Study on Symbiodinium diversity in scleractinian corals of Dong-Sha Atoll

Dong-Sha Atoll is the northernmost and well developed atoll in the South China Sea. For further study on reef-building corals and zooxanthellae (symbionts) diversity in coral hosts and their correlation with temperature fluctuation within different depths is necessary, to enact the conservation strategy in Dong-Sha Atoll, and important basic data to understand coral reef resistance and resilience to the gradual elevated seawater temperature caused by climate change.
Coral samples were collected in 2009 and 2010 period in patch reefs of Dong-Sha lagoon from 2 different depths (Reef Top 1-5 meters and Reef Base 10–15 meters). A total of 903 samples belong to 7 families and 21 genera of reef-building corals were collected. Symbiodinium diversity was assessed using restriction fragment length polymorphism (RFLP) of the nuclear large subunit ribosomal DNA (nlsrDNA) of Symbiodinium DNA. In the present study, the results showed that the reef-building corals in Dong-Sha were associated with only two Symbiodinium clades: clade C and clade D. From the analysis of 771 samples collected in 2009, the Symbiodinium clade C, D and C+D proportion was 71.34%, 19.71% and 8.95% respectively. The results were similar to the pattern observed in corals from the Indo-Pacific where the Symbiodinium diversity of coral reefs is dominated by Symbiodinium clade C. Symbiodinium distribution analysis between Reef Top and Reef Base showed that clade D proportion in Reef Top (24.48%) was higher than Reef Base (14.88%)(chi-square, p=0.007). This might be due to the higher annual average seawater temperature at the Reef Top compared to Reef Base. In 2010 bleaching event caused by La Niña, the clade C proportion in bleached corals was up to 84.78% compared to clade D proportion of 10.87%. However, 62.79% of the non-bleached corals that were associated with clade D. This may support that Symbiodinium clade D can cope with coral bleaching caused by high seawater temperature.