Thiosocius teredinicola gen. nov., sp. nov., a sulfur-oxidizing chemolithoautotrophic endosymbiont cultivated from the gills of the giant shipworm, Kuphus polythalamius

Abstract

A chemolithoautotrophic sulfur-oxidizing, diazotrophic, facultatively heterotrophic, endosymbiotic bacterium, designated as strain 2141T, was isolated from the gills of the giant shipworm Kuphus polythalamius (Teredinidae: Bivalvia). Based on its 16S rRNA sequence, the endosymbiont falls within a clade that includes the as-yet-uncultivated thioautotrophic symbionts of a marine ciliate and hydrothermal vent gastropods, uncultivated marine sediment bacteria, and a free-living sulfur-oxidizing bacterium ODIII6, all of which belong to the Gammaproteobacteria. The endosymbiont is Gram-negative, rod-shaped and has a single polar flagellum when grown in culture. This bacterium can be grown chemolithoautotrophically on a chemically defined medium supplemented with either hydrogen sulfide, thiosulfate, tetrathionate or elemental sulfur. The closed-circular genome has a DNA G+C content of 60.1 mol% and is 4.79 Mbp in size with a large nitrogenase cluster spanning nearly 40 kbp. The diazotrophic capability was confirmed by growing the strain on chemolithoautotrophic thiosulfate-based medium without a combined source of fixed nitrogen. The bacterium is also capable of heterotrophic growth on organic acids such as acetate and propionate. The pH, temperature and salinity optima for chemolithoautotrophic growth on thiosulfate were found to be 8.5, 34 °C and 0.2 M NaCl, respectively. To our knowledge, this is the first report of pure culture of a thioautotrophic animal symbiont. The type strain of Thiosocius teredinicola is PMS-2141T.STBD.0c.01a^T (=DSM 108030^T).

Keywords: Bivalve; Kuphus polythalamia; Thioautotrophic symbiont; chemolithoautotrophic symbiosis; giant shipworm; sulfur-oxidizing chemosymbiosis.

Fig. 1.

Phylogeny of Thiosocius teredinicola strains and other related symbionts and free-living bacteria based on Bayesian inference analysis of 16S rRNA sequences. The tree presented is an excerpt of a Bayesian tree (Fig. S1) reconstructed using 1172 nucleotide positions employing GTR+I+Γ as the substitution model in MrBayes version 3.2.6. Chain length was set to 4 million, subsampling every 2000 generations and discarding the first 20 % of the analytical results as burn-in. Posterior probability values are indicated for each node. The scale bar represents nucleotide substitution rate per site. Closed circles, bacterial isolates; open circles, uncultivated symbionts; asterisks, environmental clones; closed square, sequence recovered from K. polythalamius gill metagenome [9].

Fig. 2.

Transmission electron micrographs of Thiosocius teredinicola strain 2141T. (a) Longitudinal section showing nucleoid (n), sulfur globule (sg), carboxysomes (cs), outer membrane (om), and inner membrane (im). (b) Negatively-stained cell showing the presence of a single polar flagellum. Cells were grown chemolithoautotrophically in STBA liquid culture medium. Bars, 500 nm.