Diffusive and advective fluid flow shapes chemoautotrophic bacterial communities and sulfur mineralogy in hydrothermal sediments off Milos

Hydrothermal fluid flow not only shapes mineral deposition on the ocean floor but also creates ecological niches by altering temperature and energy availability. In these niches, microbial life thrives and has an additional, often unrecognized impact on mineral formation. In a newly discovered vent field in medium depths off Milos, Greece, we show how contrasting hydrothermal regimes host fundamentally different bacterial metabolisms. Diffusive flow fosters acidic, sulfate-rich conditions that promote kaolinization and pyrite formation. Fatty acid {delta}13C values down to -39{per thousand} indicate acetyl-CoA-based sulfate reduction as the main metabolism, likely contributing to pyrite formation. In contrast, vigorous venting delivers hot, acidic, carbon-rich, and sulfate-depleted fluids that sustain the activity of sulfide-oxidizing chemoautotrophs. Fatty acid {delta}13C values of up to -4{per thousand} and elemental sulfur accumulation provide evidence of their activity. Without such hydrothermal influence, little microbial activity and only quartz-rich sediments can be observed. Combined multivariate lipid analyses highlight Eh as the strongest environmental control, with increasing average chain length of fatty acids as an adaptation to (hydro-)thermal stress. These results illustrate how different types of fluid flow influence the activity of chemoautotrophic bacterial communities that are involved in the formation of characteristic sulfur minerals in hydrothermal environments.