Seasonal variation of coral photophysiology at extreme environmental conditions in a high-latitude back reef

Zooxanthellate scleractinian corals inhabiting back reefs or intertidal shallow reefs, both of which experience extreme diurnal and seasonal fluctuations, have been hypothesized to possess specific acclimation strategies to these environmental changes and potentially exhibit high tolerance to bleaching events. However, the seasonal dynamics of physiological activities under natural field conditions remain poorly studied. In this study, we focused on a high-latitude reef in Okinawa and investigated monthly changes in photochemical efficiency ( F v / F m ), zooxanthellae density, chlorophyll pigments, and Symbiodiniaceae types of the coral Acropora digitifera from a shallow back reef. We also examined the monthly variation in photosynthetic activities between 2 locations within colonies (center vs. peripheral branches). For branches at the colony center, F v / F m values, zooxanthellae densities, and chlorophyll a (chl a ) concentrations were negatively correlated with both seawater temperature and total light intensities 1 wk and 1 d prior to sampling, whereas chlorophyll c 2 (chl c 2 ) concentrations showed positive correlations. In addition, significant inter-colony differences in F v / F m were observed; approximately half of the colonies showed sharp decreases in July followed by recovery in August, while the remaining colonies exhibited no significant decline in July. These inter-colonial differences were not explained by variation in Symbiodiniaceae types. Furthermore, zooxanthellae density and chl a and chl c 2 concentrations were significantly higher in central branches than in peripheral branches, with greater seasonal variation. This study highlights the potential inter- and intra-colonial variation in the photophysiological activity of corals inhabiting environmentally harsh reef habitats, providing important insights into the physiological diversity and resilience of corals.