Microbial Sulfate Reduction in Basaltic Fluids on the Flanks of Juan de Fuca Ridge

Microbial sulfate reduction was characterized in low-temperature hydrothermal fluids from the upper ocean crust on the Juan de Fuca Ridge flanks. Basaltic fluids were collected from two CORK observatories situated at increasing distance from the ridge axis. Microbial sulfate reduction was detected in both young (CORK 1025C) and aged (CORK 1301A) basaltic porewaters. Sulfate reduction rates (SRR) in fluids collected from the young upper basement varied between 0.024-0.085 nmol cm-1 d-1 at in situ temperatures of 38°C, whereas SRR in aged crustal porewaters ranged between 0.007-0.009 nmol cm-1 d-1 at in situ temperatures of 65 °C. Further inspection of the temperature dependency of sulfate reduction at +10°C to +86°C showed that the maximum rates of sulfate reduction in fluids collected from 1025C, 0.25 nmol cm-3 d-1, were measured at temperatures of 44°C within the mesophilic range. Maximum SRR in aged crustal porewaters from 1301A however, 0.023 nmol cm-3 d-1, were observed at 77°C well in the thermophilic range. These results suggest different physiological responses of dominant microorganisms in aging fluids subjected to higher temperatures in contrast to young and colder fluids. Amendments of short-chain volatile fatty acids and H2 to aged fluids stimulated SRR of certain sulfate reducers, particularly at high temperatures. Although there is strong molecular and chemical evidence for the presence of active SRM in deep subseafloor basaltic fluids, a quantitative assessment of rates of sulfate reduction is currently missing and the nature of the electron donor remains uncertain. In the present study, we explore the temperature characteristics and metabolic capacities of SRM in basaltic fluids along the flanks of the Juan de Fuca Ridge.