Metal sulfides form key components of metalloenzyme active sites that function in the conversion of light or electrical energy to potential energy in the form of chemical bonds. The need to understand the underlying physical and chemical principles that govern how microbes capture, convert, and store energy via metalloenzymes (electrocatalysts) has led to broad scientific interest in their biosynthesis. Methanogens have served as model systems for understanding electrocatalytic conversion of CO2 to CH4, N2 to NH3, and H2 to H+, with key electron transfers taking place at metal sulfide metalloclusters containing Fe, Ni, Mo, and/or Co. As such, methanogens require copious amounts of these metals, as well as sulfide, for biosynthesis of active site metalloclusters and other cofactors. Paradoxically, Fe, Ni, Mo, and Co react with sulfide to form insoluble sulfide minerals such as pyrite (FeS2) that are thought to unavailable to anaerobic methanogens. If follows that FeS2 is the most abundant sulfide mineral in Earth’s crust and is a major reservoir of trace metals (e.g., Ni, Mo, and Co) of critical importance to national defense, energy production and storage, and nearly every sector of the economy. The identification of enzymes and pathways that allow methanogens to bio-mine Fe, Ni, Mo, Co and S from pyrite mineral sources would be a significant advancement allowing for the cost-effective recovery and/or conversion of this raw substrate into metal-based catalysts for electrochemical bioenergy generation.
Data from the Phase 1 EPSCoR project indicate that several model methanogens can convert Earth abundant and low-cost FeS2 into molecular biocatalysts that interconvert CO2 to CH4, N2 to NH3, and H2 to H+. Results demonstrate that FeS2 is reductively dissolved to yield mineral surface associated pyrrhotite (Fe1-xS), that equilibrates to form aqueous Fe(II) and HS- and/or iron monosulfide (FeSaq) clusters that are assimilated. Differential transcriptomic and proteomic experiments identified a suite of proteins putatively involved in FeS2 reduction and Fe/S acquisition, trafficking, and storage. Our Phase 2 ESPCoR renewal will build on these paradigm challenging insights to forge a new understanding of the mechanisms used by methanogens to reductively dissolve FeS2 and assimilate FeSaq to meet biosynthetic demands. To maximize impact and to improve mechanistic understanding at the interface of the physical bio- and geo-sciences, the effort will combine physiological, molecular, biochemical, computational, and spectroscopic approaches with a suite of cutting edge physical science techniques to characterize the mechanism(s) of enzymatic FeS2 reduction as well as proteins involved in Fe/S acquisition, trafficking, and storage. Data from the Phase 1 award also suggest that methanogens can bio-mine Ni and Mo from FeS2 to meet biosynthetic demands for these elements. This motivates additional experiments to characterize the mechanisms used by methanogens to acquire Ni, Mo, and other trace metals (i.e., Co) from mineral sulfides. Further, using state of the art imaging, spectroscopic, and computational approaches, we will probe the reaction mechanisms, rates, and chemical transformations at the surface of FeS2 during reduction, focusing on molecular interactions at the mineral-water-cell interface.
The research team includes expertise in physiology, transcriptomics, genomics, and genetics (Boyd, Spietz), proteomics and protein structural dynamics (Bothner), Fe and S bioinorganic chemistry (Broderick, Shepard), mechanistic enzymology (DuBois), computational and synchrotron-based surface science (Szilagyi), and mineralogy and thermodynamics (Mogk). They plan to integrate this expertise and uniquely focus it to create a blueprint for understanding and improving the synthesis of metalloclusters in model electrocatalytic enzyme systems. Further, this work seeks to establish new approaches to bio-mine trace metals from pyritic ore, with future application to rare Earth elements of national strategic importance. Energy research and mitigating pollutants from mining operations are major focal points of Montana’s EPSCoR initiative. As such, this work aligns with the goals of Montana’s EPSCoR jurisdiction and will i) contribute to building Montana research capacity and competitiveness, ii) foster new research initiatives that will develop human and technical capabilities at our institution, and iii) create new collaborations and opportunities for funding among team members.
