Photoacoustics: a novel application to the determination of photosynthetic efficiency in zooxanthellate hermatypes

We present here a novel application of photoacoustics to the monitoring and study of the physiological status of zooxanthellate corals. Until now, the method has only been applied in homogeneous phytoplankton cultures and field samples. Corals are among the world's most productive ecosystems because they host symbiotic dinoflagellates (zooxanthellae) that provide them with a large amount of photosynthates for their energy requirements. Coral reefs face unprecedented pressures on local, regional, and global scales due to climate change and anthropogenic disturbances. Responses to such stress are often a decrease in the photosynthetic efficiency of the symbiotic dinoflagellates, as well as bleaching, which involves the mass expulsion of these symbionts or loss of their pigments. Photosynthesis is a sensitive indicator of stress in plants and plays a central role in the nutrition of symbiotic invertebrates. Our aim was to examine the applicability of photoacoustics, developed by us for ecological work with phytoplankton, to the study of symbiotic dinoflagellates in situ. We have determined areal chlorophyll content and light energy storage efficiency with three zooxanthellate coelenterates, two corals and one hydrozoan, hosting symbiotic algae. We also present the effect of temperature elevation on the decrease in photosynthetic efficiency of the symbiotic coral Stylophora pistillata determined by photoacoustics. Our results demonstrate the potential, power, and convenience of photoacoustics in following bleaching-related changes in coral pigmentation, in the photosynthetic energy storage efficiency of corals, and in its usefulness in diagnosing its health in relation to environmental factors, in the example presented here, seawater warming.