Heat stress alters the microenvironment of corals: oxygen depletion and oxidative damage in the gastrovascular cavity of two tropical species

Current climate changes cause increasing stress to corals, which can disrupt host-symbiont relationships, leading to bleaching and mortality. While stress responses are well-documented at the coral colony and reef scales, less is known about how stress responses affect different compartments within the coral holobiont. Here, we investigated the impact of thermal stress on macro– and microscale compartments, including the endoderm-lined gastrovascular cavity (GVC) and the ectoderm-lined external tissue surface, in two tropical corals, Caulastrea curvata and Galaxea fascicularis. We assessed the coral physiological status via respirometry, variable chlorophyll fluorometry, O2 and H2O2 microsensor analyses, chlorophyll measurements, and oxidative stress biomarkers during a 14-day thermal stress exposure. Under heat stress, both species showed a reduced O2 concentration in the GVC by up to 90% in the light, reaching hypoxic (< 50 µmol O2 L−1) to anoxic levels, while the O2 concentration in the external surface tissue of C. curvata decreased by 20%. Moreover, no changes in the H2O2 dynamics of the coral’s external tissue surface were detected. Lipid peroxidation in the gastrodermis of both species increased significantly by 90–198% indicating oxidative damage, although antioxidant enzyme activity (SOD and CAT), chlorophyll content, and bulk metabolic rates (respiration and photosynthesis) remained stable. Our findings indicated that thermal stress could affect the microenvironment of corals, particularly in the GVC, without visible bleaching or major disruptions in bulk physiology. This suggested the importance of microenvironmental metrics for the early detection of coral physiological stress.