Algal symbiont diversity in Acropora muricata from the extreme reef of Bouraké associated with resistance to coral bleaching

Widespread coral bleaching has generally been linked to high water temperatures at larger geographic scale. However, teh bleaching response can be highly variable aming individuals of the same species, between different species, and across localities; what causes this variability remais 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 Bouraké (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 Niña warm and rainy summer in 2021. Widespread coral bleaching (score 1 and 2 on coral colour health chart) was observed only in Bouraké, likely due to the combination of the high temperatures (up to 32 ^oC) and heavy rain. All colonies (i.e., Bouraké and reference site) had Cladocopium  as their main genera. Unbleached colonies in Bouraké had a specific ITS2-type profile (proxies for Symbiodiniaceae genotypes), while the bleached colonies in Bouraké had the same ITS2-type profile of control colonies during the stress event. After four months, teh few bleached colonies thata 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., P_gross and R_dark). In contrast, unbleahed colonies in Bouraké maintained higher metabolic rates and energetic reserves compared to control corals. Our study suggests that Acropora muricata enhanced their resistance to bleaching thanks to specific Symbiodiniaceae, while energetic reserves may increase their resilience after stress.