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

Visser, Anna-Neva; Wankel, Scott D.; Niklaus, Pascal A.; Byrne, James M.; Kappler, Andreas A.; Lehmann, Moritz F.

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 the coupling of heterotrophic denitrification with the abiotic oxidation of structurally bound or aqueous Fe(II) by reactive intermediate nitrogen (N) species (chemodenitrification). The extent to which chemodenitrification is caused (or enhanced) by ex vivo surface catalytic effects has not been directly tested to date. To determine whether the presence of either an Fe(II)-bearing mineral or dead biomass (DB) catalyses chemodenitrification, two different sets of anoxic batch experiments were conducted: 2 mM Fe(II) was added to a low-phosphate medium, resulting in the precipitation of vivianite (inline-formulaFe3(PO4)2), to which 2 mM nitrite (inline-formula M2inlinescrollmathml chem normal NO normal 2 - 25pt16ptsvg-formulamathimg52c649ee5f83d3a23956e1a8d03f4531 bg-17-4355-2020-ie00001.svg25pt16ptbg-17-4355-2020-ie00001.png ) was later added, with or without an autoclaved cell suspension (inline-formula M3inlinescrollmathml normal 1.96 × normal 10 normal 8 61pt14ptsvg-formulamathimgef7b01727a061b507801367686e20e82 bg-17-4355-2020-ie00002.svg61pt14ptbg-17-4355-2020-ie00002.png  cells mLinline-formula−1) of Shewanella oneidensis MR-1. Concentrations of nitrite (inline-formula M5inlinescrollmathml chem normal NO normal 2 - 25pt16ptsvg-formulamathimgbcdbde0566f3e4be69cabaa2d184dd7a bg-17-4355-2020-ie00003.svg25pt16ptbg-17-4355-2020-ie00003.png ), nitrous oxide (inline-formulaN2O), and iron (inline-formulaFe2+, inline-formulaFetot) were monitored over time in both set-ups to assess the impact of Fe(II) minerals and/or DB as catalysts of chemodenitrification. In addition, the natural-abundance isotope ratios of inline-formula M9inlinescrollmathml chem normal NO normal 2 - 25pt16ptsvg-formulamathimg19cb9306f275cfd5c1fc7eac2dd7137a bg-17-4355-2020-ie00004.svg25pt16ptbg-17-4355-2020-ie00004.png and inline-formulaN2O (inline-formulaδ15N and inline-formulaδ18O) were analysed to constrain the associated isotope effects. Up to 90 % of the Fe(II) was oxidized in the presence of DB, whereas only inline-formula∼65 % of the Fe(II) was oxidized under mineral-only conditions, suggesting an overall lower reactivity of the mineral-only set-up. Similarly, the average inline-formula M14inlinescrollmathml chem normal NO normal 2 - 25pt16ptsvg-formulamathimg03b6852a356c49e6d8252ab141f4b6ac bg-17-4355-2020-ie00005.svg25pt16ptbg-17-4355-2020-ie00005.png reduction rate in the mineral-only experiments (inline-formula0.004±0.003 mmol Linline-formula−1 dinline-formula−1) was much lower than in the experiments with both mineral and DB (inline-formula0.053±0.013 mmol Linline-formula−1 dinline-formula−1), as was inline-formulaN2O production (inline-formula204.02±60.29 nmol Linline-formula−1 dinline-formula−1). The inline-formulaN2O yield per mole inline-formula M26inlinescrollmathml chem normal NO normal 2 - 25pt16ptsvg-formulamathimg5659f1ec0d19e14da6dd21bc92929b68 bg-17-4355-2020-ie00006.svg25pt16ptbg-17-4355-2020-ie00006.png reduced was higher in the mineral-only set-ups (4 %) than in the experiments with DB (1 %), suggesting the catalysis-dependent differential formation of NO. N-inline-formula M27inlinescrollmathml chem normal