Fluid chemistry evolution in deep-sea hydrothermal environments: Unraveling mineral-fluid-microorganism interactions through continuous culture experiment

Abstract

This study investigates minerals and microorganisms effects on fluid chemistry through a continuous enrichment culture in a gas-lift bioreactor during the MoMARsat’19 cruise. A sulfate-based chimney and buoyant hydrothermal fluid, both collected in situ at the Aisics vent of the Lucky Strike hydrothermal field, were incubated for 18 days under physico-chemical conditions simulating those of diffuse hydrothermal vents. We present the evolution of elemental and Sr, and Li isotopic compositions of the bioreactor fluid, alongside Bacteria and Archaea diversity, and analyze the mineral saturation state of the fluid through geochemical modeling. Our results show that the microbial diversity in the bioreactor reflects that of the sulfate-based chimney. During the initial 168 h, minerals precipitation/dissolution primarily controlled the elemental and Sr isotopic composition of the fluid. From 168 h to 264 h, sulfate-reducing Archaea (Archaeoglobi) disappeared in favor of sulfur-reducing Archaea (Thermoprotei and Thermococci). This shift coincides with a drastic increase in trace element concentrations and less radiogenic ⁸⁷Sr/⁸⁶Sr ratios, suggesting a possible microbial influence on the fluid. From 264 h onwards, with stable sulfur-reducing archaeal diversity, mineral saturation state primarily controls the elemental composition of the fluid. However, the observed increase in the ⁸⁷Sr/⁸⁶Sr ratio and δ⁷Li correlates with changes in bacterial diversity, notably an increase in Deinococci abundance. This study reveals that in a bioreactor simulating diffuse vent environments related to the sulfur cycle: (i) both microorganism and mineral influence fluid chemistry over time, (ii) shift in microbial diversity appear to affect trace metal concentrations and isotopic signatures, and (iii) the ⁸⁷Sr/⁸⁶Sr ratio serves as a tracer for mineral-fluid interactions and may be a tracer for microorganism-fluid interactions.