Biophotonic function of the calcium carbonate skeleton in Lithothamnion crispatum: A possible adaptation of coralline algae to low-light environments

: Coralline algae (Corallinophycidae, Rhodophyta) have adapted to a broad range of marine habitats, including low-light mesophotic zones, yet the potential role of their high-Mg calcite skeleton in light harvesting remains poorly investigated. Here, we examine the skeletal architecture of Lithothamnion crispatum rhodoliths through X-ray micro-computed tomography (μ-CT) and scanning electron microscopy (SEM), revealing a distinct Voronoi-like tessellation of the epithallial cells associated with a nearly hyperuniform arrangement of submicrometric pores. This structural organization can promote the penetration of scattered light into the thallus, enhancing photon availability in deeper tissues. To further assess the optical implications of the skeleton morphology, we integrated full-wave finite element method (FEM) and ray-tracing simulations, demonstrating that the combination of calcite birefringence and quasi-ordered cell filaments facilitates the superposition of the optical field with chloroplast-rich regions, particularly within a spectral range relevant to deep-water photosynthesis. Our findings highlight for the first time a potential biophotonic function of coralline algal skeletons, opening new perspectives on the role of biomineralization in light manipulation and energy capture in mesophotic habitats. This may help explain the remarkable evolutionary success of coralline algae in low-light environments compared to fleshy macroalgae.