynthesis that is surprising given the proposed physiological mechanisms behind the technology’s benefit. The influence of photosynthesis may be related to our results on algae growth between treatments. Our results show that while algae grew in all tanks throughout the course of the experiment, it grew more in the control tank as well as more thickly. The depression in algal growth caused by the presence of the electrical current may allow for increased photosynthesis and hence increased calcification. While other factors are no doubt at play, the effect of the electrical current on algal growth and its relationship to the claims of growth enhancement represents an important new area of study.
The lack of spatial differences in coral growth between tiles suggests that orientation to the cathode does not influence growth. That is, tiles on the edges do not grow less than tiles located more centrally or vice versa. There is, of course, variation in growth between the tiles, but no spatial pattern to this variation, which means that the variation is due to other factors and not the position of the tile in relation to the cathode.
The lack of spatial difference in growth within tiles shows that position on the tile doesn’t impact growth. For example, corals along the edge do not grow more than corals in the middle. There is also no overall pattern of corals growing poorly being next to corals growing well, which would suggest competition between the individuals. The lack of
33 spatial differences in growth within the tank allows us to more confidently examine differences in growth between the tanks.
Our results are not consistent with claims made about the benefits of electrolysis on coral growth. Growth between treatments was significantly different but while claims suggest a 3 to 5-fold increase in growth (Goreau 2014), corals exposed to an electrical current grew less than control corals. Control corals had the highest mean growth of the three groups. Growth in the controls was significantly higher than the growth of one group of experimental corals, but not significantly greater than the growth of the other experimental group.
Analysis of the different size classes shows a similar pattern. There was no significant difference for corals of size class 1-1.5mm and 1.5-2mm. For the size class 1-
1.5m, the experimental group has a slightly elevated mean growth compared to the control, but the second experimental group has a lower mean growth than the control.
For the size class 1.5-2mm, both experimental groups exhibited lower growth than the control. Growth was, however, significantly different for corals of initial size 2-2.5mm.
The unpowered corals were significantly different from the powered corals while the powered corals were not significantly different from each other. The unpowered corals grew significantly more than the powered coral, which is the opposite effect proposed by the technology’s proponents.
Thus, with the exception of the one experimental group in size class 1-1.5mm, the control group always has higher growth than the experimental group whether or not the difference is significant. Given the short duration of the experiment, it is possible that these differences may have become significant over time. The results suggest that the
34 electric current may actually depress growth. While this depression is insignificant for
Tank 6, it is significant for Tank 4, which may point to factors other than the electric current that are contributing to the lower growth. These possible factors will be discussed further in depth later.
An electric current that is too high may cause the depression in growth seen in the experimental corals. The experimental corals were receiving 0.6-0.615A and a current density of 2.04A/m2. This current density is well above the threshold found by Kihara et al. to cause detrimental effects. Their study found that detriments to growth began to occur around current densities of 100mA/m2 (Kihara et al. 2013) and our current density is several orders of magnitude above that level. Stromberg et al. also found detriments to growth above current levels of 0.12A (Stromberg et al. 2010), which again is much lower than current levels in our study. Given how much higher our current levels are than thresholds where current begins to harm the corals, it would be expected that we would have seen a large depression in the growth of the experimental corals. This was not the case. In fact, growth was only significantly lower than controls for one of the experimental groups. It is possible that response to current is species specific and some species may exhibit a greater tolerance to electrical currents than others. For example,
Borell et al. investigated the effect of the electrolytic technology on two species of coral:
Acropora pulchra and Acropora yongei. A.yongei had lowest levels of growth and highest rates of mortality on the cathode while A. pulchra had controls with the lowest growth. Mortality rates were equal for A. pulchra across treatments (Borell et al. 2010).
The differences in response between species suggest that A. pulchra has a greater tolerance and even benefits from the electric field whereas A. yongei has a low tolerance.
35 Thus, the coral species in our study, Ballanophyllia elegans, may have higher tolerance to electrical currents than species used by Kihara et al. and Stromberg et al. This higher tolerance resulted in a lower detrimental impact on growth in our study than would be expected based on the results of those studies.
