Trophic Potential and Photoecology of Endolithic Algae Living within Coral Skeletons

Abstract.The biomass of the endolithic algaeOstreobium quekettiiPhyllosiphoniaceae living within skeletons of the scleractinansMycedium elephantotusandLeptoseris fragilisaverages 300 μg protein. cm‐2. This represents approximately 7% of the protein of the zooxanthekie‐containing tissue ofM. elephantotusand approximately 38% of that ofL. fragilis. Oxygen production Pmaxnetof0. querkettiiin bare skeletons ofM. elephantotusaveraged 0.7 μg O2.cm‐2· h‐1measured in large skeletal fragments. This amount is approximately 6% of the productivity of the zooxanthellaeSymbiodinium microadriaticumliving in the same scleractinian species at the same depth Pmaxnet11 μg O2· Cm‐2· h‐1. Light compensation ofO. quekettii‐ within skeletons ‐ was reached at approximately 10 and saturation at 35 40 μE·m‐2· s‐1. Algae within theM. elephantotusskeletons receive a maximum of 4–6% of the ambient irradiance, which is approximately 0.9 μE · m‐2· s‐1approximately 0.04% surface irradiance at a depth of 88 m. InL. fragilisat a depth of 145 m, the photon flux decreases to 0.3 μE·m‐2· s‐1, which is less than 0.004% of surface intensity. With increasing depth, the ratio of Chlbto Chlaincreased in endolithic algae colonizingL. fragilis, indicating improvement of light harvesting under low light conditions. In free‐livingO. quekettiicultured at irradiance levels from 0.5–60 μE·m‐2· s‐1, the concentrations of chlorophylls increased and that of siphonein and β‐carotene decreased with decreasing photon flux.