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 (Fe₃(PO₄)₂), to which 2 mM nitrite (${\mathrm{NO}}_{\mathrm{2}}^{-}$) was later added, with or without an autoclaved cell suspension ($\sim \mathrm{1.96}\times {\mathrm{10}}^{\mathrm{8}}$ cells mL⁻¹) of Shewanella oneidensis MR-1. Concentrations of nitrite (${\mathrm{NO}}_{\mathrm{2}}^{-}$), nitrous oxide (N₂O), and iron (Fe²⁺, Fe_tot) 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 ${\mathrm{NO}}_{\mathrm{2}}^{-}$ and N₂O (δ¹⁵N and δ¹⁸O) were analysed to constrain the associated isotope effects. Up to 90 % of the Fe(II) was oxidized in the presence of DB, whereas only ∼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 ${\mathrm{NO}}_{\mathrm{2}}^{-}$ reduction rate in the mineral-only experiments (0.004±0.003 mmol L⁻¹ d⁻¹) was much lower than in the experiments with both mineral and DB (0.053±0.013 mmol L⁻¹ d⁻¹), as was N₂O production (204.02±60.29 nmol L⁻¹ d⁻¹). The N₂O yield per mole ${\mathrm{NO}}_{\mathrm{2}}^{-}$ 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-${\mathrm{NO}}_{\mathrm{2}}^{-}$ isotope ratio measurements indicated a clear difference between both experimental conditions: in contrast to the marked ¹⁵N isotope enrichment during active ${\mathrm{NO}}_{\mathrm{2}}^{-}$ reduction (${}^{\mathrm{15}}{\mathit{\epsilon }}_{{\mathrm{NO}}_{\mathrm{2}}}=+\mathrm{10.3}$ ‰) observed in the presence of DB, ${\mathrm{NO}}_{\mathrm{2}}^{-}$ loss in the mineral-only experiments exhibited only a small N isotope effect (<+1 ‰). The ${\mathrm{NO}}_{\mathrm{2}}^{-}$-O isotope effect was very low in both set-ups (${}^{\mathrm{18}}{\mathit{\epsilon }}_{{\mathrm{NO}}_{\mathrm{2}}}$ <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 ${\mathrm{NO}}_{\mathrm{2}}^{-}$ isotope systematics suggest, transiently, a small inverse isotope effect (i.e. decreasing ${\mathrm{NO}}_{\mathrm{2}}^{-}$ δ¹⁵N and δ¹⁸O with decreasing concentrations), which was possibly related to transitory surface complexation mechanisms. Site preference (SP) of the ¹⁵N isotopes in the linear N₂O molecule for both set-ups ranged between 0 ‰ and 14 ‰, which was notably lower than the values previously reported for chemodenitrification. Our results imply that chemodenitrification is dependent on the available reactive surfaces and that the ${\mathrm{NO}}_{\mathrm{2}}^{-}$ (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.

中文翻译：

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反应性表面对亚硝酸盐与亚铁离子非生物反应及相关氮，氧同位素动力学的影响

依赖于厌氧硝酸盐的Fe（II）氧化（NDFeO）在各种水生环境中广泛存在，并且在铁和氮的氧化还原动力学中起主要作用。然而，真正的酶促自养型NDFeO的证据仍然有限，替代解释涉及通过反应性中间氮（N）物种将异养型反硝化与结构结合的Fe（II）或水溶液Fe（II）的非生物氧化偶联（化学脱氮）。迄今为止，尚未直接测试离体表面催化作用导致化学脱氮的程度（或增强程度）。为了确定是否存在含Fe（II）的矿物或死生物质（DB）催化化学氮化，进行了两组不同的缺氧分批实验：将2 mM Fe（II）添加到低磷酸盐培养基中，Fe ₃（PO ₄）₂），其中加入2 mM亚硝酸盐（${\mathrm{没有}}_{\mathrm{2}}^{--}$）随后添加，有无高压灭菌的细胞悬液（$〜\mathrm{1.96}\times {\mathrm{10}}^{\mathrm{8}}$希瓦氏菌MR-1的 细胞mL ^-1）。亚硝酸盐浓度（${\mathrm{没有}}_{\mathrm{2}}^{--}$），氧化亚氮（N ₂ O）和铁（Fe 2 ⁺，Fe _tot）在两种设置下均随时间进行监测，以评估Fe（II）矿物和/或DB作为化学脱氮催化剂的影响。此外，自然丰度同位素比为${\mathrm{没有}}_{\mathrm{2}}^{--}$和Ñ ₂ Ó（δ ¹⁵ Ñ 和δ ¹⁸ ö）进行了分析，以限制关联的同位素效应。在存在DB的情况下，多达90％的Fe（II）被氧化，而仅纯 矿物条件下只有约 65％的Fe（II）被氧化，这表明纯矿物结构的总体反应性较低。同样，平均${\mathrm{没有}}_{\mathrm{2}}^{--}$仅含矿物质的实验（0.004±0.003  mmol L ^-1  d ^-1）的还原速率远低于含矿物质和DB的实验（0.053±0.013  mmol L ^-1  d ^-1）的还原速率，N ₂ O也是如此生产（204.02±60.29nmol  L ^-1  d ^-1）。所述Ñ ₂ ö每摩尔产率${\mathrm{没有}}_{\mathrm{2}}^{--}$在仅矿物的装置中，减少的NO（4％）比用DB的实验中的减少（1％）高，表明NO的催化依赖性差异形成。N-${\mathrm{没有}}_{\mathrm{2}}^{--}$同位素比测量表明两种实验条件之间存在明显差异：与活性期间显着的¹⁵ N同位素富集相反${\mathrm{没有}}_{\mathrm{2}}^{--}$ 减少（${}^{\mathrm{15}}{\mathit{\epsilon }}_{{\mathrm{没有}}_{\mathrm{2}}}=+\mathrm{10.3}$ ‰）在有DB的情况下观察到， ${\mathrm{没有}}_{\mathrm{2}}^{--}$纯矿物实验中的失重仅表现出很小的N同位素效应（< +1  ‰）。的${\mathrm{没有}}_{\mathrm{2}}^{--}$两种设置中的-O同位素效应都很低（${}^{\mathrm{18岁}}{\mathit{\epsilon }}_{{\mathrm{没有}}_{\mathrm{2}}}$ < 1  ‰），这很可能是由于与周围水进行大量的O同位素交换所致。此外，在低周转条件下（即在纯矿物实验中以及最初在DB实验中），观察到${\mathrm{没有}}_{\mathrm{2}}^{--}$ 同位素系统学研究表明，短暂的反同位素效应（即减小 ${\mathrm{没有}}_{\mathrm{2}}^{--}$ δ ¹⁵ Ñ和δ ¹⁸ ö随浓度），这是可能与暂时性表面络合机制。两种结构中线性N ₂ O分子中¹⁵ N同位素的位 点偏好（SP）介于0‰和14‰之间，明显低于该值先前报道用于化学氮化。我们的结果表明化学脱氮取决于可用的反应性表面，并且${\mathrm{没有}}_{\mathrm{2}}^{--}$ （而不是N ₂ O）同位素特征可用于区分矿物催化的化学氮化，死微生物生物量催化的化学氮化和可能的真酶NDFeO。