Symbiont intragenus diversity provides resistance to coral bleaching under extreme environmental conditions in Acropora muricata
Widespread coral bleaching has generally been linked to high water temperatures at larger geographic scales. However, the bleaching response can be highly variable among individual of the same species, between different species, and across localities; what causes this variability remains unresolved. Here, we tracked bleached and non-bleached colonies of Acropora muricata to see if they recovered or died following a stress event inside the semi-enclosed lagoon of Bourake (New Caledonia), where corals are long-term acclimatized to extreme conditions of temperature, pH and dissolved oxygen, and at a nearby control reef. We describe Symbiodiniaceae community changes, metabolic responses, and energetical reserves (12 physiological traits evaluated) during La Nina warm and rainy summer in 2021. Widespread coral bleaching (score 1 and 2 on coral colour health chart) was observed only in Bourake, likely due to the combination of the high temperatures (up to 32 degree) and heavy rain. All colonies (i.e., Bourake and reference site) had Cladocopium spp. as their main genera. Unbleached colonies in Bourake had a specific ITS2-type profile (proxies for Symbiodiniaceae genotypes), while the bleached colonies in Bourake had the same ITS2-type profile of control colonies during the stress event. After four months, the few bleached colonies that survived acquired the same ITS2 type profiles of the unbleached resistant colonies. In terms of physiological performances, all bleached corals showed metabolic depression (e.g., Pgross and Rdark). In contrast, unbleached colonies in Bourake maintained higher metabolic rates and energetic reserves compared to control corals. Our study suggests that some colonies of Acropora muricata might enhance their resistance to bleaching thanks to specific Symbiodiniaceae, while energetic reserves may increase their resilience after stress.