The influence of photoacclimation on the microscale oxygen environment and microbial niches in the coral Galaxea fascicularis

Light availability is a key environmental driver for symbiotic corals, as it regulates photosynthesis by endosymbiotic algae and hence strongly influences holobiont physiology. Light levels control symbiont photosynthetic oxygen production, which in turn shapes the internal and external physicochemical microenvironment of corals, potentially impacting the microbial communities associated with different anatomical compartments within coral polyps. Here, we experimentally investigated the effects of sustained exposure to contrasting light regimes on the oxygen microenvironment in the surface mucus layer (SML) and the gastrovascular cavity (GVC) of the coral Galaxea fascicularis. We found that sustained light-driven changes in photosynthetic activity can significantly alter internal oxygen conditions, but that responses varied across individual polyps, suggesting fine-scale physiological heterogeneity. Both SML and GVC samples were dominated by the genus Endozoicomonas , a key coral-associated taxon commonly found in intratissue aggregates. Endozoicomonas relative abundance declined in the GVC fluid under high light, but microbial community composition remained otherwise remarkably stable across light treatments. Our results underscore the need to further explore compartment-specific coral responses to environmental change.