Efficient light harvesting in deep-water zooxanthellate corals
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The vertical zonation of dominant megabenthic, photosynthetic taxa suggests that differential photosynthetic capabilities enable specialized, low-light zooxanthellate corals to dominate at depths where shallow-water corals become light limited. This study examines the ecophysiology of deep-water (68-113 m) Leptoseris spp. and shallow-water (2-15 m) Porites spp. zooxanthellate corals from Hawai’i by comparing spectral absorbance properties and photosynthetic pigment concentrations to the available light spectra in their respective environments. Photo synthetically active radiation reaching Leptoseris spp. was 3 to 11% of surface irradiance compared to 41 to 90% reaching Porites spp. Optical measurements indicated that Leptoseris spp. exhibited lower reflectance (i.e. higher absorptance) compared to Porites spp. and were chromatically adapted to the wavelengths of photons available at depth. Despite the decreased spectral reflectance, deep-water Leptoseris spp. exhibited significantly lower areal photosynthetic pigment concentrations than did shallow-water Porites spp. Based on morphological comparisons of the skeletons of both coral genera, we hypothesize that Leptoseris spp. skeletons may cause incident light to travel through the coral tissue several times, thereby increasing photon-pigment interactions without increasing pigment concentrations. This superior light harvesting efficiency exhibited by Leptoseris in an energy limited environment (enabled by skeletal design rather than pigment physiology) may in part explain why the dominant genus of reef-building corals in Hawai’i cannot compete successfully with specialized low-light corals at extreme depths.