While the above discussion points to the necessity of using lower current densities, studies have shown beneficial effects at current levels equal to and much greater than ours. Sabater and Yap (2002; 2004) used current levels from 2.5A up to
4.16A with beneficial effects on both girth and longitudinal growth for treated corals compared to untreated corals. These differences may point to the species they used,
Porites cylinrica Dana, having a very high tolerance to electrical current. Based on these results, response seems to be dependent on level of current applied and species.
However, variation in response of species cannot be explained just by variation in electric current levels or differences in how species tolerate electricity. Growth benefits have been found at higher current levels (Sabater and Yap 2002; Borell et al. 2002) while other studies simultaneously claim detriments to growth at higher current levels (Kihara et al. 2013; Stromberg et al. 2010). While difference in species tolerance explains some of this variation, there is an overall trend to the species that seem to be responding well and those that are not. A main difference between these studies that has yet to be addressed is that some use warm-water photosynthetic species and some use cold-water, non-photosynthetic species. Sabater and Yap (2002; 2004), Kiraha et al. (2013), and
Borell et al. (2010) conducted their experiments using warm-water, photosynthetic coral species. Our study and Stromberg et al. conducted experiments using cold-water, non- photosynthetic species. Only studies using warm-water, photosynthetic species have
36 shown any significant growth benefit from the technology. This benefit is not just due to lower current levels being used in studies of warm-water species as compared to their cold-water counterparts. Current levels have varied significantly within studies done on warm-water species from .02A to 4.16A. Despite this variation, there is an overall pattern of growth benefit. For example, Borell et al. found high growth rates for both A. pulchra and A. yongei inside the electric field. A. pulchra also showed high growth rates on the cathode while A. yongei did not grow well on the cathode (Borell et al. 2010). As previously discussed, A. yongei’s poor performance on the cathode may be due to harmful levels of electric current for the species. Overall, however, the study shows a significant increase in growth in corals exposed to an electrical current based on growth for A. pulchra both on the cathode and inside the electric field and high growth for A. yongei within the electric field. Sabater and Yap’s results show the same overall pattern for another species, P. cylindrica. P. cylindrica showed significantly higher growth for treated than untreated corals (Sabater and Yap 2002), despite having run the experiment at the highest current levels of any study that has explored the technology. Lastly, Kihara et al. found a large increase in growth for one species at lower current densities.
In contrast, growth is either not impacted or negatively impacted for the two studies using cold-water corals. Our study of B. elegans found lower growth for both experimental groups compared to controls although only one experimental group was significantly different. Our results suggest not only a null effect of the electrical current, but also a detrimental effect of the electrical current. Stromberg et al. had similar results despite using lower current levels. All experimental groups with the exception of one were lower than control corals in Stromberg et al.’s study of L. pertusa albeit not
37 significantly (Stromberg et al. 2010). Thus, in addition to variation caused by species tolerance and current level, there also seems to be overall trends of the technology being effective for warm-water corals, but ineffective or even detrimental to cold-water corals.
Based on the proposed physiological mechanisms behind the technology, the difference in response of cold-water and warm-water corals is surprising. If increased availability of ions around the cathode, increased pH, or ATP stimulation were responsible (Borell at al. 2010; Sabater and Yap 2002; Goreau and Hilbertz 2012;
Stromberg et al. 2010; Goreau 2014; Goreau 2012), results would be expected to be similar regardless of whether the organisms were photosynthetic or not. In other words, growth rates would occur independently of photosynthetic capacity of the organism
(Borell et al. 2010). Benefits for cold-water corals may occur more slowly due to calcium carbonate being more soluble in cold water (Goreau 2012), but they would still occur. However, benefits are not occurring for cold-water species, which suggests that photosynthesis may play an important role in determining whether or not the technology is beneficial.
Our results for algae growth are interesting because they indicate a communal level benefit that may be occurring for the corals due to the technology that could influence physiological processes at the individual level. At the start of the experiment, there is no significant difference in algae between the tanks. By the end of the experiment, however, there is a significant difference in the overall algal cover between the three tanks. The unpowered tank has the highest algal cover and is significantly different from both powered tanks. The powered tanks have lower algal cover and are not significantly different from each other. Given that there was no initial difference
38 between the tanks, algae has grown more in the powered tank than it has in the unpowered tanks. The difference between the final and initial algal score can help illuminate how much algal cover changed during the experiment. A larger difference means a greater change in algal cover. The unpowered tank had a significantly larger difference (19.83) than the powered tanks (5.28; 6.57.) Not only do these numbers show that the unpowered tank has grown significantly more than the powered tanks, they also show that the algae did not grow very much in the powered tanks.
