Biogeochemistry of Hydrothermal Chimney Environments: Continuous-Flow Experiments at in situ Temperature and Pressure

Recent interest in the existence of a subsurface microbial biosphere at hydrothermal vents has resulted in a plethora of new questions that might best be answered using interdisciplinary techniques that combine geochemistry, microbial ecology, and molecular biology. Ideally, such studies will quantitatively address issues concerning what organisms exist in the subsurface, what metabolisms are sustained in the hydrothermal environment, and what effects these active organisms might have on the nearby fluid and rock. We present a new experimental approach to studying these questions that enables monitoring of an active hydrothermal community of microbes in the presence of chimney material at in situ temperature and pressure. This apparatus is designed as a continuous-flow reactor from which fluid samples can be extracted during the course of the experiment to measure chemistry and biomass, and at the termination of an experiment solids can be extracted for analysis of mineralogical changes and microbial identification. Results of a series of experiments conducted using hydrothermal chimney material (solids and microbial community) collected from 21° N and 9° N East Pacific Rise are presented. At 70° C, a seawater-based fluid with additional NO₃^-, CO₂(aq), and H₂(aq) was reacted with chimney material from L vent, 9° N EPR. The fluid lost significant NO₃^-, PO₄^3-, and gained SO₄^2- even after accounting for the contribution from anhydrite dissolution. No significant sulfide or iron was observed in the fluid. Analysis of the DNA extracted from the solids at the termination of the experiment using partial 16S-rRNA sequence data revealed that the dominant bacteria were S-oxidizing tube worm endosymbionts, a S/NO₃^- reducing member of the Deferribacter genus, and a H₂-oxidizing/NO₃^- reducing strain of Aquifex. Mineral analysis from before and after the experiment indicates the loss of pyrrhotite (FeS) and anhydrite (CaSO₄), and the gain of an Fe-oxide phase tentatively identified using magnetic remnance and Mossbauer as goethite (FeOOH), responsible for the minimal Fe in solution. An abiotic control experiment was conducted under the same conditions, where the chimney material was first freeze-dried under vacuum, then autoclaved to sterilize without producing any artificial mineral changes. This experiment shows no loss of NO₃^-, PO₄^3-, no additional SO₄^2- gain after anhydrite dissolution, and a steadily increasing dissolved iron concentration, implying the net dissolution of pyrrhotite. Results from additional experiments testing different fluid chemistry, temperature, and source chimney similarly show linkages between the measured fluid chemistry, the identity of the dominant organisms in the experiment, and bulk changes in the mineralogy.