PHOTOSYNTHESIS AND PRODUCTION OF HYDROGEN PEROXIDE BY <i>SYMBIODINIUM</i> (PYRRHOPHYTA) PHYLOTYPES WITH DIFFERENT THERMAL TOLERANCES¹

<jats:p>Occurrences whereby cnidaria lose their symbiotic dinoflagellate microalgae (<jats:italic>Symbiodinium</jats:italic> spp.) are increasing in frequency and intensity. These so‐called bleaching events are most often related to an increase in water temperature, which is thought to limit certain <jats:italic>Symbiodinium</jats:italic> phylotypes from effectively dissipating absorbed excitation energy that is otherwise used for photochemistry. Here, we examined photosynthetic characteristics and hydrogen peroxide (H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub>) production, a possible signal involved in bleaching, from two <jats:italic>Symbiodinium</jats:italic> types (a thermally “tolerant” A1 and “sensitive” B1) representative of cnidaria–<jats:italic>Symbiodinium</jats:italic> symbioses of reef‐building Caribbean corals. Under steady‐state growth at 26°C, a higher efficiency of PSII photochemistry, rate of electron turnover, and rate of O<jats:sub>2</jats:sub> production were observed for type A1 than for B1. The two types responded very differently to a period of elevated temperature (32°C): type A1 increased light‐driven O<jats:sub>2</jats:sub> consumption but not the amount of H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> produced; in contrast, type B1 increased the amount of H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> produced without an increase in light‐driven O<jats:sub>2</jats:sub> consumption. Therefore, our results are consistent with previous suggestions that the thermal tolerance of <jats:italic>Symbiodinium</jats:italic> is related to adaptive constraints associated with photosynthesis and that sensitive phylotypes are more prone to H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> production. Understanding these adaptive differences in the genus <jats:italic>Symbiodinium</jats:italic> will be crucial if we are to interpret the response of symbiotic associations, including reef‐building corals, to environmental change.</jats:p>