Chemical Microenvironment of the Coral Gastrovascular Cavity and Its Associated Bacterial Diversity

Coral gastrovascular cavities (GVCs) host diverse microorganisms and, as semi-closed compartments, are characterized by elevated nutrient concentrations and pronounced diel fluctuations in oxygen (O2) 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 by measuring hydrogen (H2), nitric oxide (NO), and nitrous oxide (N2O) production, indicative of microbial fermentation, nitrogen fixation and denitrification. Changes in GVC H2 concentrations after feeding suggest that feeding modulates substrate availability, which in turn influences microbial activity in this compartment. In bleached corals, the GVC remained consistently hypoxic and acidic, and anaerobic metabolic processes persisted across the diel cycle. These patterns suggest a shift in energy and nutrient cycling pathways within the bleached coral holobiont. 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.