The effect of heterotrophy on photosynthesis and tissue composition of two scleractinian corals under elevated temperature

Exogenous food can increase protein levels of coral host tissue, zooxanthellae densities, chlorophyll (chl) concentrations and rates of photosynthesis and is thought to play an important role in the resilience of bleached corals. There is however no information about the effect of heterotrophy on the bleaching susceptibility of corals under elevated temperature conditions. This study investigates potential interactions between food availability, basal metabolic functions (photosynthesis and respiration), energy status (lipid concentrations), total protein concentrations and the bleaching susceptibility (loss of chl and/or zooxanthellae) of the scleractinian corals Stylophora pistillata (Esper) and Galaxea fascicularis (Linnaeus) in response to elevated temperature (daily temperature rises of 3-4 °C) over 15 days. Feeding experiments were carried out in which the corals were either fed daily with zooplankton or starved. Compared to fed corals, starvation of both species resulted in a significant decrease in daily photosynthetic oxygen evolution over time. Gross (Pg) and net (Pn) photosynthetic production of starved corals of both species between 10:00-11:00 hrs had declined by ~50% at day 15 while there were no marked changes in Pg and Pn of fed corals. After 15 days, starved S. pistillata contained significantly lower zooxanthellae densities, lipid and protein concentrations than fed corals. Starved G. fascicularis also displayed a decrease in zooxantllae densities which was accompanied by a significant decline in algal chl concentrations. Contrary to S. pistillata, feeding treatment had no effect on the lipid concentrations of G. fascicularis. Total protein concentrations however were significantly lower in straved than in fed G. fascicularis. Furthermore, starvation resulted in a significant decrease in respiration of S. pistillata during the last four days of the experiment while treatment had no effect on the respiration rates of G. fascicularis. Overall the oxygen consumption of S. pistillata of both treatments was about 39-67% higher than the respiration of G. fascicularis indicating that low metabolic rates may have allowed starved G. fascicularis to conserve energy reserves over the course of the experiment. The combined results reveal a strong positive relationship between food availability, sustained photosynthetic activity and reduced loss in pigmentation of both species under elevated temperature conditions.