Photophysiology and hydrogen peroxide generation of the dinoflagellate and chlorophyte symbionts of the sea anemone Anthopleura elegantissima

Associating with algal symbionts is considered largely beneficial for cnidarians such as corals and sea anemones, yet there are potential costs of hosting symbionts, such as the production of reactive oxygen species. We compared the photophysiology and H 2 O 2 production rates of  Symbiodinium muscatinei  and  Elliptochloris marina , the dinoflagellate and chlorophyte symbionts, respectively, of the temperate sea anemone  Anthopleura elegantissima.  Analyses of photosystem II ( PSII ) function in the two symbionts, including maximum quantum yield and relative electron transport rates, were consistent with prior studies indicating that  E. marina  has lower photosynthetic performance than  S. muscatinei  at high temperature and irradiance. The efficiency of  PSII  in both symbionts was positively affected by the addition of exogenous catalase, suggesting that both symbionts experience H 2 O 2 -mediated declines in  PSII  efficiency. We found that  S. muscatinei  produced more H 2 O 2  than  E. marina  across all treatments, with the highest production under high light and temperature. Results were similar in experiments involving both isolated symbionts and symbionts residing within intact tentacles, indicating that the potential stress of symbiont isolation was not a significant factor biasing our results. Despite the lower typical densities of  S. muscatinei  relative to  E. marina , extrapolations of cell-specific H 2 O 2  production rates to the intact symbiosis suggest that  S. muscatinei  imposes a greater H 2 O 2  burden on  A. elegantissima.  Even with this burden, however, the considerably higher productivity and fitness of  S. muscatinei- bearing anemones documented in prior studies suggests that the net benefit of hosting  S. muscatinei  exceeds that of  E. marina.