Low Symbiodiniaceae diversity in a turbid marginal reef environment

The coastal waters of Singapore support coral reefs that are biodiverse but characterized by high turbidity and sedimentation. Here, we used internal transcribed spacer two (ITS2) amplicon sequencing to investigate the Symbiodiniaceae communities associated with this marginal reef system, as turbid reefs may serve as potential refugia from future thermal stress. Using the analytical framework SymPortal, we identified a predominance of Cladocopium among the five coral species studied across six reef sites. Durusdinium was present in comparatively lower abundances and was composed of multiple Durusdinium trenchii strains. In contrast to other marginal environments, the Cladocopium communities exhibited low diversity and lacked the host-specificity of strains reported elsewhere. Nevertheless, we identified a site-specific strain across three species, which was supported by sequencing of the non-coding region of the psbA minicircle (psbA^ncr). The overall low diversity of the symbiont communities suggests that, although Singapore’s reefs may provide habitat for a diverse coral assemblage, the strong selective pressure exerted by the prevalent turbidity likely limits the diversity of the associated symbiont community.