Elevated temperatures and ultraviolet radiation cause oxidative stress and inhibit photosynthesis in symbiotic dinoflagellates

Elevated temperatures and solar ultraviolet (UV) radiation have been implicated as causes for the loss of symbiotic algae in corals and other invertebrates with photoautotrophic symbionts (i.e. bleaching). Significantly higher cellular concentrations of superoxide radicals and hydrogen peroxide are observed when cultures of Symbiodinium bermudense are exposed to elevated temperatures with and without exposure to UV radiation. This increase in oxidative stress is accompanied by a reduction 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 from oxidative stress during exposure to UV radiation and elevated temperatures (31 degrees C). The addition of exogenous scavengers of active oxygen, however, improves photosynthetic performance, but not to pre-exposure rates in zooxanthellae exposed to both elevated temperature and UV radiation, confirming a role for oxidative stress in the inhibition of photosynthesis by UV radiation and elevated temperatures. After exposure of zooxanthellae to UV radiation and elevated temperatures, an action spectrum for the inhibition of photosynthesis shows significantly greater wavelength-dependent effects of UV radiation between 290 and 375 nm than for zooxanthellae exposed to UV radiation alone.