Symbiotic unicellular dinoflagellates of the genus Symbiodinium are best known for their association with scleractinian corals. The symbionts are essential to the functioning of the holobiont by providing their hosts with an important part of their energetic demands . A wide range of reef-dwelling organisms, including the orders Scleractinia, Alcyonacea, Actinaria, Hydroida, Milleporina, Stolonifera, Veneroida, Zoanthidae, Corallimorpharia and Foraminiferida [2–4], depend on their endosymbionts for sufficient energy uptake in oligotrophic tropical seas.
The genus Symbiodinium consists of nine broad genetic clades, A-I , that were first revealed with small subunit (SSU) rDNA markers . The more variable internal transcribed spacer unit (ITS) 1 and 2 revealed various genetically and ecologically distinct Symbiodinium types within these clades: e.g. C1, C3, C21 etc. [3, 7–13]. Because of the multi-copy nature and the high intragenomic variance of the ribosomal DNA region, Symbiodinium types can contain co-dominant repeats in their genome that manifest as additional bands in the fingerprint pattern and are referred to as intragenomic variants: e.g. C1a, C1b, C1c etc. . While inter-clade differences are substantial and comparable to order-level differences in non-symbiotic dinoflagellate groups , genetic distances within Symbiodinium clades are generally small. Despite this, closely related Symbiodinium types often relate to distinct ecological diversification and influence functional characteristics such as photosynthetic efficiency, growth or thermal tolerance of the host [7, 15, 16].
The importance of Symbiodinium for the holobionts’ (host plus symbionts) stress response is illustrated by the difference in vulnerability to increasing sea surface temperatures (SST) of same host, different symbiont combinations [15, 17, 18]. While many factors threaten the persistence of coral reefs, increasing SST is regarded as a primary threat to coral reefs by causing disruption of the symbiosis (coral bleaching) and leading to substantial mortality of reef invertebrates over the last two decades [19, 20]. The identification of physiological differences in thermal tolerance and bleaching susceptibility between Symbiodinium types at the ‘type’ level [17, 18] combined with rising SST’s underline the importance of understanding Symbiodinium diversity and this has spurred a broad research interest over the past decade [15, 18, 21–25].
In a recent meta-analysis of Symbiodinium data compiled from literature (Tonk et al. unpub. data), 62 different Symbiodinium types were identified from 207 host species on the Great Barrier Reef (GBR). Due to its many environmentally distinct areas and broad geographic range (spanning approximately 2300 km and including 10% of coral reefs worldwide) the GBR offers a unique opportunity to study patterns of Symbiodinium diversity. With continued efforts to define Symbiodinium communities an increasing number of novel types are described [26, 27]. As information on cnidarian-Symbiodinium symbioses is steadily increasing and expanding its documented geographic extent, it becomes more difficult to compare new with existing Symbiodinium data. Although Symbiodinium sequences are readily available from generic genetic databases, their usefulness is impeded by the lack of a general consensus on the classification of Symbiodinium species. Coupled with the variety of different markers used to identify the genus Symbiodinium (internal transcribed spacer region (ITS) 1 and 2, large ribosomal subunit region (LSU) D1/D2, chloroplast 23S rDNA and psbA minicircle) it becomes more compelling to assimilate this vastly growing knowledge base into a single, searchable resource.