pH measurements in the cœlenteron (pHcoel) of a reef coral, Stylophora pistillata, were performed using the microsensor technique. Seven samples grown in long-term coral culture facilities on glass slides at the Centre Scientifique de Monaco were used for this study. Measuremetns were made using motorised micromanipulator for precise movements of the microsensor on the order of micrometres. We first performed depth profiles in the polyps and in the coenosarc in tissue with a high dinoflagellate density, under light conditions to determine the variation of pHcoel. We then measured pHcoel of polyps and coenosarc at eight light intensities, from darkness to strong illumination, which allowed us to derive a pHcoel-irradiance curve and evaluate the role of photosynthesis in influencing pH in the coelenteron. Finally, we measured pHcoel in the coenosarc under light and dark conditions in two regions of interest characterised by visually different densities of dinoflagellates residing within the coral tissue: a high dinoflagellate density (HDD) tissue at the centre of the microcolonies and a low dinoflagellate density (LDD) tissue at the edge of microcolonies. For each sample, at least three replicate measurements were performed under all conditions.

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