Mesophotic Coral Ecosystems

Flat skeleton Leaf E ect of morphology on light harvesting

Leptoseris structure

Porites structure

Sunlight

Water column

Tissue

Coral skeleton

Source: Enríquez et al. 2005, Kahng et al. 2012a, Kahng 2014

Figure 2.10. The absorption of light is influenced by the micromorphology of coral and algal skeletons.

ucture acclimatization strategies (both ecological and biological). These include the following (reviewed in Kahng et al. 2010, 2014): • Minimizing self-shading and maximizing surface area at a colony morphology level (e.g. horizontally flattened or encrusting colony morphologies), at a cellular level (e.g. monolayered zooxanthellate), and possibly at a subcellular level. • Reducing the amount of tissue biomass, surface area and respiratory demand to increase growth efficiency. • Reducing skeletal mass per unit colony area to reduce energy requirements. • Optimizing skeletal light-scattering properties (Figure 2.10). The reflective properties of calcium carbonate play an important role in increasing the light-harvesting efficiency of mesophotic corals (Enríquez et al. 2005, Kahng et al. 2012a, Kahng 2014) and may also occur in other organisms, such as calcareous green algae and coralline red algae. For a plant leaf (or non-calcareous macroalgae), light passes through the tissue only once and, unless absorbed by pigments, is lost. In contrast, the skeleton of a coral can reflect light back through the tissue, thereby increasing the probability of absorption. Light-harvesting efficiency is not only influenced by skeletal composition, but can also be affected by the light-scattering properties of skeletal micromorphology. Internal scattering can increase the probability of light absorption, independent of pigment concentration, by increasing the photon path length within the coral tissue (Figure 2.10). become light-limited on a slope that is shaded for much of the day (Figure 2.11). Mesophotic corals exhibit several adaptations relative to dependence on low light at depth, one of which is the switch from autotrophic (i.e., energy from light) to heterotrophic (i.e., energy from consumed foods) nutrition. This has been demonstrated using stable isotope techniques in scleractinian corals, Montastraea cavernosa (Lesser et al. 2010) and in a facultative zooxanthellate gorgonian from a temperate ecosystem (Gori et al. 2012). Specifically, planktonic resources, which are often higher on mesophotic reefs (e.g. Lesser and Slattery 2013) due to upwelled nutrients (Leichter and Genovese 2006, Leichter et al. 2007), are captured by the coral’s tentacles, thereby offsetting the lmss of energy from phototrophic sources.

Location can also affect the amount of ambient light available for mesophotic corals and algae. On flat or gently sloping areas, sessile organisms can be exposed to diffuse low light throughout the day, but on a steep slope, light is limited because the slope obstructs the light for a portion of the day (Brakel 1979). Thus, an MCE in clear water may have ample light at a given depth in areas with flat open seafloor, but may

Figure 2.11. A near-vertical mesophotic reef slope on thewestern side of Tobi (Hatohobei) Island, Palau at 55 m in depth. This area is heavily shaded during morning periods when the sun is in the east, casting a shadow across the area (photo Patrick L. Colin).

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 16