Effect of feeding on the carbon and oxygen isotopic composition in the tissues and skeleton of the zooxanthellate coral $Stylophora\ pistillata$

International audienceThe effect of feeding on the carbon isotopic composition of zooxanthellae, animal tissue and skeleton was investigated in the scleractinian coral Stylophora pistillata. Two sets of corals were grown with filtered seawater under controlled conditions. One group of colonies was fed $Artemia$ sp. nauplii and compared to a control group that was starved. Fed corals exhibited higher concentrations of chlorophyll (60% more), soluble protein (4 times more) and calcification rates (29% more) than starved colonies. The net photosynthetic rate was higher in starved than in fed corals (18.53 $\pm$ 6.99 and 6.78 $\pm$ 2.06 $\mu$mol O$_2$ cm$^{-2}$ h$^{-1}$) respectively), whereas dark respiration was not significantly different (8.74 $\pm$ 2.27 and 6.66 $\pm$ 0.40 $\mu$mol $_2$ cm$^{-2}$ h$^{-1}$). The average $\delta^{13}$C value of $Artemia$ sp. nauplii used for feeding was -12 $\delta^{13}$C was significantly heavier in zooxanthellae than in animal tissues, for both fed (-10.1 vs -11.7‰) and starved colonies (-10.9 vs -13.2‰). Artemia sp. carbon was incorporated into the coral tissue as shown by the heavier $\delta^{13}$C in fed than in starved colonies (- 11.7 to -13.2‰, respectively), although there was no difference in the $\delta^{13}$C of the zooxanthellae fraction. Skeletal $\delta^{13}$C was similar in fed and starved colonies (mean -4.6‰). Skeletal $\delta^{18}$O composition was, however, significantly different between the 2 treatments (-4.24 to -4.05‰ for fed and starved colonies, respectively), which may have been due to differences in the calcification rates of fed and starved corals. These data are used to establish a conceptual model of the carbon flow between the various compartments of a symbiotic coral. It suggests that the skeletal $\delta^{13}$C is not sensitive to heterotrophic food supply