IMAGING OF OXYGEN DYNAMICS WITHIN THE ENDOLITHIC ALGAL COMMUNITY OF THE MASSIVE CORAL PORITES LOBATA1

We used transparent planar oxygen optodes and a luminescence lifetime imaging system to map (at a pixel resolution of <200 μm) the two‐dimensional distribution of O₂ within the skeleton of a Porites lobata colony. The O₂ distribution was closely correlated to the distribution of the predominant endolithic microalga, Ostreobium quekettii Bornet et Flahault that formed a distinct green band inside the skeleton. Oxygen production followed the outline of the Ostreobium band, and photosynthetic O₂ production was detected at only 0.2 μmol photons m^?2 · s^?1, while saturation occurred at ～37 μmol photons m^?2 · s^?1. Oxygen levels varied from ～60% to 0% air saturation in the illuminated section of the coral skeleton in comparison to the darkened section. The O₂ production within the Ostreobium band was lower in the region below the upward facing surface of the coral and elevated on the sides. Oxygen consumption in darkness was also greatest within the Ostreobium zone, as well as in the white skeleton zone immediately below the corallites. The rate of O₂ depletion was not constant within zones and between zones, showing pronounced heterogeneity in endolithic respiration. When the coral was placed in darkness after a period of illumination, O₂ levels declined by 50% within 20 min and approached steady‐state after 40–50 min in darkness. Our study demonstrates the use of an important new tool in endolith photobiology and presents the first data of spatially resolved O₂ concentration and its correlation to the physical structures and specific zones responsible for O₂ production and consumption within the coral skeleton.