ARTICLE
MICROALGAE ASSISTED AQUACULTURE SUSTAINABLE SOLUTION FOR SUSTAINABLE AQUACULTURE
Kaleeswaran, K, Bharathi, S and Cheryl Antony Tamil Nadu Dr. J. Jayalalithaa Fisheries University, Nagapattinam.
Introduction juveniles, while in the second phase, these juveniles are further reared to marketable sizes. The first larval Aquaculture is also known as ‘underwater agriculture’ rearing phase is crucial and its success determines and most aquaculture production systems basically the success of grow-out farming operations. The larval have two phases. The first phase involves rearing of rearing phase requires controlled rearing conditions larvae (either hatchery produced or wild caught) to and in-depth knowledge on the environmental and
Fig. 1: Pure culture of brown microalgae Chaetoceros calcitrans under laboratory conditions
VOL 3 ISSUE 11 NOVEMBER 2020 Aquaculture Spectrum 23 they are usually adapted to start feeding the small sized microalgae or live feed that is suitable to the mouth size of the reared animal. Although the amount The idea behind this concept of feed required during this phase is very little and cost is to transform the organic is not a major issue, the quality of feed provided is very compounds in the effluents of important. eutrophic culture systems into microalgae biomass and to Depending on the species reared, microalgae is administered as feed in the following ways; utilize this value-added biomass to partly replace artificial feeds, (1) Microalgae is exclusively provided as feed which additionally enhances throughout the life stages including larval, juvenile the immunity in farmed aquatic and adult stages of clams, mussels, oysters and other animals. bivalves which are filter feeders. (2) Microalgae and zooplankton are given either together or sequentially during the larval rearing of species such as shrimp, crab, milkfish, carps etc. nutritional requirement of the reared species. The Microalgae either directly serve as food to the small highest drop in survival is observed during this phase. fry or indirectly via zooplankton which consume the Feeding is the most important aspect in larval rearing microalgae. and after the absorption of the yolk sac in the larvae,
Figure 2: Schematic diagram of microalgae assisted aquaculture (Source: Han et al., 2019)
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(3) Microalgae are provided as food for the zooplankton production and the treated effluent is recycled to use (mainly rotifer Brachionus plicatilis), which are in turn again in the aquaculture systems. provided to the larvae and juveniles of fish such as Seabass, Sea bream, White bream, Japanese Amberjack Advantages of Microalgae based etc. Aquaculture Various algal culture media and culture technologies 1. Microalgae are autotrophs that produce oxygen have been evolved and commercially adopted in through photosynthesis and therefore serve as a different countries for production of microalgae. bio-pump for aeration and assist in reducing the Presently, even powder and pellet forms of algae such cost towards electricity for aeration, besides helping as Spirulina and Haematococcus pluvialis are available in maintaining stable water quality of the aquatic commercially. environment. In aquaculture, the water used for rearing the stocks 2. As water quality is greatly controlled, the water are generally discharged into the environment during replacement is not needed in the aquaculture system. water exchange as well as while harvesting. This water 3. The growth of algal species may reduce the growth of (effluent) is rich in organic and inorganic nutrients like other harmful and unwanted micro-organisms. Nitrogen, Carbon, Phosphorous and Potassium. These nutrients are essential for the growth of microalgae. The 4. Micro-algal feed increases the immunity of aquatic effluents from aquaculture systems can be effectively animals. Thus, it reduces the usage of chemicals and utilized to culture microalgae and after microalgae are healthcare products. harvested, the remaining wastewater could be recycled and used again in the aquaculture systems. Microalgae Microalgae cultivation systems assisted aquaculture deals with the conversion of organic and inorganic nutrients in the effluents which Though various microalgae cultivation systems have are eutrophic in condition, into value-added microalgae been developed, the Raceway system and Revolving biomass. Algal Biofilm (RAB) are the two systems suitable for microalgae assisted aquaculture wastewater The idea behind this concept is to transform the organic remediation. compounds in the effluents of eutrophic culture systems into microalgae biomass and to utilize this value- Raceway culture system: added biomass to partly replace artificial feeds, which A raceway system has a closed circulation channel additionally enhances the immunity in farmed aquatic with a depth of 0.5 to 1 meter and having one or two animals. paddle wheels to drive water circulation. Raceway The concept of microalgae assisted aquaculture is systems are highly suitable because it involves low explained in Figure 2. It involves the following: investment cost and is suitable to treat large volumes of aquaculture wastewater and to produce microalgae (1) Microalgae are inoculated into fish