Seasonal mesophotic coral bleaching of Stylophora pistillatain the northern Red Sea

Coral bleaching occurs when environmental stress induces breakdown of the coral-algae symbiosis and the host initiates algae expulsion. Two types of coral bleaching had been thoroughly discussed in the scientific literature; the first is primarily associated with mass coral bleaching events; the second is a seasonal loss of algae and/or pigments. Here, we describe a phenomenon that has been witnessed for repeated summers in the mesophotic zone (40-63 m) in the northern Red Sea: seasonal bleaching and recovery of several hermatypic coral species. In this study, we followed the recurring bleaching process of the common coral Stylophora pistillata. Bleaching occurred from April to September with a 66% decline in chlorophyll a concentration, while recovery began in October. Using aquarium and transplantation experiments, we explored environmental factors such as temperature, photon flux density and heterotrophic food availability. Our experiments and observations did not yield one single factor, alone, responsible for the seasonal bleaching. The dinoflagellate symbionts (of the genus Symbiodinium) in shallow (5 m) Stylophora pistillata were found to have a net photosynthetic rate of 56.98-92.19 mmol O₂ cm^-2 day^-1. However, those from mesophotic depth (60 m) during months when they are not bleached are net consumers of oxygen having a net photosynthetic rate between -12.86 - (-10.24) mmol O₂ cm ^-2 day^-1. But during months when these mesophotic corals are partially-bleached, they yielded higher net production, between -2.83-0.76 μmol O₂ cm^-2 day^-1. This study opens research questions as to why mesophotic zooxanthellae are more successfully meeting the corals metabolic requirements when Chl a concentration decreases by over 60% during summer and early fall.