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https://doi.org/10.5194/bg-2020-73
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-73
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 17 Mar 2020

Submitted as: research article | 17 Mar 2020

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This preprint is currently under review for the journal BG.

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

Anna-Neva Visser1,4, Scott D. Wankel2, Pascal A. Niklaus3, James M. Byrne4, Andreas A. Kappler4, and Moritz F. Lehmann1 Anna-Neva Visser et al.
  • 1Department of Environmental Sciences, Basel University, Bernoullistrasse 30, 4056 Basel, Switzerland
  • 2Woods Hole Oceanographic Institution, Woods Hole, 360 Woods Hole Rd, MA 02543, USA
  • 3Department of Evolutionary Biology and Environmental Studies, Universityof Z├╝rich, Winterthurerstrasse 190, 8057 Z├╝rich, Switzerland
  • 4Department of Geosciences, T├╝bingen University, H├Âlderlinstrasse 12, 72074 T├╝bingen, Germany

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 ├Ś 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 (­ŁŤ┐15N and ­ŁŤ┐18O) 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 (−15╬ÁNO2 = +10.3 ÔÇ░) observed in the presence of DB, NO2 loss in the mineral-only experiments exhibited only a small N isotope effect (< +1 ÔÇ░). The nitrite O isotope effect was very low in both setups (18╬ÁNO2 < 1 ÔÇ░), most likely due to substantial O isotope exchange with ambient water. Moreover, during the low-turnover conditions (i.e., in the mineral-only experiments, as well as initially in experiments with DB), the observed nitrite isotope systematics suggest, transiently, a small inverse isotope effect (i.e., decreasing nitrite ­ŁŤ┐15N and ­ŁŤ┐18O with decreasing concentrations), possibly related to transitory surface complexation mechanisms. Site preference (SP) of the 15N isotopes in the linear N2O molecule for both setups ranged between 1 to 7 ÔÇ░, notably lower than previously reported for chemodenitrification. Our results imply that chemodenitrification is dependent on the available reactive surfaces, and that the NO2 (rather than the N2O) isotope signatures may be useful for distinguishing between chemodenitrification catalysed by minerals, chemodenitrification catalysed by dead microbial biomass, and possibly true enzymatic NDFeO.

Anna-Neva Visser et al.

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Anna-Neva Visser et al.

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Short summary
We investigated how dead cells and Fe(II) minerals enhance the chemical reaction between Fe(II) and nitrite, which has been reported to produce high levels of the greenhouse gas N2O. Nitrite reduction is significantly enhanced if both additives are present, whereas the reaction is less pronounced if only Fe(II) minerals are present. Overall, both reaction systems show distinct differences, a minor N2O production and our results indicate that the abiotic production of N2 is indeed occurring.
We investigated how dead cells and Fe(II) minerals enhance the chemical reaction between Fe(II)...
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