Corals are known to be the typical hosts for symbiotic dinoflagellates of the genus Symbiodinium which are functionally diverse 1. This diversity reflects the functional ecology of different Symbiodinium types that establish different coral-symbiont partnerships in habitats of different conditions. Accordingly, genetic diversity of endosymbionts associated with corals is considered critical for coral life. Many studies had revealed that Symbiodinium clades have different thermal tolerance in which Symbiodinium D is the most thermotolerant and C is the most thermosensitive 2. Though, under environmental stressors, members within the same clade do not necessarily acquire similar physiological responses 3. At finer levels, Symbiodinium types within each clade possess differential secondary thermal tolerances 14. As a consequence, bleaching events are severe at reefs dominated by thermosensitive types 567. Yet, determining the definite thermal tolerance that outlines the susceptibility of corals to bleaching, of many types is still in its infancy 3. However, inspecting genetic composition of coral’s endosymbiotic systems is considered crucial for understanding susceptibility of corals to environmental stressors and predicting the consequences of increasing temperature on coral reefs.

Red Sea coral reefs represent the upper northern limits for coral reefs distribution. Although Red Sea coral reefs are considered unique reefs, they had received little concern about their endosymbiotic systems 8. Only few studies on the genetic diversity of Symbiodinium at the northern tip of the Gulf of Aqaba and at the eastern coasts had been published 91011. Despite that the Egyptian coast of the Red Sea extends approximately 1800 km 12, genetic structure of endosymbiotic systems along this western part had not resolved yet, especially in scleractinian corals which are considered the main reef builders. Here, we assess Symbiodinium diversity on the reef building corals host species and one species of fire coral that exist along Egyptian coast of the Red Sea. Coral samples were collected from shallow (1-5) and mid-depth (5-10) habitats. We address geographical differences between coral populations of western Egyptian coast and eastern Saudi Arabian coast of Red Sea in terms of phylogenetic diversity of their dinoflagellate symbionts.

Based on 18S nrDNAs, three different banding patterns were produced by Symbiodinium harbored studied corals. The first banding pattern was characterized by two bands of 1173 and 800 pbs, while the second pattern consisted of three bands of 800, 678, and 288 pbs Figure 2 The third was consisted of a combination of the first and second banding patterns Figure 3 The first and second banding patterns are corresponding to previously published RFLP profiles for Symbiodinium clades C and A, respectively, while the third one is a combination of clades A and C (A+C).

MEGABLAST search on NCBI GenBank for ITS2 sequences of all Symbiodinium isolated from different coral species confirmed that they relate either to clade A or C. Further sequence data showed that each Symbiodinium clade represented by only one subclade (Figure 4) For clade A, all ITS2 sequences were highly similar to Symbiodinium A1 type (Tridacna maxima, Egypt, Red Sea, GU069005) with a maximum identity of 99%, E value of 3e-120, and query cover of 100%. On the other hand, for clade C, 10 sequences were highly similar to Symbiodinium C1 type (Polycyathus sp., Taiwan, JQ180021) with a maximum identity of 99%, E value of 2e-136, and query cover of 100%. Sequences of isolated Symbiodinium from studied corals were archived in the NCBI GenBank with accession numbers showed in Table 1.

Of nine examined species (n=98), the current genetic survey revealed that 61% of coral samples hosted Symbiodinium C, 33% associated with Symbiodinium A, and only 6% formed symbiosis with A+C combination. Symbiodinium C1 was recorded in acroporid, poritid, faviid, and pocilloporid species, while clade A1 was only detected in the three studied pocilloporid species as well as a fire coral M. dichotoma (Figure 5).

Phylogenetic analysis of Symbiodinium sp. associated with scleractinian corals along wide geographic localities in the Red Sea revealed low diversification in endosymbionts (Figure 6). Nevertheless, eastern coast of the Red Sea showed higher Symbiodinium diversity than western side. Regarding host flexibility, pocilloporid corals hosted more phylogenetically diverse Symbiodinium types. For instance, P. verrucosa harbored Symbiodinium A1, D1a, B1, C15, or C85; while S. pistillata harbored Symbiodinium closely related to A1, F2, C72, or C3. In contrast, Acropora spp. were associated mainly with Symbiodinium C1.

In spite of the great concern paid for coral reef ecosystem, Symbiodinium genetic diversity along the Red Sea had been overlooked. So, the current study introduced an overview on Symbiodinium genetic diversity in seven species of scleractinian corals and one species of fire coral along the Egyptian Red Sea coast. The present study revealed that Symbiodinium population associated with Red Sea corals were primarily composed of C and A clades. In general, these two clades were recorded to have a global distribution 20. Among different types of Symbiodinium under each clade, only C1 and A1 types were recorded with the dominance of C1 type comprising 61% of the recorded symbionts in all studied corals. C1 was detected to be the most dominant and widely distributed type because it has the ability to associate with a wide array of host species and a great adaptation with different environmental conditions 2122.

