Impact of reactive surfaces on the abiotic reaction between nitrite and ferrous iron and associated nitrogen and oxygen isotope dynamics

Abstract. Anaerobic nitrate-dependent Fe(II) oxidation (NDFeO) is widespread in various aquatic environments, and plays a major role in iron and nitrogen redox dynamics. However, evidence for truly enzymatic, autotrophic NDFeO remains limited, with alternative explanations involving coupling of heterotrophic denitrification with abiotic oxidation of structurally-bound or aqueous Fe(II) by reactive intermediate N species (chemodenitrification). The extent to which chemodenitrification is caused, or enhanced, by ex vivo surface catalytic effects has, so far, not been directly quantified. To determine whether the presence of either a Fe(II)-bearing mineral or dead biomass (DB) catalyses chemodenitrification, two different sets of anoxic batch experiment were conducted: 2 mM Fe(II) was added to a low-phosphate medium, resulting in the precipitation of vivianite (Fe3(PO4)2), to which later 2 mM nitrite (NO2−) were added, with or without an autoclaved cell suspension (~ 1.96 × 108 cells ml−1) of Shewanella oneidensis MR-1. Concentrations of nitrite, nitrous oxide (N2O) and iron (Fe2+, Fetot) were monitored over time to assess the impact of Fe(II) minerals and/or DB as catalysts of chemodenitrification in the two setups. In addition, the natural-abundance isotope ratios of NO2− and N2O (𝛿15N and 𝛿18O) were analysed to constrain associated isotope effects. Up to 90 % of the Fe(II) was oxidized in the presence of DB, while only ~ 65 % were oxidized under mineral-only conditions, suggesting an overall lower reactivity of the mineral-only setup. Similarly, the average NO2− reduction rate (0.004 ± 0.003 mmol L−1 day−1) in the mineral-only experiments was much lower compared to experiments with mineral plus dead biomass (0.053 ± 0.013 mmol L−1 day−1), as was N2O production (204.02 ± 60.29 nmol/L*day). The N2O yield per mole NO2− reduced was higher in the mineral-only setups (4 %) compared to the experiments with dead biomass (1 %), suggesting the catalysis-dependent differential formation of NO. N-NO2− isotope ratio measurements indicated a clear difference between both experimental conditions: in contrast to the marked 15N isotope enrichment during active NO2− reduction (−15eNO2 = +10.3 ‰) observed in the presence of DB, NO2− loss in the mineral-only experiments exhibited only a small N isotope effect (