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Project Progress

Subglacial sulfate production in excess of the oxidative (O₂) potential of incoming waters, and in the absence of sulfate bearing minerals, suggests a mechanism for anaerobic pyrite (FeS₂) oxidation in circumneutral subglacial systems (1). Ongoing investigations at RG provide new evidence supporting the role of FeS₂ -driven chemosynthesis as a mechanism for primary production in cold-dark environments, and a solution to the subglacial sulfate mass balance discrepancy. Novel work builds off findings by Mitchell et al. (2) that reveal microbial communities associated with FeS₂ incubated in the RG subglacial outflow are most closely related to those extracted from RG subglacial sediments. Furthermore, the biomass associated with each incubated mineral phase (i.e. pyrite, olivine, quartz, calcite, magnetite and hematite) increases as a function of iron weight percent (2). Together these results suggest FeS₂-based chemolithotrophy may play an important role in shaping and fueling the RG subglacial community (Fig. 2). The biogeochemical and geobiochemical mechanisms underlying FeS₂-driven chemosynthesis and its influence on aerobic and anaerobic FeS₂ oxidation rates under circumneutral pH conditions and near-freezing temperatures remain enigmatic.

Progress to date supplies evidence associating FeS₂ oxidation with chemosynthetic primary productivity and carbonate dissolution in subglacial sediments sampled from RG. Quantification and sequencing of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) transcripts suggests that populations closely affiliated with Sideroxydans lithoautotrophicus, an iron-sulfide oxidizing autotrophic bacterium, are abundant constituents of microbial communities at RG. Microcosm experiments indicate sulfate production during biological assimilation of radiolabeled bicarbonate. Geochemical analyses of subglacial meltwater indicate that increases in sulfate are associated with increased calcite and dolomite dissolution. Collectively, these data suggest a role for biological FeS₂ oxidation in driving primary productivity and mineral dissolution in a subglacial environment and provide the first rate-estimate for bicarbonate assimilation in these ecosystems. Evidence for lithotrophic primary production in this contemporary subglacial environment provides a plausible mechanism to explain how subglacial communities could be sustained in near isolation from the atmosphere during glacial-interglacial cycles (Boyd et al., 2014). This work was published in Applied and Environmental Microbiology by Boyd et al., in 2014.

Efforts were taken to isolate a relevant chemolithoautotroph from RG to further elucidate the biogeochemical and geobiochemical mechanisms of chemosynthetic primary production in the subglacial environment. These efforts yielded an environmentally relevant RG isolate most closely related to the chemolithoautotroph Thiobacillus denitrificans. Isolate growth tested under both aerobic and anaerobic conditions reveal a highly adapted bacterium capable of chemosynthesis through the oxidation of aqueous sulfur intermediates and framboidal FeS₂ coupled to O₂, NO₃- and NO₂- reduction at 4 oC.

Abiotic, aerobic FeS₂ oxidation at circumneutral pH proceeds through an initial, incomplete oxidation step that produces aqueous thiosulfate (S[~2~O₃²), a metastable reduced sulfur intermediate. We tested the ability of our RG isolate to grow on aqueous S₂O₃^2- in both aerobic and anaerobic medium at 4 oC, conditions relevant to the subglacial system. Growth curves based on geochemical data exhibit the ability of this isolate to couple S₂O₃^2- oxidation with O₂, and NO₃- and NO₂- reduction in aerobic and anaerobic systems, respectively. Redox transformations do not occur in parallel abiotic assays, supporting the role of this isolate in driving S₂O₃^2- oxidation and thus, sulfate production. Cell count data taken alongside geochemical data confirms the production of new cells under all conditions.

Isolate chemosynthesis was also sustained via oxidation of framboidal FeS₂ in circumneutral, aerobic and anaerobic media using O₂ and NO₃- as oxidants, respectively, at 15 oC. Furthermore these chemosynthetic pathways are capable of enhancing the rate of framboidal FeS₂ oxidation relative to abiotic processes. This is most noted in the anaerobic systems where little to no abiotic FeS₂ oxidation occurs. Results from this work provide evidence for chemosynthesis-driven anaerobic FeS₂ oxidation in the subglacial environment. Geochemically, both anaerobic FeS₂ and S₂O₃^2- -driven metabolisms exhibited by the RG isolate are capable of satisfying the mass balance discrepancy on sulfate production in the subglacial environment.

Energy requirements for cell production at 4 oC and 15 oC, on aqueous thiosulfate and framboidal FeS₂, respectively, will be determined based on cell count data and the thermodynamics of substrate oxidation in aerobic and anaerobic assays. Broad implications include the energetics of cold adapted, FeS₂-based chemosynthetic life and its potential role as a primary producer in cold-dark environments. Ongoing work includes evaluating the geobiochemical mechanisms of framboidal FeS₂ oxidation under both aerobic and anaerobic conditions. This will be achieved through the use of a semi-permeable membrane to create a physical barrier that prevents cell-mineral contact.

1. M. Tranter et al., Hydrol. Process. 16: 959 (2002).
2. A. C. Mitchell et al., Geol., 41:855 (2013).
3. T.L. Hamilton et al., ISME J, 7: 1402 (2013).