Status of Symbiodiniaceae abundance in hard corals during the 2020 bleaching event along the Egyptian Red Sea coast

In the present study, the density of endosymbiotic dinoflagellates (Family: Symbiodiniaceae) has been used as a measure to investigate the effect of summer heat stress on the hard corals inhabiting the Egyptian coast of the Red Sea. Count data were collected between September and October 2020 throughout a microscopic examination of eight keystone coral genera (n= 301) sampled from three sectors (contain six sites), two reef localities (inshore and offshore), two depth ranges (0-5 m and 10-15 m), and four bleaching severity states (from 0% unbleached to 51-99% highly bleached colonies). The results indicated that coral samples collected from the southern sites (Sector_3) were associated with lower densities of Symbiodiniaceae compared to coral samples collected from the northern (Sector_1) or mid sites (Sector_2). Also, samples collected from the surface water had demonstrated lower Symbiodiniaceae density compared to those collected from the deep. Conversely, our results indicated that there was no effect for the distance from the shore on the Symbiodiniaceae density in the examined samples. Furthermore, our analysis revealed that Millepora was particularly associated with the lowest Symbiodiniaceae density among the other genera. On the other hand, the density of Symbiodiniaceae cells decreased – as expected – with increasing bleaching severity in the study samples (n= 131). However, this discrimination between bleached and unbleached colonies was largely restricted to acute bleaching severities. Taken together, these results suggest that while the corals at the northern Egyptian coast of the Red Sea look more tolerant, corals at the southern reefs may be threatened by the summer heat stress.