In hospite Symbiodinium photophysiology and antioxidant responses in Acropora muricata on a coast-reef scale: implications for variable bleaching patterns

Understanding susceptibility to bleaching may indicate how corals cope with increasing seawater temperatures resulting from climate change. In Belle Mare lagoon, Mauritius, Acropora muricata colonies at reef sites but not near the coast exhibited bleaching. We compared seawater temperatures and light intensity both on a summer day and a winter day at a reef station and a near-coast station. The total phenolic contents, non-enzymatic antioxidant activities of the coral holobiont, in situ photophysiological parameters of in hospite Symbiodinium as well as their cladal diversity were assessed. Both sites had comparable maximum temperatures but fluctuations were higher in the near-coast station (2–3 °C) compared to the reef station (1–1.5 °C) on both days. Light intensity was higher on the summer day than the winter day at both stations. Higher total phenolic contents and ferrous reducing antioxidant potential were observed in near-coast colonies than in reef colonies on the summer day. Only Clade A-like Symbiodinium sp. variants were detected at both locations, but higher maximum relative electron transport rates and maximum non-photochemical quenching were measured in near-coast colonies. Our data show that the near-coast colonies exhibited enhanced photophysiological responses, antioxidant activities and increased total phenolic contents in response to higher thermal and light fluctuations. This acclimatization may explain the variability in bleaching along a coast-reef scale.