An isotopic investigation of nitrite oxidation by ligand-bound Mn(III)

Redox transformations of nitrogen (N) play a critical role in determining its speciation and biological availability, thus defining the magnitude and extent of productivity in many ecosystems. A range of important nitrogen transformations often co-occur in regions hosting other redox-active elements, including sulfur, iron, and manganese (Mn), especially along sharp redox gradients within aquatic sediments. This proximity in “redox real estate” produces conditions under which multi-element interactions and coupled cycling are thermodynamically favored. While previous work has reported anoxic nitrification linked to the presence of manganese (Mn) oxides in sediments, a clear connection between the cycling of Mn and N has remained elusive. Soluble Mn(III), which is stabilized via ligand-complexation, has recently been shown to represent the dominant dissolved Mn species in many environments. Here, we examined the reactivity of ligand-stabilized Mn(III) with nitrite, using natural abundance stable nitrogen and oxygen isotopes to explore reaction dynamics under a range of conditions. Oxidation of nitrite to nitrate by Mn(III)-pyrophosphate proceeded abiotically under both oxygen replete and nitrogen-purged conditions. Kinetics and isotope systematics of this reaction were measured over a range of pH (5–8), with reaction rates decreasing with increasing pH. Under all treatments, an inverse kinetic isotope effect of −19.9 ± 0.7‰ was observed for N, remarkably similar to previously documented fractionation by nitrite-oxidizing organisms. Experiments using ¹⁸O-labeled water confirmed that the source of the additional oxygen atom was from water. These findings suggest that nitrite oxidation in environments hosting abundant ligand-bound Mn(III), including porewaters, estuaries, and coastal waters, may be facilitated in part by abiotic reactions with Mn, even under functionally anoxic conditions.