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., Pgross and Rdark). 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.