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Effects of Changing Seawater Temperature on Photosynthesis And sm69n3347 8/9/05 11:00 Página 347 SCI. MAR., 69 (3): 347-354 SCIENTIA MARINA 2005 Effects of changing seawater temperature on photosynthesis and calcification in the scleractinian coral Galaxea fascicularis, measured with 2+ O2, Ca and pH microsensors* FUAD A. AL-HORANI Marine Science Station, P.O. Box 195, 77110, Aqaba-Jordan E-mail: [email protected] SUMMARY: Single polyps of Galaxea fascicularis were fixed to glass vials with underwater epoxy resin. After regenera- tion into microcolonies they were used for microsensor measurements of photosynthesis and calcification under different -3 –1 incubating temperatures. Gross photosynthesis was found highest at temperatures of 23 and 26ºC (ca. 0.022 mole O2 m s ), close to the ambient temperature (i.e. 26ºC). At 35°C, gross photosynthesis was irreversibly inhibited as the microcolonies bleached. The net photosynthesis rapidly decreased with temperature and became negative at temperatures higher than 29ºC. 2+ 2+ Profiles of O2 and Ca showed a strong effect of temperature on them. The concentrations of Ca measured on the polyp surface also showed temperature dependence of Ca2+ uptake. In the dark and below 29ºC, the surface Ca2+ concentration was temperature independent. During illumination, the surface Ca2+ concentration showed a dip at 26 ºC (ca. 8.7 mM), indicat- ing maximum uptake rates at ambient temperature. However, at 32ºC and higher, Ca2+ was slightly higher at the tissue sur- face than in the seawater, in both light and dark, resulting from calcium dissolution. The surface pH in light increased grad- ually from 8.3 to 8.6 with increasing temperature to 23ºC, and thereafter remained constant to 29ºC. At 32 ºC, the pH became slightly acidic compared with the water phase, probably due to a decrease in the uptake of CO2 by photosynthesis. The largest difference in pH between light and dark incubations was at temperatures between 23 and 29ºC (7.5-7.7 in dark versus 8.6- 8.7 in light), which indicate high rates of photosynthesis and respiration in this temperature range . It is concluded that pho- tosynthetic activity in the coral is maintained up to rather high temperatures (32ºC), but corals at super-optimum tempera- tures (above 26ºC) consume more O2 than they produce, decalcify and produce CO2. Keywords: temperature, coral, gross photosynthesis, net photosynthesis, calcification, Galaxea fascicularis, microsensors.. RESUMEN: EFECTOS DEL CAMBIO EN LA TEMPERATURA DEL AGUA EN LA FOTOSÍNTESIS Y LA CALCIFICACIÓN DEL CORAL ESCLE- 2+ RACTINIARIO GALAXEA FASCICULARIS MEDIDOS CON MICROSENSORES DE O2, Ca Y pH. – Se fijaron pólipos de Galaxea fascicu- laris sobre viales de cristal con resina epoxy. Una vez generaron pequeñas colonias se utilizaron como sensores para medir los cambios en la fotosíntesis y calsificación a diferentes temperaturas. Los valores de fotosíntesis total (bruta) fueron más -3 -1 elevados a temperaturas entre 23 y 26°C (ca. 0,022 mole O2 m s ), similares a los del ambiente (26°C). A valores de 35°C, la fotosíntesis total se inhibió irreversiblemente y las pequeñas colonias se blanquearon. La fotosíntesis neta disminuyó rápi- 2+ damente con la temperatura hasta llegar a valores negativos en temperaturas mayores de 29°C. Los perfiles de O2 y Ca mos- traron un elevado efecto de la temperatura. Las concentraciones de Ca2+ medidas en la superficie de los pólipos también mos- traron una influencia en la incorporación de Ca2+. En condiciones de oscuridad y temperatura por debajo de 29°C, la con- centración de Ca2+ era dependiente de la temperatura. En periodos iluminados, el Ca2+ concentración mostró un pico 26°C (ca. 8,7 mM) hecho que indicaba una tasa de incorporación a temperatura ambiente. hecho que indica que las máximas incor- poraciones se efectuaron a temperatura ambiente. Sin embargo, a 32°C o más, los valores de Ca2+ fueren ligeramente altos en la superficie del tejido que en agua de mar, en ambas situaciones de iluminación y oscuridad, resultado de la disolución de la disolución de Calcio. El pH en periodos iluminados se incrementó ligeramente de 8.3 a 8.6, incrementándose la tem- peratura se y permaneció constante hasta 23°C aunque permenecía constante hasta los 29°C. A 23°C el pH se transformó en un pH ligeramente ácido, probablemente debido a la disminución de en la tasa de retención de CO2 mediante la fotosín- tesis. Las mayores diferencias en pH en peiodos iluminados y oscuros fue de 23-29°C (7.5-7.7 en oscuridad versus 8.6-8.7) *Received June 21, 2004. Accepted January 28, 2005. EFFECTS OF TEMPERATURE ON CORALS 347 sm69n3347 5/9/05 19:16 Página 348 lo que infica un alta tasa de fotosíntesis y respiración a estos datos en este margen de temperaturas. La conclusión del tra- bajo es que, la actividad fotosintética en el coral es claramente elevada y se mantiene alta a temperaturas hasta alcanzar los 32°C, pero los corales, en el periodo óptimo de temperaturas de verano (por