Depth-dependent microskeletal features modify light harvesting in corals.

Coral skeletal morphology modulates light exposure in symbiotic algae, especially in light-limited environments like mesophotic reefs. However, quantifying light capture within complex coral structures remains challenging. Here, we used optical coherence tomography and high-resolution X-ray scanning to explore depth-dependent bio-optical properties of shallow and mesophotic corals from the Gulf of Eilat/Aqaba, Red Sea. We identified two distinct skeletal layers: a highly scattering superficial layer and a deeper, more light-penetrating layer. Mesophotic corals showed higher scattering coefficients and a lower anisotropy of scattering values, yielding increased reflectivity. Regardless of depth, coenosteum grooves facilitated forward scattering, while protruding features such as spines and septa increased surface reflectivity and isotropic scattering. Light simulations demonstrated an enhanced fluence rate at the skeleton-water interface, with mesophotic corals enhancing the available light up to 2.7-fold. These findings suggest that microskeletal heterogeneity fine-tunes light capture at the microenvironmental scale, thereby enhancing light-harvesting efficiency across depth.