Results for each of the algae classes help to illuminate where the changes in algal cover are occurring. Initially, there is no significant difference between the tanks for areas of no algae (Class 0), areas of thin to moderate algae (Class I), and areas of thick algae (Class II). There is no significant difference at the end of the experiment for the areas of no algae, but the areas with no algae decreased more in the unpowered than the powered tanks. The uncovered area only decreased by 9.5% and 10.7% in the powered tanks, but decreased by 31.2% in the unpowered tank. This suggests a greater growth of algae in the unpowered tank with the result that areas of no algal cover have been overgrown more than in the powered tanks. There is also no significant difference in thin to moderate algae cover between the tanks although the unpowered tank and one powered tank decreased slightly with the unpowered tank decreasing more. Since the uncovered areas are decreasing in all tanks, this suggests that the thin to moderate algae is growing and turning into areas of thick algae. The thick algae is significantly different between the tanks at the end of the experiment with the unpowered tank having a higher proportion of the thick algae. In addition, the thick algae increased by much more (36%) in the unpowered tank than it did in the powered tanks (8%, 13%). Based on the changes
39 in all algae classes, areas of no algae and thin to moderate algae cover are growing and changing into areas of thick algae in the unpowered tanks. While algae is changing in the unpowered tanks, it is not changing very much, especially compared to the changes occurring in the unpowered tank. Overall, the results show that the unpowered tank has a greater growth of algae then the powered tanks, especially for areas of thick algal cover.
This suggests that presence of the electrical current inhibits growth of algae that occurs under normal conditions.
Changes in algal cover in the presence of the electrolytic technology are not unusual and have been alluded to in several of the studies performed on Biorock®. In their exploration of electrochemical deposition of calcium carbonate as an alternative substrate to the unnatural ones traditionally used for artificial reefs, Schuhmacher and
Schillak (1994) noticed that algae did not grow while power was turned on for the structures, but algae began to grow as soon as the power was turned off. When the power was turned back on, the algae disappeared (Schuhmacher and Schillak1994). These results corroborate the findings in our study that the electrical current inhibits the growth of the algae. Goreau noted that the main difference between Biorock® projects in
Jamaica and the natural reefs growing near them was that the natural reefs were being overgrown by algae. Algae was still present on the Biorock® projects, but it was not dominating the system and overgrowing the corals like it was on the natural reefs. When electricity was turned off, coral on Biorock® structures were eventually overgrown by algae (Goreau et al. 2004). Not only does this illustrate the ability of the electrolytic technology to prevent the overgrowth of coral by algae, but based on the high growth exhibited by the electrified corals compared to their natural counterparts, it suggests that
40 the prevention of algae and coral growth may be related. The untreated corals in Sabater and Yap’s study exhibited high rates of mortality and most of these mortalities were due to the corals being overgrown by algae. The treated corals also had some mortalities caused by algal overgrowth, but their rate of mortality was much lower (Sabater and Yap
2002). This suggests that the electrical current may be deterring the growth of the algae and therefore limiting the number of corals that die. Sabater and Yap’s longer-term study also noted that while algae were not present on the electrified structures while an electrical current was running, the algae began to grow when the electrical current was turned off. The negative impact of the algae growth can be seen in coral growth. The treated corals exhibited significantly greater girth and longitudinal growth when the electricity was turned on. However, once the electricity was turned off and the algae began to grow, significant differences in longitudinal growth disappeared and while difference in girth growth was still significant, the difference in girth growth between the treated and untreated corals was less (Sabater and Yap 2004). This suggests not only that the electrical current discourages growth of algae, but also that growth of the algae and growth of the corals are related. In a study of algae and coral competition, it was found that corals grew significantly less (3-10 times) and had significant tissue death when algae was present. The study concluded that algae pose a substantial threat to coral growth and survival, especially considering the increase in the presence of algae that is occurring on reefs (Lirman 2001). Another study explored the effect of the algae
Sargassum hystrix on the coral Porites porites and found that exposure to the algae caused a significant reduction in growth (80%) compared to control corals not exposed to algae (River and Edmunds 2001).
