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 (3 1OC). 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 wavelengthdependent effects of UV radiation between 290 and 375 nm than for zooxanthellae exposed to UV radiation alone. Over the past several years, and with increasing regularity, coral reefs have been affected globally by a phenomenon known as coral bleaching, which involves either the mass expulsion of zooxanthellae or the loss of photosynthetic pigments within individual zooxanthellae (Glynn 199 1). These events closely follow periods of warming that elevate seawater temperatures to 30-33°C (Ogden and Wicklund 1988; Glynn 199 1). Field and laboratory studies on bleaching in corals and other symbiotic cnidarians have established a causal link between temperature stress and bleaching (Jokiel and Coles 1990; Glynn and D’Croz 1990; Fitt et al. 1993) and have also suggested that corals are living close to their upper thermal limits (Jokiel and Coles 1990). Other studies have also implicated UV radiation (UV-A, 320-400 nm; UVB, 290-320 nm) as a cause of bleaching, either alone (Jokiel 1980; L esser et al. 1990; Gleason and Wellington 1993) or synergistically with elevated temperature (Lesser et al. 1990; Glynn et al. 1993). The decrease of stratospheric ozone from anthropogenic inputs of chlorinated fluorocarbons has resulted in 1 Current address and address for proofs, correspondence, and reprints: Department of Zoology and Center for Marine Biology, University of New Hampshire, Durham 03824. Acknowledgtients This work was supported by the National Science Foundation Biological Oceanography Program (OCE 9216307 and OCE 94-96082). Special thanks to Terry Cucci and Michael Sieracki for advice and assistance. I also thank P. Neale for computational assistance on the action spectra. Standards for UV-absorbing compounds were synthesized by Walter Dunlap and provided by Deneb Karentz. This is Bigelow Laboratory for Ocean Sciences contribution 95007. an increase in the flux of harmful UV-B radiation reaching the sea surface at high latitudes. The natural concentration of ozone is thinnest near the equator (Cutchis 1982); as such, tropical ecosystems have a long evolutionary history of exposure to greater fluxes of UV radiation than do ecosystems at higher latitudes (Frederick et al. 1989). Recent data describing global decreases in stratospheric ozone show a highly variable trend of 1.2 + 1.3% in the loss of ozone over equatorial regions (5’S) and 2 to 4% at 20-3O”N in the last decade (Madronich 1992), suggesting that fluxes of UV-B radiation could increase in the future for this region. Because of the high transparency of tropical ocean waters, UV radiation penetrates to depths exceeding 20 m (Jerlov 1950; Fleischmann 1989; Lesser unpubl.). These wavelengths are known to have a detrimental effect on photosynthesis and growth in zooxanthellae (Jokiel and York 1984; Lesser and Shick 1989) and on survival of coral reef epifauna (Jokiel 1980; Siebeck 1988). The continued increase in the incidence of coral bleaching will result in a decrease in growth and reproduction, and an increase in mortality rates for corals repeatedly bleached (Glynn 19 9 1). To understand the underlying mechanisms that result in bleaching, one must elucidate the responses of both the host and symbiont to the proposed causes of bleaching. Gates et al. (1992) suggested that elevated temperatures affect the host, resulting in the loss of gastrodermal cells containing zooxanthellae. Other studies suggest that the algal symbionts are the principal targets of exposure to either elevated temperatures or UV radiation alone or from exposure to both simultaneously (e.g. Lesser et al. 1990; Glynn and D’Croz 1990; Iglesias-Prieto et al. 1992) and that damage to these symbionts results in an energetic cost for the host in terms of a decrease in the translocation of photosynthate or exposure to highly reactive oxygen radicals (Lesser and Shick 1989), for which DNA, pro-