ELEVATED-TEMPERATURES AND ULTRAVIOLET-RADIATION CAUSE OXIDATIVE STRESS AND INHIBIT PHOTOSYNTHESIS IN SYMBIOTIC DINOFLAGELLATES

Elevated temperatures and solar ultraviolet (UV) radiation have been i mplicated as causes for the loss of symbiotic algae in corals and othe r invertebrates with photoautotrophic symbionts (i.e. bleaching). Sign ificantly higher cellular concentrations of superoxide radicals and hy drogen peroxide are observed when cultures of Symbiodinium bermudense are exposed to elevated temperatures with and without exposure to UV r adiation. This increase in oxidative stress is accompanied by a reduct ion in the quantum yield of fluorescence for photosystem 2 and protein -specific activities of the carboxylating enzyme, Rubisco. An increase in antioxidant enzyme activities is unable to protect these cells fro m oxidative stress during exposure to UV radiation and elevated temper atures (31 degrees C). The addition of exogenous scavengers of active oxygen, however, improves photosynthetic performance, but not to pre-e xposure rates in zooxanthellae exposed to both elevated temperature an d UV radiation, confirming a role for oxidative stress in the inhibiti on of photosynthesis by UV radiation and elevated temperatures. After exposure of zooxanthellae to UV radiation and elevated temperatures, a n action spectrum for the inhibition of photosynthesis shows significa ntly greater wavelength-dependent effects of UV radiation between 290 and 375 nm than for zooxanthellae exposed to UV radiation alone.