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.