Efficient light-harvesting of mesophotic corals is facilitated by coral optical traits

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., mesophotic). 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.

持续存活的光依赖型珊瑚礁群落广泛分布于从海平面至150米水深的各类光环境中（即中光层（mesophotic））。但中光层珊瑚如何在光照极度受限的环境下成功繁衍，目前仍有待深入研究。本研究针对4种广域深度适应性珊瑚物种开展了系统性生态生理学与生物光学研究，旨在厘清光学性状属性在迥异光照环境下对光捕获过程的功能作用。研究表明，珊瑚骨骼的光学性状可随周边光照环境发生适应性调控，以匹配共生微藻对充足光照的需求，由此形成空间利用效率优异的光合共生系统。与浅海珊瑚相比，中光层珊瑚的光吸收量最高可达其三倍，即便仅处于表层入射辐照度约3%的光照条件下，仍可展现出优异的光合响应。中光层珊瑚的光捕获能力提升，是通过光学散射在珊瑚骨骼内部重新分配光线实现的，从而促进光线传输并被色素密集的宿主组织吸收利用。本研究结果不仅为解析中光层珊瑚礁生态系统生产力背后的光捕获机制提供了基础性认知，同时也引发了学界对其抵御长期环境扰动能力的担忧。