Morpho-functional traits of the coral Stylophora pistillata enhance light capture for photosynthesis at mesophotic depths

Abstract

The morphological architecture of photosynthetic corals modulates the light capture and functioning of the coral-algal symbiosis on shallow-water corals. Since corals can thrive on mesophotic reefs under extreme light-limited conditions, we hypothesized that microskeletal coral features enhance light capture under low-light environments. Utilizing micro-computed tomography scanning, we conducted a novel comprehensive three-dimensional (3D) assessment of the small-scale skeleton morphology of the depth-generalist coral Stylophora pistillata collected from shallow (4–5 m) and mesophotic (45–50 m) depths. We detected a high phenotypic diversity between depths, resulting in two distinct morphotypes, with calyx diameter, theca height, and corallite marginal spacing contributing to most of the variation between depths. To determine whether such depth-specific morphotypes affect coral light capture and photosynthesis on the corallite scale, we developed 3D simulations of light propagation and photosynthesis. We found that microstructural features of corallites from mesophotic corals provide a greater ability to use solar energy under light-limited conditions; while corals associated with shallow morphotypes avoided excess light through self-shading skeletal architectures. The results from our study suggest that skeleton morphology plays a key role in coral photoadaptation to light-limited environments.

Modeling the radiative, thermal and chemical microenvironment of 3D scanned corals

“…Light is mainly absorbed in the coral tissue and, to some extent, in the skeleton by endolithic algae and photosynthetic bacteria (Kühl et al, 2008). However, both coral morphology (Kaniewska et al, 2011; Kaniewska & Sampayo, 2022; Kramer et al, 2022) and the inherent scattering properties of coral skeleton and tissue modulate light propagation and thus the light exposure of the microalgal symbionts (Enríquez et al, 2005; Wangpraseurt et al, 2016) (Bollati et al, 2022; Lyndby et al, 2016).…”

The role and risks of selective adaptation in extreme coral habitats

“…4a–e ). Differences in such morphological features in corals are implicated in the regulation of within-colony light levels to optimize endosymbiont light harvesting and photosynthesis 52 – 54 . Moreover, the microgeometry of the coral skeletal surface appears to regulate networks of currents at the polyps surface, controlling sharing of nutrient, O 2 and metabolites generated by the algal endosymbionts 55 .…”

Speciation across depth gradients in reef corals

“…Therefore, corals have evolved a rich morphological and physiological repertoire to cope with strong gradients in light availability and intensity owing to their sessile nature 101,102 . Both corallite shape and colony architecture respond to light intensity to optimize autotrophy, enabling corals with a wider morphological response to occupy a broader depth range 103,104 . Higher polyp density found in shallow colonies, such as in O. faveolata Shallow (Fig.…”

“…Higher polyp density found in shallow colonies, such as in O. faveolata Shallow (Fig. 2|E, S2|F), acts as a self-shading mechanism to deal with excess UV light, whereas colonies in deep environments have fewer polyps to increase light capture, thereby enhancing photosynthesis 104,105 . The higher density of polyps in the Shallow lineage, which overlaps at mid-depths with that of the Deep lineage (Fig.…”