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

Abstract

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 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.

Flexibility of nutritional strategies within a mutualism: food availability affects algal symbiont productivity in two congeneric sea anemone species

“…Symbiont densities were affected by host dietary changes, but underlying mechanisms are not well understood. It is likely that the sea anemone host benefits from a reduction in symbiont density when they are unnecessary (supplement treatment) as they can cause damage to tissue via oxygen radicals [43,44]. The host would also benefit from an increase in symbiont density or chlorophyll when heterotrophic diet decreases (reduction treatment) to compensate for lost dietary carbon as an increase in either would allow for increased translocation of photosynthetic products from the symbionts to the host ( figure 1).…”

“…Anthopleura elegantissima (a congeneric co-occurring species) can exocytose and egest algal cells to control their densities [41,42]. There are costs to maintaining high symbiont densities in this species, most notably the production of oxygen radicals (H 2 O 2 ) by photosynthesizing symbionts under intense light that damage host cells [43,44]. While the mechanisms underlying control of symbiont densities in Anthopleura spp.…”