Biogeochemistry of seafloor hydrothermal vent systems: An experimental study conducted at in situ conditions

Experimental and theoretical observations constrain the redox chemistry in shallow subsurface hydrothermal systems and examine the interactions between autotrophic microorganisms and the geochemical environment. Fundamental metabolic reactions involving H₂(aq), H₂S(aq), CH₄(aq), acetate and Fe++ fail to equilibrate over short times scales (hours to days) in experiments at 100°C even in the presence of Fe-oxides and sulfides. These observations are the foundation of a thermodynamic model depicting mixing of hydrothermal fluids with seawater in the presence of a mineral assemblage, analogous to the seafloor subsurface environment. Because of the disequilibrium of key redox reactions, available energy is predicted for many aqueous metabolic reactions below 150°C. A new flow-through reactor was designed to investigate the geochemical impact of microorganisms at hydrothermal pressure and temperature. Several experiments using natural hydrothermal chimney material from 9°N East Pacific Rise as a source of mineral substrate and microorganisms confirm the influence of metabolism on fluid chemistry. The conditions of an abiotic control experiment (70°C, 250 bars) result in anhydrite dissolution and pyrrhotite conversion to elemental sulfur. Growth of a sulfur-utilizing community of bacteria, including a previously uncultured strain of Desulfurobacterium, however, results in the microbially mediated reaction of S° to Fe-oxide. Molecular analysis revealed that the dominant metabolic types enriched under NO₃−-enrichment were Aquificales, while those predominant under S-enrichment were SO ₄= reducing Thermodesulfobacterium. Thus, variable fluid chemistry enriches unique portions of the natural population. This leads to net S-oxidation during NO₃−-enrichment, but net S-reduction in the absence of NO₃−. Another large subsurface environment that may potentially harbor a biosphere is represented in the Bent Hill Massive Sulfide deposit at Middle Valley on the Juan de Fuca Ridge as cooler fluids circulate through the mound. While it is unknown if the Bent Hill deposit contains an active subsurface biosphere, a geochemical study of the trace element distribution of natural sulfides was undertaken using laser-ablation ICP-MS. It is as yet unknown, however, if these observations represent true biosignatures or an abiotic baseline with which other deposits may be compared.