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Project Progress

During the course of the previous year, one retentostat experiment has been successfully completed. A model subsurface microorganism, the thermophilic sulfate-reducing bacterium Desulfotomaculum putei was grown until it reached maintenance metabolism. Sulfur isotopic analyses from a previous rentostat run were compared with isotopic fractionation during exponential phase growth of the same organism in batch culture experiments, where the fractionation of stable sulfur isotopes was no greater than 9.5‰. It was hypothesized that isotopic fractionation in the retentostat during nutritional deprivation would exceed those of batch cultures due to the increased importance of intermediate-valence sulfur species (eg. thiosulfate, trithionate) in the final step reduction of sulfite to sulfide. Enhanced fractionation was also likely to occur as a result of reversibility of the entire metabolic pathway. The current observed maximum fractionation in the retentostat experiments is about 13‰. Sulfur isotopic analyses during this year’s retentostat experiment where thiosulfate was prevalent, are currently underway at the Stable Isotope Laboratory at Indiana University. Another retentostat experiment is currently in progress at Princeton University and the temperature will be varied to see if the isotopic fractionation of sulfur is affected.

A manuscript describing the remote detection of life during a simulated Martian drilling mission (MARTE) is currently in a second round of editorial review. A draft of a manuscript describing the microbial community of a subsurface, sulfate reducing environment in South Africa has been completed and is being circulated to co-authors for comments.