Species richness and generalists–specialists mosaicism of symbiodiniacean symbionts in corals from Hong Kong revealed by high-throughput ITS sequencing

Hong Kong is considered to be a marginal area for coral growth due to its subtropical geography with relatively low winter sea temperatures. Corals can only form non-reefal communities and are believed to host only a low diversity of symbionts with limited flexibility in their symbiont changes. Whether these previously predicted low symbiont diversity is true or is simply a technical artifact and whether these symbionts are generalists or specialists have not fully resolved. To address these issues, we investigated symbiodiniacean diversity and community structure of 30 species of Hong Kong corals using high-throughput sequencing of the nuclear ribosomal RNA gene ITS2. We found high Symbiodiniaceae species richness, with each coral hosting multiple distinct ITS2 symbiont types. Using SymPortal pipeline to alleviate effects of intraspecific ITS polymorphism, the 4662 Symbiodiniaceae ITS2 sequence variants (DIVs) found in our samples were collapsed into 13 distinct ITS2-type profiles, covering Symbiodinium, Breviolum, Cladocopium, and Durusdinium genera, seven of which were coral specific. Cladocopium goreaui was the most diversified (six profiles) and prevalent lineage, dominating the symbiont communities in 29 of the 30 corals species examined. The stress-tolerant Oulastrea crispata was exceptional as its symbionts were dominated by Durusdinium eurythalpos (D13-D13b-D12-D13c profile). Interestingly, Cladocopium C15 was diversified into two ITS2-type profiles, one being Porites lobata specific, while the other was associated with both P. aranetai and P. lutea. Overall, most corals harbor a single dominant generalist symbiont, while some corals host both generalist and specialist symbionts. This work lays the foundation for future research to understand how the generalist and specialist as well as dominant and rare symbionts contribute to the responses and resilience of their host corals against environmental fluctuations in a marginal coral ecosystem, like that in Hong Kong.