Experimental assessment of organic carbon fluxes in the scleractinian coral Stylophora pistillata during a thermal and photo stress event

We assessed pico- and nanoplankton grazing rates as well as dissolved free amino acid (DFAA) uptake rates by the symbiotic coral Stylophora pistillata exposed to thermal and photo stress with concomitant bleaching. The aim was to determine whether these types of food sources could maintain the daily energetic requirements of this coral species. Moreover, the total organic carbon (TOC) flux was measured to quantify bulk C loss or gain. Under control conditions (27 degrees C and 200 mu mol photons m(-2) s(-1)), autotrophic C acquisition covered more than 90% of the respiratory needs of non-bleached corals. Another 10.6% of the respiratory needs were covered by pico- and nanoplankton grazing. Net TOC flux rates were negative, indicating substantial TOC uptake by the corals. After the stress (31 degrees C and 300 mu mol photons m(-2) s(-1)), the contribution of autotrophic C to the respiratory demand decreased to 64% in bleached corals. Pico- and nanoplankton grazing covered only 2 and 7% of the respiratory needs during and after the stress, respectively. These findings demonstrate a substantial stress-induced impact on auto- and heterotrophic capacities for energy acquisition in this species. Although no significant change occurred in the DFAA uptake rates, a significant change in the TOC flux direction was observed, which resulted in TOC net release. Consequently, autotrophy and heterotrophy were less efficient in sustaining the respiratory needs of bleaching and bleached S. pistillata, suggesting that this coral species, and possibly other related species, can be severely endangered by reoccurring and wide-spread bleaching events.