Species and spatio-environmental effects on coral endosymbiont communities in Southeast Asia

Understanding the drivers of algal endosymbiont communities hosted by reef corals is a requisite for predicting coral resilience. For the biodiverse reefs of Southeast Asia, few studies have characterised the spatial variability of Symbiodiniaceae communities amongst reefs and investigated species and environmental effects on community structure and diversity. To profile the endosymbionts associated with reef corals inhabiting Southeast Asia, three common species, Pachyseris speciosa, Pocillopora acuta and Diploastrea heliopora, were sampled from 10 reef sites along the coasts of the Malay Peninsula. The nuclear internal transcribed spacer 2 region of Symbiodiniaceae was targeted for high-throughput sequencing, and the SymPortal framework was used to establish the identities of endosymbiont genera and types. Effects of environmental variables on endosymbiont community structure and diversity were then tested. Analyses revealed that Symbiodiniaceae diversity in this region is higher than previously known. Endosymbiont communities are structured significantly by host species and are relatively invariant in D. heliopora, with P. speciosa associating strongly with both Cladocopium and Durusdinium while P. acuta and D. heliopora are dominated by Durusdinium. Environmental parameters influence Symbiodiniaceae communities and diversity distinctly between host species. In particular, higher sea surface temperature (SST) affects endosymbiont diversity positively for P. acuta while higher SST range affects diversity negatively for P. speciosa and D. heliopora. Overall, this study has uncovered the hidden diversity of Symbiodiniaceae types previously unrecorded in the region and established a baseline for future comparative studies on how Southeast Asian reef corals acclimatise and adapt to changing environments through the natural variation of endosymbiont communities.