There is no mineral that has played a larger role in the transformation of Earth than manganese: the oxidation of Earth was initiated and depends on Mn-based biochemistry. Once the oxygen-evolving biochemistry of photosynthesis overtook the planet manganese and oxygen chemistries became important for life on Earth. These changes in the overall redox state of the ocean would dramatically influence trace metal chemistry and bioavailability, which in turn affect important biogeochemical cycles, including those of carbon and nitrogen. In addition, manganese is used as a redox-proxy for the presence of oxygen on Earth, and other planets. In order to determine manganese speciation in a geological setting, the nature and rates of biotic and abiotic transformations need to be better understood. Microbes drive the rates of manganese transformations in the environment. In many environments, microbial mandanese reduction, used as an anaerobic respiratory strategy, can account for the majority of carbon mineralization. However, we know little about the extent of microbial manganese reduction, and the identity of manganese reducing microbes.
In the last 10 years the field of Geochemistry advanced impressively to overthrow a long-held dogma that the cycling of manganese occurred only among soluble Mn(II) and solid Mn(III,IV), and demonstrated that soluble Mn(III)-ligands make up the majority of soluble Mn in the environment in anoxic or low-oxigen environments. Mn(III)-ligands can also be respired by microbes. While much of the research involving Mn reduction has centered on solid phases, I am focused on understanding the role Mn(III) as an anaerobic electron acceptor, which microbes can utilize it, and what is the biochemical mechanism to do so.