NO normal 2 - 25pt16ptsvg-formulamathimg339eb377d47c7c5a6dfc89d0f7c9bad6 bg-17-4355-2020-ie00007.svg25pt16ptbg-17-4355-2020-ie00007.png isotope ratio measurements indicated a clear difference between both experimental conditions: in contrast to the marked inline-formula15N isotope enrichment during active inline-formula M29inlinescrollmathml chem normal NO normal 2 - 25pt16ptsvg-formulamathimg674dccc3bf67b8da1350d22c64000208 bg-17-4355-2020-ie00008.svg25pt16ptbg-17-4355-2020-ie00008.png reduction (inline-formula M30inlinescrollmathml normal 15 italic ε chem normal NO normal 2 = + normal 10.3 76pt18ptsvg-formulamathimge100a46f67049d4470525858d2aa5df1 bg-17-4355-2020-ie00009.svg76pt18ptbg-17-4355-2020-ie00009.png  ‰) observed in the presence of DB, inline-formula M31inlinescrollmathml chem normal NO normal 2 - 25pt16ptsvg-formulamathimgc3f13250287d383abd1d82d99b476719 bg-17-4355-2020-ie00010.svg25pt16ptbg-17-4355-2020-ie00010.png loss in the mineral-only experiments exhibited only a small N isotope effect (inline-formula<+1 ‰). The inline-formula M33inlinescrollmathml chem normal NO normal 2 - 25pt16ptsvg-formulamathimg22e42d77b88b98d0a8d28299e9ddcc02 bg-17-4355-2020-ie00011.svg25pt16ptbg-17-4355-2020-ie00011.png -O isotope effect was very low in both set-ups (inline-formula M34inlinescrollmathml normal 18 italic ε chem normal NO normal 2 33pt18ptsvg-formulamathimge681c0e49bb49c4787bfef0e7a5ec82d bg-17-4355-2020-ie00012.svg33pt18ptbg-17-4355-2020-ie00012.png inline-formula<1 ‰), which was most likely due to substantial O isotope exchange with ambient water. Moreover, under low-turnover conditions (i.e. in the mineral-only experiments as well as initially in experiments with DB), the observed inline-formula M36inlinescrollmathml chem normal NO normal 2 - 25pt16ptsvg-formulamathimg85e7cdf58766541dc4d261bc53a3af3a bg-17-4355-2020-ie00013.svg25pt16ptbg-17-4355-2020-ie00013.png isotope systematics suggest, transiently, a small inverse isotope effect (i.e. decreasing inline-formula M37inlinescrollmathml chem normal NO normal 2 - 25pt16ptsvg-formulamathimg90891ad79e9f720d673a4d32bf9e2963 bg-17-4355-2020-ie00014.svg25pt16ptbg-17-4355-2020-ie00014.png inline-formulaδ15N and inline-formulaδ18O with decreasing concentrations), which was possibly related to transitory surface complexation mechanisms. Site preference (SP) of the inline-formula15N isotopes in the linear inline-formulaN2O molecule for both set-ups ranged between 0 ‰ and 14 ‰, which was notably lower than the valuespage4356 previously reported for chemodenitrification. Our results imply that chemodenitrification is dependent on the available reactive surfaces and that the inline-formula M42inlinescrollmathml chem normal NO normal 2 - 25pt16ptsvg-formulamathimgef655b7de5138e3536c4788b60d2d1fc bg-17-4355-2020-ie00015.svg25pt16ptbg-17-4355-2020-ie00015.png (rather than the inline-formulaN2O) isotope signatures may be useful for distinguishing between chemodenitrification catalysed by minerals, chemodenitrification catalysed by dead microbial biomass, and possibly true enzymatic NDFeO.



Visser, Anna-Neva / Wankel, Scott D. / Niklaus, Pascal A. / et al: Impact of reactive surfaces on the abiotic reaction between nitrite and ferrous iron and associated nitrogen and oxygen isotope dynamics. 2020. Copernicus Publications.


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