Sievert, Stefan M.

AbstractBathymodiolus thermophilus, a mytilid mussel inhabiting the deep-sea hydrothermal vents of the East Pacific Rise, lives in symbiosis with chemosynthetic Gammaproteobacteria within its gills. The intracellular symbiont population synthesizes nutrients for the bivalve host using the reduced sulfur compounds emanating from the vents as energy source. As the symbiont is uncultured, comprehensive and detailed insights into its metabolism and its interactions with the host can only be obtained from culture-independent approaches such as genomics and proteomics. In this study, we report the first draft genome sequence of the sulfur-oxidizing symbiont of B. thermophilus, here tentatively named Candidatus Thioglobus thermophilus. The draft genome (3.1 Mb) harbors 3045 protein-coding genes. It revealed pathways for the use of sulfide and thiosulfate as energy sources and encodes the Calvin-Benson-Bassham cycle for CO2 fixation. Enzymes required for the synthesis of the tricarboxylic acid cycle intermediates oxaloacetate and succinate were absent, suggesting that these intermediates may be substituted by metabolites from external sources. We also detected a repertoire of genes associated with cell surface adhesion, bacteriotoxicity and phage immunity, which may perform symbiosis-specific roles in the B. thermophilus symbiosis.

Summary Studies were conducted in opposing gradients of oxygen and sulfide in microslide capillaries to (i) characterize the chemical microenvironment preferred by Candidatus Arcobacter sulfidicus, a highly motile, sulfur‐oxidizing bacterium that produces sulfur in filamentous form, and (ii) to develop a model describing the mechanism of filamentous‐sulfur formation. The highly motile microorganisms are microaerophilic, with swarms effectively aggregating within oxic‐anoxic interfaces by exhibiting a chemotactic response. The position of the band was found to be largely independent of the sulfide concentration as it always formed at the oxic‐anoxic interface. Flux calculations based on steady state gradients of oxygen and sulfide indicate that sulfide is incompletely oxidized to sulfur, in line with the formation of filamentous sulfur by these organisms. It is proposed that Candidatus Arcobacter sulfidicus effectively competes with other sulfur‐oxidizing bacteria in the environment by being able to tolerate higher concentrations of hydrogen sulfide (1–2 mM) and by possessing the ability to grow at very low oxygen concentrations (1–10 μM). The formation of mat‐like structures from filamentous sulfur appears to be a population mediated effort allowing these organisms to effectively colonize environments characterized by high sulfide, low oxygen and dynamic fluid movement.