Sustained light-dependent coral reef communities can be found at a wide range of light environments, extending from the sea level to as deep as 150 m (i.e. esophotic). How mesophotic corals thrive despite extremely limited light conditions still requires further investigation. Here, we undertook a comprehensive ecophysiological and bio-optical study on four depth-generalist coral species aiming to delineate the functional role that optical trait properties have in light-harvesting, at contrasting light regimes. We show that the optical traits of coral skeletons are adjusted to their ambient light conditions and complement the microalgal demands for sufficient light, thus exhibiting a spatially efficient photosymbiotic system. In contrast to shallow corals, mesophotic corals absorbed up to three fold more light, resulting in excellent photosynthetic response under light conditions of only ~3% of the incident surface irradiance. The enhanced light-harvesting capacity of mesophotic corals was achieved by redistributing light in the coral skeleton through optical scattering, thereby facilitating light transport and absorption by densely pigmented host tissue. Our findings provide fundamental insight into the light-harvesting mechanisms underlying the productivity of mesophotic coral reef ecosystems, yet also raise concerns regarding their ability to withstand prolonged environmental disturbances. A free Plain Language Summary can be found within the Supporting Information of this article.
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