encima de 26°C) consume mas O2 que el que produce, se descalcifica y produce CO2. Palabras clave: temperatura, coral, fotosíntesis total, fotosíntesis neta, calcificación, Galaxea fascicularis, microsensores. INTRODUCTION severity, due mainly to high water temperatures associated with El Niño–related warming and a gen- Coral reefs are tropical shallow water ecosys- eral warming trend. This temperature effect on tems largely restricted to the seas between the lati- corals was first recognised after the 1982-1983 El tudes of 30° north and 30° south of the equator, Niño/Southern Oscillation (ENSO) event through where temperature ranges between 18 and 30 ºC which massive coral bleaching occurred after being (Spalding et al., 2001). Scleractinian corals are the exposed to unusually high water temperature main constituent of the coral reefs. These are ben- (Glynn, 1988). Since that event, coral bleaching has thic animals that form colonies of varying shapes, occurred on a regular basis in many places of the colours and sizes. The building block of each world (reviewed by Brown, 1997). From mid-1997 colony is called a polyp. The body of the coral is to late 1998, catastrophic bleaching with massive formed mainly from an exoskeleton of CaCO3 coral mortality has been reported from many areas deposited by the animal and a thin layer of living of the world (e.g. Wilkinson and Hodgson, 1999; tissue covering the skeleton. The corals feed on Lough, 2000; Vargas-Angel et al., 2001; Jimenez et zooplanktons and other suspended organic matter al., 2001; Carriquiry et al., 2001; Bruno et al., 2001; from the surrounding seawater (i.e. heterotrophy) Podesta and Glynn, 2001). The amount of bleaching (Porter, 1976). In addition to this, the animals live and mortality was similar to that reported in the in a symbiotic relationship with plant-like dinofla- 1982-1983 bleaching event (Glynn et al., 2001). gellates called Symbiodinium sp., commonly While many studies on the effects of temperature known as zooxanthellae, which photosynthesize on corals have focused on coral bleaching and its using light energy to produce oxygen and organic mechanism, little is known about the effects of tem- matter for the benefit of the animal (i.e. autotro- perature on the metabolic processes in coral. The phy). This symbiotic relationship is crucial for the main biogeogenic processes of corals are photosyn- survival of the coral in an environment described thesis (O2 production, CO2 binding), respiration 2+ as a desert in terms of nutrient content (Sorokin, (CO2 release) and calcification (Ca uptake, CO2 1995; Hoegh-Guldberg, 1999). release), and obviously the balance between these is Coral reefs are threatened by many anthro- crucial for the net exchange of corals and seawater. pogenic and environmental factors, some of which It is thought that calcification can balance the pH are the increased atmospheric CO2 and the associat- effect of photosynthesis, and thereby alleviate the ed decrease in aragonite and calcite saturation states, CO2 limitation for photosynthetic activity increased sea surface temperature, coral diseases, (McConnaughey, 1994). The effect of temperature eutrophication and pollution (Smith and on the metabolic rates was studied through the use Buddemeier, 1992; Brown, 1997; Hodgson, 1999; of entire colonies with inherent variation in pheno- Leclercq et al., 2000; Kleypas et al., 2001). In their type and genotype. A better comparison of experi- weakened conditions, coral reefs will be less able to ments became possible through the use of micro- cope with rising sea levels and other anthropogenic colonies originating from the same parent colony stresses, which are expected in the middle of this (Al-Moghrabi et al., 1993). The new application of century (Langdon et al., 2000). By the end of the microsensors permitted the measurement of many century, global warming is expected to kill most of parameters at a µm scale and with high spatial and the presently existing coral reefs (Pockley, 1999). temporal resolution (De Beer et al., 2000; Kühl et One of the consequences of the increased sea sur- al., 1995; Al-Horani et al., 2003a; Al-Horani et al., face temperature is coral bleaching, by which the 2003b; Al-Horani et al., 2005). In this study, the two coral turns pale or white in colour as a result of los- advancements were combined to investigate the ing its symbiont and/or the pigmentation. Coral impact of temperature on the physiology of the scle- bleaching events have increased in frequency and ractinian coral Galaxea fascicularis. 348 F.A. AL-HORANI sm69n3347 5/9/05 19:16 Página 349 MATERIALS AND METHODS Experimental set-up Coral sample Coral colonies were placed in a polycarbonate flow cell for microsensor measurements. Filtered Galaxea fascicularis colonies (6 parent colonies) seawater was circulated between the flow cell and a were collected by SCUBA diving from about 5 m 10 l reservoir at a constant flow rate (420 ml/min).
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