41 Furthermore, electrolysis is used in several contexts to prevent the growth of algae. Ship hulls are electrified not only to prevent corrosion, but also to prevent biofouling, which includes the growth of algae. Many patents exist for variations on the exact device and method of how the electrolytic system is applied to ships, but all claim to inhibit the growth of marine organisms (Harms and Hutchison 1980; Riffe and Carter
1997; Osborn 1972; Diprose et al.1989). Corporations also sell electrolytic systems for ships to prevent biofouling. One study examined the effectiveness of an electrochemical system to prevent the growth of micro and macroorganisms in seawater cooling pipelines for power plants. The system was successful in preventing growth of organisms on the electrode surface (Wake et al. 2006), which is comparable to the prevention of algal growth on the cathode in Biorock® studies. Another study was conducted on the use of electrolysis to kill algae in ships’ ballast water. It was found to be effective with no living algae cells present in the ballast water after 72 hours (Dang et al. 2006). Finally,
Alfafara et al. explored its use as a means of removing algae from eutrophied lakewater.
The study found that it was effective due to the interaction between electroflotation where bubbling up of gases produced by electrolysis removes particles, and electroflocculation where particles are removed by the addition of coagulating metal ions.
Electroflotation alone could not completely remove the algae with only 40-50% removal
(Alfafara et al. 2002). Electroflotation and not electroflocculation is probably occurring on Biorock® structures, which would explain why in our study algae did grow to some extent on the powered structures and why some treated corals died of algae overgrowth in
Sabater and Yap’s study (2004).
42 The above discussion illustrates the ability of electrolysis to prevent the growth of algae and its potential connection to increased growth in corals on electrified reefs, especially since corals have been documented to grow less in the presence of algae. This is especially important when one considers that overgrowth by algae is one of the foremost issues facing coral reefs and that degraded reefs tend to have a predominant presence of algae (Goreau et al. 2004; Moberg and Folke 1999). The widespread nature of the algal problem could explain why Biorock® projects have been claimed to be successful all over the world despite enormous variability in local species and environmental stresses. However, the question remains of the mechanism by which prevention of algae growth could result in increased coral growth. It is possible that algae decrease zooxanthellae’s ability to photosynthesize by shading. Since calcification is directly related to photosynthesis, this would impact corals’ ability to calcify and grow.
The mechanism behind the relationship between photosynthesis and calcification is not fully understood, but one possible mechanism is decreased availability of phototrophic carbon for the corals. Excess carbon from photosynthesis is provided to the corals by the zooxanthellae, which may be used by the corals for the energy-intensive process of calcification. Decreasing light decreases productivity and decreases carbon provided to the corals. The corals must then seek other sources of nutrition, which requires energy and decreases coral growth. Thus, decreased photosynthesis results in decreased coral growth (Muller-Parker and D’Elia 1997). By preventing the growth of algae, Biorock® can increase light levels, increase photosynthesis, and increase coral growth compared to nearby corals affected by algae, which would result in the faster growth rates of coral on
Biorock® compared to natural corals per the technology’s claims.
