title: Endozoicomonas Are Specific, Facultative Symbionts of Sea Squirts.
abstract: 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. Representatives of the Endozoicomonas (Gammaproteobacteria, Hahellaceae) clade were detected in the ascidian species Ascidiella aspersa, Ascidiella scabra, Botryllus schlosseri, Ciona intestinalis, Styela clava, and multiple Ascidia/Ascidiella spp. In total, Endozoicomonas was detected in more than half of all specimens screened, and in 25-100% of the specimens for each species. 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.
authors.affiliation: Department of Bioscience, Center for Geomicrobiology and Section for Microbiology, Aarhus University Aarhus, Denmark.
authors.forename: Lars
authors.initials: L
authors.lastname: Schreiber
authors.affiliation: Department of Bioscience, Center for Geomicrobiology and Section for Microbiology, Aarhus University Aarhus, Denmark.
authors.forename: Kasper U
authors.initials: KU
authors.lastname: Kjeldsen
authors.affiliation: Section of Genetics, Ecology and Evolution, Department of Bioscience, Aarhus University Aarhus, Denmark.
authors.forename: Peter
authors.initials: P
authors.lastname: Funch
authors.affiliation: Department of Bioscience, Center for Geomicrobiology and Section for Microbiology, Aarhus University Aarhus, Denmark.
authors.forename: Jeppe
authors.initials: J
authors.lastname: Jensen
authors.affiliation: Department of Marine Sciences, University of Gothenburg Gothenburg, Sweden.
authors.forename: Matthias
authors.initials: M
authors.lastname: Obst
authors.affiliation: Department of Biology and Marine Biology, Center for Marine Science, University of North Carolina Wilmington Wilmington NC, USA.
authors.forename: Susanna
authors.initials: S
authors.lastname: López-Legentil
authors.affiliation: Department of Bioscience, Center for Geomicrobiology and Section for Microbiology, Aarhus University Aarhus, Denmark.
authors.forename: Andreas
authors.initials: A
authors.lastname: Schramm
keywords: Endozoicomonas; ascidians; marine; mucin; sea squirts; symbiosis; tunicates