Investigating the Role of Salinity in the Thermotolerance of Corals

Abstract

Coral reefs are in global decline due to ocean warming and ocean acidification. While these stressors are commonly studied in climate change predictions, salinity, although being an important environmental factor, is not well understood. The response of the coral holobiont (the association of the coral host, its algal endosymbiont and a suit of other microbes) to changes in salinity and the contribution of each holobiont compartment underlying the necessary osmoadaptation remain especially elusive. Interestingly, we find some of the most thermotolerant corals in some of the most saline seas, e.g. the Red Sea and the Persian Arabian Gulf. This observation sparked the hypothesis of a link between osmoadaptation and coral thermotolerance. Here, we set out to elucidate the putative effects of high salinity on conveying thermotolerance and thereby a possible link to bleaching in the context of the coral holobiont. For this, we conducted a series of heat stress experiments at different salinities in the coral model Aiptasia and subsequently validated our findings in corals from the central Red Sea. We confirm a role of osmoadaptation in increased thermotolerance and reduced bleaching in Aiptasia and Red Sea corals. This salinity-conveyed thermotolerance was characterized by a reduction in algal endosymbiont loss, photosystem damage and leakage of damaging reactive oxygen species (ROS) in high salinity. Further analysis of the osmoadaptation response using targeted GC-MS uncovered high levels of the sugar floridoside at high salinity only in holobionts that show the salinity-conveyed thermotolerance. The increase of floridoside, an osmolyte capable of scavenging ROS, and the concurrent reduction of ROS argues for a mechanistic link of increased thermotolerance and reduced bleaching in high salinities. In addition, the restructuring of the microbiome at high salinity that aligned with the difference in thermotolerance in Aiptasia may be indicative of a microbial contribution towards a more beneficial holobiont composition. Hence, emphasizing the potential cumulative contribution of each holobiont compartment during stress-resilience, as well as highlighting the overall role of osmoadaptation in the thermotolerance of corals.