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

Abstract

Manganese (Mn) oxides are strong oxidants and sorbents of nutrients and contaminants, thus their formation mechanisms impact multiple biogeochemical cycles. Manganese in oxic surface waters is often found as 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 oxide reduction by hydrogen peroxide associated with photolysis of organic matter and direct organic matter-mediated reduction within sunlit waters. Nevertheless, Mn oxides can persist in some surface environments, which indicates an incomplete understanding of controls on Mn oxide distributions. Here, we couple field- and lab-based analyses to explore Mn oxide distributions, Mn oxidation rates, and underlying controls on Mn oxide formation within Siders Pond, a brackish and meromictic pond on Cape Cod (Massachusetts, USA) during the summer and fall of 2020. Manganese oxides were observed consistently in sunlit surface waters with concentrations declining to undetectable at the base of the chemocline. Surface Mn oxide concentrations were highest in late summer, reaching concentrations of ∼1 μM, and lowest in late fall, reaching only ∼50 nM. Minerals identified using synchrotron-based absorbance measurements were structurally similar to δ-MnO2 and feitknechtite. Substantial light-mediated Mn oxidation only took place in live incubations of Siders Pond water while net Mn reduction proceeded in killed incubations. Thus, total particle-mediated oxidation by microbes and minerals combined outpaced photoreduction, leading to net accumulation of Mn oxides within Siders Pond. Our results identify important roles for microbial- and mineral-mediated oxidation in determining Mn oxide distributions within surface waters of a natural setting, a finding that may help explain comparable distributions in other locations.