Genetic diversity of potentially free-living Symbiodinium in the Xisha Islands, South China Sea: Implications for the resilience of coral reefs

Abstract

Free-living Symbiodinium have been the focus of considerable attention among coral reef researchers because they represent a source of symbionts for both recently recruited corals and adult corals that are subject to environmental stress (e.g. climate change). We evaluated the genetic diversity of free-living Symbiodinium in seawater collected from the Xisha Islands in the South China Sea using sequence analysis of the hypervariable region of the chloroplast 23S. We sequenced a total of 267 cloned gene fragments representing Symbiodinium in clades A, B, C, D, F, and G, of which 34 were distinct types. The diversity of free-living Symbiodinium types was high at all sites. The majority of Symbiodinium sequences belonged to clade C. There was no significant difference in Symbiodinium sequence types among sites or depths. Our results highlight the high diversity of free-living Symbiodinium pools in the coral reef environment and provide a foundation for future studies of the spatiotemporal distribution of Symbiodinium. Such information is critical to understand and predict the influence of symbiotic dinoflagellates on the adaptation of their host to environmental changes.

Genetic diversity of free-living Symbiodinium in the Caribbean: the importance of habitats and seasons

“…It is important to note that some of the Symbiodinium types detected in this and previous studies may not necessarily be ''free-living'' but transient since they could be recently released from corals. Adding to the contribution of previous studies, we revealed that not only the common clades of Symbiodinium A, B, C, and to a lesser extent G are found in the free-living environment, but also discovered, for the first time for a Caribbean reef environment, the presence of Symbiodinium clades F and H. The detection of members of clades F and H as free-living (Zhou et al 2012;Huang et al 2013; this study) thus implies a pool of symbionts that might be shared among hosts from different phyla in the Caribbean. The discovery of new Symbiodinium clades in the environment in the present study highlights the value of using culture-independent techniques to assess diversity, compared to culturing-based surveys conducted in the past that have underestimated the ''real'' diversity in the reef environment (Coffroth et al 2006).…”

“…However, over the last 30 yr, we have learned so little of the ecological aspects of this important phase of the Symbiodinium life cycle. Having said that, various studies have documented, through time snapshot explorations, the composition of Symbiodinium in various habitats within the reef (Carlos et al 1999;Gou et al 2003;Coffroth et al 2006;Thornhill et al 2006;Koike et al 2007;Hirose et al 2008;Littman et al 2008;Manning and Gates 2008;Porto et al 2008;Adams et al 2009;Hansen and Daugbjerg 2009;Reimer et al 2010;Venera-Ponton et al 2010;Takabayashi et al 2012;Zhou et al 2012;Huang et al 2013;Yamashita and Koike 2013;Jeong et al 2014). Nevertheless, none of these studies, except one ) over a timeframe of 2 d, used systematic sampling approaches to understand the spatial and temporal distribution of Symbiodinium in the free-living environment.…”

“…Despite reports of Symbiodinium-like DNA in environments outside of hosts (hereafter referred to as free-living or environmental Symbiodinium) dating back to the late 1970s and early 1980s (Loeblich and Sherley 1979;Chang 1983), freeliving Symbiodinium did not receive much attention until the late 1990s. The first molecular identification of freeliving Symbiodinium in 1999 (Carlos et al 1999) was followed by investigations of their occurrence in the water column, sediments, and on macroalgae (Gou et al 2003;Coffroth et al 2006;Thornhill et al 2006;Koike et al 2007;Hirose et al 2008;Littman et al 2008;Manning and Gates 2008;Porto et al 2008;Adams et al 2009;Hansen and Daugbjerg 2009;Reimer et al 2010;Venera-Ponton et al 2010;Takabayashi et al 2012;Zhou et al 2012;Huang et al 2013;Yamashita and Koike 2013;Jeong et al 2014). These studies found that freeliving Symbiodinium occurred in diverse habitats; however, they did not address how Symbiodinium assemblages are structured across habitats.…”