ponds or rearing biomass for aquaculture feed. Two parameters are systems. Here, they digest a part of the waste organic very important in this raceway culture systems. One matter released by fish into the system thus helps in is location and depth of raceway to allow the light purification of aquaculture system. permeability which helps for photosynthesis and (2) The effluent water from fish ponds is used to growth of microalgae. Second, the relationship between cultivate microalgae in separate tanks using various microalgae and bacteria in the raceway pond system methods. should be fully understood. Studies indicate that the microalgae-bacteria consortium helps to remove 28% (3) Microalgae are then harvested in ecofriendly and COD, 53% BOD, 31% Total Nitrogen (TN) and 64% cost-effective manner. Total Phosphorus (TP) in aquaculture wastewater which (4) The harvested algal biomass is utilized as feed resides in a 12 m³ raceway system. Though this system in the fish culture, thus reducing the cost of fish involves low investment cost and operational cost,
VOL 3 ISSUE 11 NOVEMBER 2020 Aquaculture Spectrum 27 A raceway culture system for Microalgae the harvesting of the microalgae biomass is a time- The RAB system has the components of flat rotating consuming process and energy-intensive. discs, a cylindrical drum that is rotated from its center axis or a conveyor belt rotated by a drive shaft. The Revolving Algal Biofilm (RAB) attached material in the RAB system alternatively rotates between the air and wastewater. One type of The RAB system involves growing and production RAB design is the Rotating Biological Contractor (RBC) of microalgae on a film. It is a suitable technology which has been widely used in wastewater treatment. because it does not require any additional land area as In this design, a drum continuously rotates between the available ponds or raceway systems can be utilized. liquid and air phase using a drive shaft and the algal Harvesting of microalgae is also very simple, as the cells are attached to the drum. The algal cells attached algae attached to the film are simply scrapped off by to the drum can be easily scrapped off for harvesting. using a scrapper and it involves no cost for harvesting.
Figure 3: Revolving Algal Biofilm with rotating drum Source:https:// Figure 4: Revolving Algal Biofilm system with vertical discs. Source: https:// mafiadoc.com/rotating-algal-biofilm-reactor-for-biomass-growth-semantic-scho www.aiche.org/chenected/2013/11/rotating-algal-biofilm-reactor-on- lar_5bafb2cf097c470b348b4579.html location
VOL 3 ISSUE 11 NOVEMBER 2020 Aquaculture Spectrum 28 Some studies report that by using rotating discs as developed techniques should meet out three important attaching material, very high density of algal biomass requirements; i) harvesting at low cost ii) non-use of (20 g of biomass per m² of disc surface per day) can be toxic or undesired chemicals for harvesting as they achieved. could affect both the feed quality as well as the aquatic animals reared and iii) harvesting should be efficient Algal cells get attached to various surfaces by secreting and time-effective. Fungi-assisted harvesting and extracellular polymeric substances (EPS). They take flotation are the two important and suitable techniques much time to attach on to the surface of drum or for harvesting the algal biomass. These two techniques discs. Cotton based duct canvas and ropes can be can fulfill the above-mentioned criteria. used as attaching material around the drum or disc. These materials show superior algal attachment Fungi-assisted harvesting: characteristics. Studies by other scientists indicate that the nutrient removal rates in the RAB system is Fungi-assisted microalgae harvesting means the 2.1 g per m² per day of total dissolved Phosphorus addition of filamentous fungi in the algal culture and 14.1 g per m² per day of total dissolved Nitrogen medium either as fungal pellets or fungal spores. Micro- and the biomass production is 31 g per m² per day. As algal cells attach or get entrapped in these fungal harvesting process of RAB system does not involve the pellets which can then be easily harvested by simple usage of any chemicals, the harvested biomass has a filtration. Using this technique, over 95% of microalgae high safety level for aquaculture use. can be harvested. However, the production of fungal pellets involves high costs and recent studies indicate Micro-algae biomass harvesting that the co-cultivation of fungal spores with microalgae techniques: in wastewater or culture medium can simplify the harvesting process and reduce the cost of harvesting. Several techniques such as centrifugation, filtration, Another advantage of this co-cultivation technique gravity-driven sedimentation, flocculation by positively is promoting the nutrients recovery from aquaculture charged ions, flotation and harvesting by edible wastewater with high contents of solid organics. Table1 fungi have been developed for microalgae harvesting. lists out some examples of fungi-assisted microalgae However, not all these can be adopted for harvest harvesting. microalgae biomass in the aquaculture industry. The
