Oxygen Radical Production in the Sea Anemone Anthopleura Elegantissima and Its Endosymbiotic Algae

Host animals in algal-invertebrate endosymbiotic associations are exposed to photosynthetically generated hyperoxia while in sunlight, conditions conducive to photodynamic excitations and production of cytotoxic oxygen-derived radicals such as the superoxide anion and the hydroxyl radical (.OH). All previous evidence of oxyradical production in symbiotic associations has been circumstantial. We here present direct evidence, from electron paramagnetic resonance studies on tissue homogenates of the photosymbiont-containing sea anemone Anthopleura elegantissima (Brandt), of substantial light-dependent .OH and production that is abolished by dichlorophenyldimethylurea (DCMU), an inhibitor of photosynthesis. Shade-adapted A. elegantissima lacking endosymbiotic algae likewise show .OH production upon illumination. The latter flux is not dependent on photosynthesis, and DCMU has no effect. Rather, .OH production in apozooxanthellate anemones is via direct photoexcitations. The selective reaction of dimethyl sulfoxide (DMSO) with .OH to form methane sulfinic acid allows quantification of .OH produced in vivo. Such in vivo measurements confirm the production of .OH in both host and algae in illuminated zooxanthel-late anemones, where the amount of .OH in the zooxanthellae is disproportionately large relative to their fractional contribution to the biomass of the symbiosis. In vivo studies using DMSO also suggest a photochemical production of .OH in apozooxanthellate anemones exposed to simulated sunlight enriched in ultraviolet (UV) wavelengths, and the enhancement by UV light of .OH production in zooxanthellate individuals. Such chronic radical exposure necessitates defenses against photooxidative stress, a cost that is seldom considered in these mutualistic symbioses.