Ascidians are marine filter feeders and harbor diverse microbiota that can exhibit a high degree of host-specificity. Pharyngeal samples of Scandinavian and Mediterranean ascidians were screened for consistently associated bacteria by culture-dependent and -independent approaches. The retrieved Endozoicomonas 16S rRNA gene sequences formed an ascidian-specific subclade, whose members were detected by fluorescence in situ hybridization FISH as extracellular microcolonies in the pharynx.
Two strains of the ascidian-specific Endozoicomonas subclade were isolated in pure culture and characterized. Both strains are chemoorganoheterotrophs and grow on mucin a mucus glycoprotein. The strains tested negative for cytotoxic or antibacterial activity. Based on these observations, we propose ascidian-associated Endozoicomonas to be commensals, living off the mucus continuously secreted into the pharynx.
Members of the ascidian-specific Endozoicomonas subclade were also detected in seawater from the Scandinavian sampling site, which suggests acquisition of the symbionts by horizontal transmission.
The combined results indicate a host-specific, yet facultative symbiosis between ascidians and Endozoicomonas. Several species of ascidians Tunicata, Ascidiacea , commonly referred to as sea squirts, have been shown to produce bioactive secondary metabolites Faulkner, , which are hypothesized to protect from biofouling and predation e. At least in the case of the ascidian Lissoclinum patella , it was shown that these bioactive compounds are in fact produced by its bacterial symbiont Prochloron didemni Lewin, ; Schmidt et al.
The detection of defensive microbial symbionts in other marine invertebrates, such as corals, sponges and bryozoans Faulkner, , as well as the potential to use the produced bioactive compounds as anticancer drugs Simmons et al. The known diversity of ascidian-associated bacteria comprises members of several phyla including Actinobacteria, Bacteroidetes, Cyanobacteria, and Firmicutes, as well as Alpha-, Gamma-, and Delta-proteobacteria Schuett et al.
