Heterotrophic feeding and symbiotic state shape the chemical microenvironment, microbiome composition, and activity in the coral gastrovascular cavity

Coral gastrovascular cavities (GVCs) host diverse microorganisms and, as semi-closed compartments, are characterized by elevated nutrient concentrations and pronounced diel fluctuations in oxygen (O 2 ) and pH. However, their broader chemical microenvironment and associated microbial activities remain poorly understood. Here, we provide direct evidence for active anaerobic metabolism in the GVC of Astraeosmilia curvata by measuring hydrogen (H 2 ), nitric oxide (NO), and nitrous oxide (N 2 O) production, indicative of microbial fermentation and denitrification. Changes in GVC H 2 concentrations in Astraeosmilia curvata and Goniastrea sp. after food particle ingestion suggest that coral feeding activity modulates substrate availability, which in turn influences microbial activity in this compartment. In bleached A. curvata corals, the GVC remained consistently hypoxic and acidic, and anaerobic metabolic processes persisted across the diel cycle, whereas in healthy corals, the GVC became hyperoxic under light, and anaerobic metabolism was suppressed. These patterns suggest a shift in energy and nutrient cycling pathways within the bleached coral holobiont. Amplicon sequencing and PICRUSt2-based functional predictions revealed that the shifts in bacterial community composition correlated with microenvironmental chemical gradients, including the enrichment of putative denitrifying taxa (Alcanivoracaceae, Xanthobacteraceae) in polyps exhibiting elevated NO concentration, alongside predicted metabolic pathways involved in nitrogen and hydrogen metabolism. Together, our findings identify the GVC as a dynamic site of microbial activity that may play an important role in holobiont carbon and nitrogen cycling, with potential implications for nutrient balance and coral resilience. IMPORTANCE Understanding interactions among the coral animal host, Symbiodiniaceae, and associated bacteria within specific coral compartments is essential for elucidating the underlying mechanisms affecting the ecophysiology of reef-building coral holobionts, both under ambient and environmental stress conditions. However, most studies in the literature have focused on studying coral-microbe interactions at larger spatial scales, and especially the gastrovascular cavity (GVC) of corals remains underexplored despite its key role in coral processes, such as prey digestion, circulation of resources between polyps, sexual reproduction, and as the pathway for algal symbiont expulsion and colonization. By combining microsensor profiling with microvolume sampling and molecular analysis of the gastrovascular cavity, this study demonstrates strong chemical dynamics in single coral polyps, where spatio-temporal changes in O 2 availability enable active fermentation and denitrification within the GVC. Our findings suggest that the GVC is a hotspot of both aerobic and anaerobic microbial metabolism, with potential importance for energy and nutrient cycling in the coral holobiont.