EFFECTS OF MODERATE HEAT STRESS AND DISSOLVED INORGANIC CARBON CONCENTRATION ON PHOTOSYNTHESIS AND RESPIRATION OF SYMBIODINIUM SP (DINOPHYCEAE) IN CULTURE AND IN SYMBIOSIS

The influence of temperature and inorganic carbon (C-i) concentration on photosynthesis was examined in whole corals and samples of cultured symbiotic dinoflagellates (Symbiodinium sp.) using combined measurements from a membrane inlet mass spectrometer and chl a fluorometer. In whole corals, O-2 production at 26 degrees C was significantly limited at C-i concentrations below ambient seawater (similar to 2.2 mM). Further additions of C-i up to similar to 10 mM caused no further stimulation of oxygenic photosynthesis. Following exposure to 30 degrees C (2 d), net oxygen production decreased significantly in whole corals, as a result of reduced production of photosynthetically derived oxygen rather than increased host consumption. Whole corals maintained a rate of oxygen evolution around eight times lower than cultured Symbiodinium sp. at inorganic carbon concentrations < 2 mM, but cultures displayed greater levels of photoinhibition following heat treatment (30 degrees C, 2 d). Whole corals and cultured zooxanthellae differed considerably in their responses to C-i concentration and moderate heat stress, demonstrating that cultured Symbiodinium make an incongruous model for those in hospite. Reduced net oxygen evolution, in whole corals, under conditions of low C-i (< 2 mM) has been interpreted in terms of possible sink limitation leading to increased nonphotochemical energy dissipation. The advantages of combined measurement of net gas exchange and fluorometry offered by this method are discussed.