43 Based on this mechanism, zooxanthellae in corals on Biorock® should behave differently than natural corals. Two studies on the technology have explored these physiological responses. Borell et al. (2010) measured chlorophyll fluorescence, zooxanthella densities, and chlorophyll a for corals exposed to an electrical current for two species, Acropora yongei and Acropora pulchra. A. pulchra had significantly higher zooxanthellae densities on the cathode than the controls. Fluorescence did not vary significantly between treatments for A. pulchra and chlorophyll a was not significantly different between the cathode and control. Growth was significantly higher on the cathode than in the controls. In contrast, A. yongei had significantly lower densities of zooxanthellae on the cathode than in the control, significantly lower fluorescence than the cathode, and no significant difference in chlorophyll a. Growth was significantly lower on the cathode. As discussed previously, A. yongei appears to have a lower tolerance for the electrical current and this low tolerance may explain the opposite impact of the electrical current than in A. yongei. Low zooxanthellae density and decreased fluorescence are signs of physiological stress in corals (Borell et al. 2010), which corroborates the idea that the electrical current is stressful for A. yongei. Overall, however, Borell et al. (2010) concluded that zooxanthellae density was related to coral growth on the cathode. Goreau et al. (2004) performed a similar study on six types of coral grown on Biorock® and measured zooxanthellae density, chlorophyll density, and mitotic indices of zooxanthellae. The Biorock® corals had higher zooxanthellae densities and higher mitotic indices. Half had higher chlorophyll concentrations than controls and half had lower chlorophyll than controls (Goreau et al. 2004). Lower chlorophyll is common in corals exposed to high light conditions (Muller-Parker and D’Elia 1997). The
44 results of both of these studies corroborate the hypothesis that coral growth on the cathode is related to photosynthetic performance of the zooxanthellae. In addition, both of these studies do not support the idea that photosynthesis and calcification should be decoupled if the mechanism is increased ion availability, increased pH, or ATP stimulation. Thus, there is an apparent link between changes in photosynthesis in
® Biorock corals, coral growth, and algae growth.
The photosynthetic component of Biorock® is interesting in the context of our research. Although our results do corroborate the prevention of algal growth by the electrolytic technology, coral growth tended to be higher in the controls. While the impact on growth is the opposite of what may be expected, our coral species, B. elegans, is not photosynthetic. In addition, B. elegans does not like to grow in high light conditions, preferring shade. So, not only may the lack of algal growth not have affected the treated corals, it may have been a detriment. The algae in control tank may have shaded the corals and kept them in the lower light conditions that they prefer. Decreased light could account for the increase in growth for the control tank if it made environmental conditions for the control corals less stressful. This may also explain the lack of significant difference in growth for L. pertusa, another cold-water non- photosynthetic species (Stromberg et al. 2010) and the overall trend of benefits being seen for warm-water photosynthetic species and not for cold-water non-photosynthetic species.
In summary, our results for coral growth do not support growth enhancement claims about the technology. In the context of other studies, they do raise interesting questions about species-specific tolerance to current densities as well as the appropriate
45 current densities to expose the corals to. While these two factors may explain some of the variation in response to the technology, there seem to be patterns of beneficial impacts for photosynthetic species, but not for non-photosynthetic species. This trend may be related to the technology’s ability to prevent algal growth, which may have a positive impact on photosynthesis and the corals ability to calcify. This benefit, however, would not impact non-photosynthetic species. In addition, if current levels are too stressful for a species, the benefit would be negated, resulting in the variation seen even within studies performed on photosynthetic species. At the same time, enhanced calcification due to enhanced photosynthesis caused by decreases in algal cover may only be part of the mechanism. An interaction may exist between increased ion availability at the surface of the cathode and increased photosynthesis. Enhanced photosynthesis provides additional energy for calcification while increased ion availability provides additional building blocks for the growth process. This interaction may ultimately be responsible for the enhanced growth. Based on limitations of our study and others, it is important for further research to clarify the mechanisms.
Limitations of our study include short duration and a lack of tanks. Positioning of the tanks may have caused variation in environmental conditions that could cause some of the differences in coral growth seen in the study. We did not have enough tanks to adequately randomize them and limit the influence of confounding variables. Our study is limited by time constraints as well, which only allowed for a short duration of the study. Future studies should be careful to minimize these constraints. A new study should be conducted with three replicates for each treatment: three control tanks and three experimental tanks. If availability of tanks allows, there should be more
46 experimental tanks, allowing for three replicates at various current densities. For example, three tanks running at one current density and three tanks running at another current density. Position of the tanks should be randomized in order to minimize the influence of variation in environmental conditions. The tanks should run for six months and careful note should be taken when accretion stops. After six months with the electrical current on, it should be turned off and the experiment should be run for another six months. This will allow for comparison between mineral accretion with an electrical current, electrical current without continued accretion, and no electrical current. Coral and algae growth should be measured weekly or monthly along with water temperature, pH, carbonate chemistry, ion concentration, and other environmental parameters.
Chlorophyll and zooxanthellae density should also be monitored. Hopefully this study will clarify interactions between current density, ion availability, algae, and photosynthesis that may be important factors in determining the effectiveness of the
Biorock® technology.
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