Particle- and Light-Mediated Processes Control Seasonal Manganese Oxide Cycling in a Meromictic Pond

Manganese (Mn) oxides are strong oxidants and sorbentsof nutrientsand contaminants, thus their formation mechanisms impact multiplebiogeochemical cycles. Manganese in oxic surface waters is often foundas dissolved Mn species, such as Mn-(II) and Mn-(III)-ligand complexes,rather than Mn oxides. This is believed to be a result of Mn oxidereduction by hydrogen peroxide associated with photolysis of organicmatter and direct organic matter-mediated reduction within sunlitwaters. Nevertheless, Mn oxides can persist in some surface environments,which indicates an incomplete understanding of controls on Mn oxidedistributions. Here, we couple field- and lab-based analyses to exploreMn oxide distributions, Mn oxidation rates, and underlying controlson Mn oxide formation within Siders Pond, a brackish and meromicticpond on Cape Cod (Massachusetts, USA) during the summer and fall of2020. Manganese oxides were observed consistently in sunlit surfacewaters with concentrations declining to undetectable at the base ofthe chemocline. Surface Mn oxide concentrations were highest in latesummer, reaching concentrations of similar to 1 mu M, and lowestin late fall, reaching only similar to 50 nM. Minerals identified usingsynchrotron-based absorbance measurements were structurally similarto delta-MnO2 and feitknechtite. Substantial light-mediatedMn oxidation only took place in live incubations of Siders Pond waterwhile net Mn reduction proceeded in killed incubations. Thus, totalparticle-mediated oxidation by microbes and minerals combined outpacedphotoreduction, leading to net accumulation of Mn oxides within SidersPond. Our results identify important roles for microbial- and mineral-mediatedoxidation in determining Mn oxide distributions within surface watersof a natural setting, a finding that may help explain comparable distributionsin other locations.