From Coral Color to Tissue Heating: How Host Pigments Redirect Light Within the Tissue

Coral host pigments strongly modify the internal light environment of corals, yet their influence on heat dissipation in coral tissue remains poorly understood. Here, we combined scalar irradiance and temperature microsensor measurements with mechanistic multiphysics modelling to quantify how chromoproteins (CPs) and fluorescent proteins (FPs) regulate light propagation and tissue heating in corals. Measurement-informed simulations of coupled radiative and heat transfer showed that high host pigment concentrations increase broadband scattering while strongly absorbing light within their characteristic spectral bands, reducing light penetration into deeper tissue layers and producing localized energy deposition. Under high symbiont densities, tissue heating was governed by absorption by both symbionts and host pigments, whereas in bleached corals, host pigments became the dominant source of heat generation. Heat generated by CP absorption was concentrated within the epidermis, while symbiont absorption caused deeper heating within the gastrodermis. Unlike CPs, FPs additionally redistributed a fraction of the absorbed photons through lower energy fluorescence emission, increasing scalar irradiance at longer wavelengths and shifting energy deposition deeper into the tissue, thereby enhancing subsurface heating relative to the corresponding non-fluorescent CP scenario. In the model, this pathway changes scalar irradiance and predicted energy deposition relative to the same FP parameterization with fluorescence disabled. The magnitude of this effect varied with FP concentration, spatial organization (granular or diffuse), and symbiont density. Model outputs were supported by surface light and temperature microsensor measurements across sampled coral phenotypes and symbiont densities. Together, our results demonstrate distinct modeled optical pathways for CPs and FPs that regulate not only light propagation but also the spatial distribution of heat generation within coral tissues. By balancing light attenuation, spectral redistribution, and heat generation, host pigmentation is thus likely to influence photoacclimation, bleaching susceptibility, and recovery of the coral-algal symbiosis.