Harvesting Microalgae Fungi Conditions efficiency Fungal pellets; 30 - 340C; pH 4 - 5; 98.2% Agitation speed 120-160 rpm. Chlorella sp. Penicillium sp. Fungal spores; 400C; pH 7; 99.3% Agitation speed 160 rpm. Fungal spores; Heterotrophic culture; Aspergillus oryzae 93% 250C; Agitation speed 150 rpm; 3 day. 250C; pH 5 - 6; Agitation speed Aspergillus sp. Almost 100% Chlorella vulgaris 100 rpm; 2 day. Fungal spores; 270C; pH 5; Aspergillus niger >60% Agitation speed 150 rpm; 3 day. Aspergillus fumigatus >90% Scenedesmus Fungal pellets; 280C; Agitation Aspergillus fumigatus >90% quadricauda Pyrocystis lunula Aspergillus fumigatus Around 30% speed 150 rpm; 2 day Table 1: Fungi assisted microalgae harvesting
VOL 3 ISSUE 11 NOVEMBER 2020 Aquaculture Spectrum 29 Flotation: Microalgae as feed in aquaculture: Flotation is an economically feasible method to harvest Microalgae are an important source of nutrition for microalgae. The process involves introduction of fine air both finfish and shellfish either directly or indirectly. bubbles continuously in the wastewater or in the culture In the first scenario, they are consumed directly by fish medium by mild aeration. As the air bubbles attach fry, larvae or juveniles that are being reared. Indirectly, to the micro-algal cells, they rise to the surface which microalgae are initially consumed by other organisms facilitate easy harvest. in the food chain such as rotifers, copepods and cladocerans, which in turn serve as food for fish larvae. Modified flotation: They are considered as the best feed source as they do not pollute aquatic systems and maintain water A negative repulsive charge on the surface of micro-algal quality when compared to artificial feeds. They are a cells is the main reason for them being in a state of rich source of protein, lipid, and carbohydrate and are suspension. By combining the flotation and flocculation also capable of synthesizing some value-added products techniques, the negative charge on micro-algal cells like antioxidants and pigments. Table 2 lists out some can be partly neutralized. Thus, it helps to increase potential algal strains to be cultured for wastewater the harvesting rate. Natural polymers can be used as remediation. flocculating agents instead of chemicals which could have negative impacts on the aquatic system and fishes. Nutritional benefits from microalgae
Culture Protein Lipid Carbo-hydrate Strain Value-added compounds medium (%) (%) (%) Medium with EPA and DHA Thraustochytrium sp. NA 38.95 NA glycerol (37.88% of total lipid) Cane Polyunsaturated fatty acids Chlorella zofingiensis NA 30-50 NA molasses (36.89–49.16% of fatty acid profile) Soybean oil Scenedesmus sp. extraction 53.3 33.4 NA EPA (15.89% of fatty acid profile) effluent Modified Dietary fiber Galdieria sulphuraria Allen 32.5 1.77 62.9 (54.1% of carbohydrate) medium Cane Chlorella zofingiensis NA 30-50 NA Astaxanthin (13.6 mg/l) molasses Haematococcus OHM NA NA NA Astaxanthin (>15 mg/l) Pluvialis medium Haematococcus Primary NA NA NA Astaxanthin (80 mg/l) Pluvialis treated water Essential amino acids Botryococcus braunii NA 39.9 34.4 18.5 (45.2 g/100 g protein) Essential amino acids Tetraselmis chuii NA 46.5 12.3 25 (45.5 g/100 g protein) Phaeodactylum Essential amino acids NA 39.6 18.2 25.2 tricornutum (45.2 g/100 g protein) Porphyridium Essential amino acids NA 31.6 13.7 45.8 aerugineum (63.9 g/100 g protein) Table 2: Nutritional profile of microalgae in different culture medium
VOL 3 ISSUE 11 NOVEMBER 2020 Aquaculture Spectrum 30 feed: in the year 2016. As the sector aims at enhancing production further to cater to the growing global Microalgae contain protein ranging between 26.5% requirement and also to compensate for the decline and 53.3% on a dry weight basis. The food conversion in fisheries production, environmental conservation ratio (FCR) of micro-algal feed is better than the and sustainability issues have come to the fore in traditional feeds. Poly Unsaturated Fatty Acids like recent years. The need for intensification of production Eicosapentaenoic acid (EPA) and Docosahexaenoic using advanced technologies while minimizing land acid (DHA) are present in higher amount in and water utilization is very much necessary now. microalgae than traditional fish feed ingredients like Aquaculture requires high quality feed and the use of soybean and peanut meal. Natural pigments such as microalgae to meet these nutritional requirements and astaxanthin, xanthophyl and carotene are also present atleast partly replace the expensive microencapsulated in microalgae. Astaxanthin pigment is an essential diets is gaining significance. Algal biofilm culture nutrient for salmonid fishes that determines the flesh systems have drawn new interest for algae production color and is also known to strengthen the immunity and it replaces the conventional raceway ponds for of fishes. The microalgae of Haematococcus sp. can algae production, as it minimizes harvesting efforts and contain 2 – 5% astaxanthin in dry weight. costs. Utilization of effluent water from aquaculture production systems also makes microalgae assisted Conclusion: aquaculture concept a very sustainable option Aquaculture contributed 46.81% (80 million MT) to to enhance algal biomass as well as aquaculture the total global fish production of 170.9 million MT production.
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