الفهرس 
CHAPTER 1: GENERAL INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
The Red Sea is considered one of the few environments that accommodate a wide diversity of thermotolerant corals. However, the recent bleaching events had revealed that some Red Sea coral species may be under threat and may face a more dramatic future. At the western coast, little is known on the thermal tolerance of most corals and data available on Symbiodiniaceae diversity and coral bleaching are very limited. Here, we aim to investigate the potential effect of the ocean warming on hard corals along the Egyptian coast of the Red Sea.
Accordingly, the genetic diversity of Symbiodiniaceae community associated with 32 hard coral species (six families) collected from the shallow water of Port Ghaleb inshore reef between October and November 2017 had been resolved. Three Symbiodiniaceae genera had been recorded in coral samples where Cladocopium (associated primarily with Acroporidae, Poritidae, Merulinidae, and Mussidae) was the most dominant followed by Symbiodinium (associated primarily with Acroporidae, Pocilloporidae, Milleporidae). Durusdinium was less common and was mainly associated with Merulinidae and Poritidae. In total, we identified 46 ITS2 type profiles annotated by 75 unique DIVs and 22 majSeqs. The specificity of the endosymbiotic systems, however, was high and the co-occurrence of different ITS2 type profiles was uncommon in the same colony.
In the present study we additionally investigate the pattern of bleaching in hard corals inhabiting the Egyptian coast of the Red Sea (from Hurghada, at the north, to Wadi El-Gemal, at the south) during the 2020 summer heat stress period. Field data on the cover, number of colonies, and severity of the bleaching were collected from three geographical sectors (each contains two study sites), two depth ranges (0-5m and 10-15m), and two reef systems (inshore and offshore reefs) during the period from September to October 2020. The results indicated that the bleaching symptoms of different severities had appeared on 32.74% (460/1405) of the total examined colonies (36.66% of the total coral cover). Field survey, however, revealed that corals in the southern reefs (Sector_3) were more susceptible to bleaching than those present in the north (Sector_1). Also, it was noted that the bleaching intensity was more concentrated between 0-5m, while colonies beyond 10m were more sheltered. In contrast, the results revealed that both inshore and offshore reefs were vulnerable to bleaching with no effect for the distance from the shore on coral resistance. On the other hand, coral genera like Millepora, Montipora, Pocillopora, Acropora, and Porites showed high bleaching cover and severity whereby they may be more threatened by the thermal stress than the others.
On the other hand, the daily high-resolution (5km) gridded data of the SST collected by NOAA’s CRW in the period from 1990-2020 was used to calculate three indices of heat stress (i.e., SSTmax, DHW, and MHW). The results indicated that although the whole study area has experienced an increase in the SST, particularly between 2010-2020, the southern reefs have been subjected to less interannual thermal variability than those in the north. However, summer SST anomalies detected in the north may have not reached the threshold of stress and the conditions may have still been in the normal range of coral’s thermotolerance. On the contrary, although the increases of the daily SST above the MMM in the southern reefs were lower, these anomalies may be on the verge of the bleaching limits.
Together, our results expand the knowledge on Symbiodiniaceae genetic structure in different coral species inhabiting the Red Sea and provide baseline information on corals endosymbionts at the Egyptian coast. In the light of recent bleaching records, the present study emphasizes that many coral species, especially those harbouring Symbiodiniaceae C41/C1 majSeq, may have thermotolerant traits in the northern Red Sea. In contrast, and more cautiously, some keystone species particularly those associated with Symbiodinium may be more vulnerable to thermal stress. Concurrently, albeit inconclusive, the present study addresses the potential that the NGS-SymPortal profiling may afford not only in illuminating the deep genetic structure of Symbiodiniaceae assemblages but as a novel analytical platform for assessing the thermal tolerance of holobiont by inspecting the symbiont ITS2 intragenomic structure.