Reductive dissolution of pyrite by methanogens and its physiological and ecological consequences

Abstract

All life requires iron and sulfur, in particular, for use in metallocofactors of enzymes that catalyze chemistry that is essential for metabolism. In anaerobic environments, iron and sulfur are typically found in their reduced forms (ferrous iron and sulfide, respectively) that will react and form insoluble iron-sulfide minerals, such as pyrite. A consequence of this is that either iron or sulfur are typically limiting in solution, raising the question as to how anaerobes acquire these essential elements under such conditions. Here, it is demonstrated that anaerobic methanogens can reduce pyrite to release iron and sulfur that are assimilated by the cells to meet biosynthetic demands. Through a combination of growth experiments, -omics analyses, and microscopy, a model for the reductive dissolution of pyrite was established. In this model, direct contact between cells and pyrite is required for mineral reduction. When in direct contact, pyrite is reduced and sulfide is released, leaving a pyrrhotite secondary mineral on the surface. Iron solubilized from pyrrhotite reacts with sulfide in the growth medium to yield aqueous iron sulfur clusters that are assimilated by cells. Cells grown on pyrite exhibit phenotypic differences in comparison to traditionally grown cells provided with ferrous iron and sulfide. At a morphological level, pyrite-grown cells were 33% smaller than traditionally grown cells and hyperaccumulated iron as an intracellular mineral. When grown under nitrogen-fixing conditions, cells grown on pyrite had higher cell densities, growth yields, and growth rates in comparison to traditionally grown cells. Molybdate transporters were down expressed in pyrite-grown, nitrogen-fixing cells relative to traditionally-grown cells, consistent with sulfide limiting molybdate availability in the latter condition. Moreover, pyrite-grown cells could fix nitrogen at ~100-fold lower molybdenum concentration than traditionally grown cells, indicating differences in molybdenum requirements based on the iron and sulfur source provided. Together, these data highlight that in contemporary anoxic environments, iron-sulfide minerals are an important and even preferred source of iron and sulfur for methanogens. These findings provide insight into how ancient methanogens could have acquired iron and sulfur on an anoxic early Earth when one or both of these elements were likely only available as metal sulfides.