Dominance of Symbiodinium C1 was explained by two arguments with the same perception. The first supposed that Symbiodinium C1 type has a generalist nature along wide geographic scales 22. The second proposed that coral-Symbiodinium symbiosis is directed toward specialist-types preference along evolutionary history of coral reefs 21. One of studies suggested that dominance of C1 type in west Pacific had been transferred to western Indian Ocean, and eventually to the Red Sea, before biogeographic partitioning between the two oceans 23. Because northern Red Sea coral reefs are considered recent reefs 24, far from major natural stressors, dominance of C1 type associated with scleractinian corals may be explained on the basis of recent low-specialist symbiosis.

Symbiodinium population at Yanbu was dominated by clade C (69%) followed by clade A (27.3%) while clade D formed only 3.7% of symbionts 11. The current results emphasized the previous mentioned dominance order of clades C and A, and added the endosymbiotic system composed of a combination from the two clades C and A which could be considered as a unique system in Red Sea corals as reported by 20. This is in contrast with corals of the Arabian Gulf where Symbiodinium D represents the dominant clade 1125. Predominance of such thermotolerant clade at the Arabian Gulf is associated with thermal tolerance of >33^°C 2. However, Symbiodinium D and B that are known to associate with scleractinian corals were not detected.

Compared with high host diversity along the Red Sea, symbiont diversity was low. The same pattern was previously recorded along other reefs at Great Barrier Reef and Indian Ocean 2627. The flexibility of P. verrucosa for harboring Symbiodinium A and C was coincident with that recorded in the central Red Sea 28. On the other hand, divergence within phylotype C may explain the dominance of this phylotype in coral reefs of the Red Sea.

Based on coral reproductive strategy, Acropora., Favia and Porites are mainly broadcast spawners and tend to utilize horizontal transmission strategy for Symbiodinium acquisition from surrounding water 1029. In contrast, S. pistillata, S. wellsi, and M. dichotoma are principally brooder species and consequently associated with Symbiodinium A by vertical mode of transmission 10. Though, S. pistillata in the Red Sea has the ability to associate with Symbiodinium C 1130. Previous studies indicated that colonies of S. pistillata collected from the Red Sea are able to harbor clades A, C, or A+C combination. They stated that 13% of S. pistillata collected from Gulf of Aqaba (17m depth) harbored clade C, while 87% of the colonies (5 and 17m depth) associated with clade A 3132. Comparatively, our results revealed that 14% of S. pistillata colonies (mainly collected from surface water at Fanous Reef during winter) were populated with clade C.

It is worth mentioning that the first stages of corals are more flexible than adult stages 33. Simultaneous or individual occurrence of clades A and C in colonies of S. pistillata and P.verrucosa may be interpreted upon assumption that acquisition of Symbiodinium C by colonies that already harbor clade A, may occur by horizontal transmission at later stages during coral ontogeny. A or C clades could be transmitted vertically from parent to juveniles that may maintain cladal structure along life cycle or switch to other clade by horizontal transmission in later stages 34.

At central Red Sea, coral reefs were affected by mass bleaching event at Thuwal during summer 2010 35. They reported that, at some sites, 80-100% of Oculinidae, Agariciidae, and Fungiidae were severely impacted by this event, while bleaching cover among Pocilloporidae, Acroporidae, and Faviidae was 19, 33, and 40%, respectively. More recently, sea surface temperature was raised up to 34°C at the Egyptian coasts of the Red Sea during August 2012. This high temperature anomaly resulted in mass bleaching event in some reef building coral species (Hanafy and Ahmed, unpublished data). Among these, Stylophora, Montipora, Porites, andMillepora were the most affected genera by thermal stress. Symbiodinium A or C harbored by Red Sea corals were considered thermosensitive clades 36. The dominance of these clades observed in the present study may indicate that coral reefs at the Red Sea are endangered by the thermal stress. Accordingly, patterns of temperature anomalies may have dramatic effects on coral reefs diversities and structures at the Red Sea as suggested by 35. However, with little knowledge on Symbiodinium genetic diversity at these levels, we could not generalize our investigation on Red Sea corals. Accordingly, intensive molecular survey with deep resolution on Symbiodinium populations may contribute in understanding of bleaching patterns along the Red Sea. In parallel, future experimental studies are required to derive comprehensive perception about thermal tolerances of these clades and/or types.

Conclusively, the current study emphasizes the need for resolving Symbiodinium genetic diversity along Egyptian coasts of the Red Sea. The level at which this diversity will be resolved is critical for identifying threats to coral reefs at the Red Sea. Identifying Symbiodinium types harbored by different hosts may contribute largely in understanding Symbiodinium dynamics and responses to environmental changes at these reefs.