Endolithic algae influence the skeletal microstructure and porosity of reef-building corals.

Coral skeletons provide habitat for a euendolithic community, forming a pigmented band within the skeleton, where Ostreobium is often a dominant group. Euendoliths actively penetrate live coral skeletons, but how they use and modify skeletal structure is not properly understood. This study explores the microstructural characteristics of skeletal microenvironments through a micro-CT technique that analyzes the "footprint" of the euendolithic community on the porosity of coral skeleton. We compared three Porites species samples, all of which exhibited a pigmented green band, based on the percentage of the relative volume of microporosity, macroporosity, total porosity, and solid volume fraction of calcium carbonate (CaCO) among three distinct zones within the coral colony: coral tissue, the green band (characterized by the eundolithic community) and the bare skeletal region. We found a significant increase in microporosity within the green band, while the opposite occurs for macroporosity that decreased within this zone, for all analyzed species. We describe a model to explain the porosity gradient along the vertical axis for Porites coral colonies and suggests that within the "green band" microenvironment, the metabolic activity of the community is the responsible for this pattern. These findings suggest potential beneficial roles of the euendolithic community within coral holobionts, including microporosity filling via re-mineralization and mitigation of macroerosion.