Evaluation of laser scanning confocal microscopy as a method for characterizing reef-building coral tissue thickness and Symbiodiniaceae fluorescence

Predicting the sensitivity of reef-building corals to disturbance, including bleaching, requires an understanding of physiological responses to stressors, which may be limited by destructive sampling and the capacity of common methodologies to characterize early life history stages. We developed a new methodology using laser scanning confocal microscopy (LSCM) to measure and track the physiological condition of corals. In a thermal stress experiment, we used LSCM to track coral condition during bleaching in adults and juveniles of two species, Montipora capitata and Pocillopora acuta. Depth of fluorescence in coral tissues provides a proxy measure of tissue thickness while Symbiodiniaceae population fluorescence relates to both population density and chlorophyll-α content. In response to thermal stress, there were significant shifts in tissue thickness and Symbiodiniaceae fluorescence with differences between life stages. This method is particularly well-suited for detecting shifts in physiological condition of living corals in laboratory studies, especially in small juvenile colonies.