The hermatypic coral Stylophora pistillata has a wide bathymetric distribution (0 to 70 m). Within this range, light intensity decreases exponentially. Deep-water colonies are generally planar in morphology, with the upper part being dark and the bottom-facing part pale. Shallow-water colonies are generally subspherical and ivory in coloration. We studied the effects of photoacclimation on photosynthesis, respiration, and calcification in S. pistillata colonies along its bathymetric range over a reef profile (5 to 65 m) in Eilat, Gulf of Aqaba, Red Sea, during winter and summer, using a submersible respirometer. Respiration rate, light-saturated rate of photosynthesis (Pmax), compensation light intensity (Ec), and light intensity of incipient saturation (Ek), all decreased with depth. In contrast, the efficiency of photosynthesis (α) increased with depth. All colonies displayed ‘light-enhanced calcification’ during daytime and decreasing calcification rates with depth. These results indicate an adjustment in harvesting and utilization of light by the algal symbionts to the light environment. At all light intensities except the lowest ones, there was a consistent ratio of calcification to photosynthesis, in agreement with the concept of light-enhanced calcification. In the deepest, low-light corals, there was no evidence for support of calcification by photosynthesis, and we assume that these colonies subsist mainly by preying on zooplankton.
Israel - Red Sea