The comparative roles of thermal inactivation of endosymbiont photosynthesis and nutrient deprivation in coral bleaching

Abstract Coral bleaching is one of the most serious events in marine ecology. It is triggered by high sea water temperatures that cause oxidative stress resulting from photodamage to symbiotic algae, consequently leading to the inactivation of their photosynthetic apparatus. Bleaching can also be caused by various environmental factors, such as diseases, chemical pollutants, and lack of nutrients. To investigate differences in photosynthetic responses of symbiotic algae between heat- and nutrient-dependent bleaching, we prepared long-term acclimatized strains of the coral, Acropora tenuis , harboring the same symbiotic algal and bacterial communities. These coral strains were exposed to natural (26 °C) and elevated (31 °C) temperatures without feeding in isolated aquaria. Photosynthetic activity of healthy symbiotic algae, involving both photosystems I and II (PSI and PSII) was monitored non-invasively during the treatment. In heat-dependent bleaching, decreased PSII activity, photosynthetic electron transport, and non-photochemical quenching of symbiotic algae occurred simultaneously with coral bleaching. No changes were observed in PSI photochemistry, suggesting that inactivation of photosynthesis affected the entire electron transport chain. In contrast, nutrient-dependent bleaching affected coral hosts, but had no direct effect on the photosynthesis of symbiotic algae. Heat-dependent bleaching was observed in all strains, but the degree of nutrition-dependent bleaching varied between strains, and trends were correlated with host growth. These results showed that there is a trade-off between the growth of the coral host and host capacity to maintain algal symbionts. These findings enhance understanding of coral host-algal endosymbionts interactions and may facilitate the selection of promising strains for coral farming.