An isotopic study of abiotic nitrite oxidation by ligand-bound manganese (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 multielement 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 O-18-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. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

Redox transformations of nitrogen are critical for determining productivity and nitrogen availability in ecosystems. Environments rich in redox-active elements like manganese may facilitate the oxidation of nitrite, with the additional oxygen atom derived from water.