Current climate changes cause increasing stress to corals, which can disrupt host-symbiont relationships, leading to bleach-ing and mortality. While stress responses are well-documented at the coral colony and reef scales, less is known abouthow stress responses affect different compartments within the coral holobiont. Here, we investigated the impact of ther-mal stress on macro– and microscale compartments, including the endoderm-lined gastrovascular cavity (GVC) and theectoderm-lined external tissue surface, in two tropical corals, Caulastrea curvata and Galaxea fascicularis. We assessedthe coral physiological status via respirometry, variable chlorophyll fluorometry, O2 and H2O2 microsensor analyses, chlo-rophyll measurements, and oxidative stress biomarkers during a 14-day thermal stress exposure. Under heat stress, bothspecies 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, nochanges in the H2O2 dynamics of the coral’s external tissue surface were detected. Lipid peroxidation in the gastrodermisof 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 find-ings indicated that thermal stress could affect the microenvironment of corals, particularly in the GVC, without visiblebleaching or major disruptions in bulk physiology. This suggested the importance of microenvironmental metrics for theearly detection of coral physiological stress.
