Is the Mehler reaction the main photoprotective mechanism occurring in Symbiodinium in hospite?

The ecological success of reef-building corals throughout tropical oligotrophic waters relies on the symbiosis between cnidarians and photosynthetic dinoflagellates of the genus Symbiodinium. This association allows the transfer of highly energetic compounds and an efficient recycling of growth-limiting nutrients. In the natural environment the holobiont have to cope with significant daily variations in light intensities that sometimes exceed Symbiodinium photosynthetic capacity. Fortunately, these organisms possess regulatory features that help to ensure that high light intensities can be endured without the accumulation of photodamage. Among them, it has been found recently that photoreduction of oxygen downstream PSI by the so-called Mehler reaction was the main alternative electron sink at the onset and steady state of photosynthesis in different cultured strains of Symbiodinium. In this study, we investigated the occurrence and the relative amplitude of this photoprotective mechanism in Symbiodinium cells living within the tropical coral Stylophora pistillata and the sea anemone Anemonia viridis. To this end, joint measurements of oxygen evolution, PSI and PSII activities were conducted. The impacts of CO2 limitation and CO2 fixation on the photoreduction of oxygen by PSI was also studied by using several inhibitors targeting the carbon concentrating mechanism and the Calvin-Benson-Bassham cycle. Conversely to cultured cells, these measurements revealed that the Symbiodinium located in S. pistillata, under control conditions, rely on other photoprotective mechanisms than the Mehler reaction to prevent over-excitation of the photosynthetic apparatus. However, we observed an increased O2 uptake capacity during photosynthesis when the efficiency of the carbon fixation was reduced in S. pistillata.