Abstract Dissimilatory iron reduction (DIR) plays an essential role in biogeochemical Fe cycling in anoxic environments. At near-neutral pH, in both biotic and abiotic systems, aqueous Fe(II) (Fe(II)aq) interacts with reactive ferric (hydr)oxides via electron transfer and atom exchange that is catalyzed by large amounts of sorbed Fe(II). This may result in substantial Fe isotope exchange, which, at equilibrium, produces up to a ∼4‰ 56Fe/54Fe fractionation between coexisting oxide/hydroxide and Fe(II)aq, depending on mineralogy. The role of biology in such systems has been interpreted to lie in the production of Fe(II) rather than a specific “vital” effect, such as enzymatic and kinetic processes. Under acidic abiotic conditions, however, the lack of sorbed Fe(II) generates little Fe isotope exchange, and, by extension, it has been expected that little exchange would occur during DIR at low pH if sorbed Fe(II) is the key component for catalyzing isotopic exchange. In this study, we explored the extent and mechanism of Fe isotope exchange between Fe(II)aq and ferric hydroxides (ferrihydrite and goethite), including determination of the 56Fe/54Fe fractionations during DIR by Acidianus strain DS80 at pH ∼ 3.0 and 80 °C, over 19 days of incubation. Significant Fe(III) reduction occurred for both minerals along with large changes in Fe isotope compositions for Fe(II)aq, indicating Fe isotope exchange. Solid-phase extractions using HCl confirmed a lack of sorbed Fe(II), which suggests that a mechanism other than sorption is required to catalyze Fe isotope exchange during DIR at low pH. Reactive Fe(III) (Fe(III)reac) extracted from the mineral surface allowed for the calculation of the Fe pools that underwent isotopic exchange. A total of ∼20% of goethite and ∼60% of ferrihydrite underwent isotopic exchange over 19 days. For goethite from biotic experiments, we calculate a Fe(III)reac-Fe(II)aq fractionation factor of 1.57 ± 0.52‰, which is larger than the abiotic equilibrium fractionation factor (∼0.73‰ at 80 °C). This result is consistent with previous work on DIR of goethite at neutral pH, where a fractionation factor larger than equilibrium was interpreted to reflect an isotopically distinct “distorted surface layer” of goethite produced during exchange with Fe(II)aq. In contrast to goethite, the difference between the Fe(III)reac-Fe(II)aq fractionation factor for ferrihydrite from our biotic reactors (2.91 ± 0.40‰) and the abiotic equilibrium fractionation factor (∼2.28‰ at 80 °C, under silica-free conditions) is smaller. Ultimately, the contrast in the extent of Fe isotope exchange between biotic and abiotic experiments emphasizes the importance of biology in promoting Fe isotope exchange in acidic systems. We speculate that the unique role of biology at low pH in catalyzing Fe isotope exchange, not seen in equivalent abiotic systems, must lie in the transport of electrons to the ferric hydroxide surface that produces Fe(II) atoms in situ. This suggests that isotopic exchange occurs on an atom-by-atom basis as Fe(III) is reduced to Fe(II), followed by the release of Fe(II) into solution. This study demonstrates that significant variations in Fe isotope compositions may be uniquely produced in acidic environments where microbial Fe cycling occurs via DIR, compared to minor isotopic variations observed previously in acidic abiotic systems.

中文翻译：

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在酸性水热环境中，嗜热嗜酸试剂在异化 Fe(III) 还原过程中稳定 Fe 同位素分馏

摘要 异化铁还原（DIR）在缺氧环境中生物地球化学铁循环中起着至关重要的作用。在接近中性的 pH 值下，在生物和非生物系统中，含水 Fe(II) (Fe(II)aq) 通过电子转移和原子交换与活性氧化铁（氢）氧化物相互作用，后者由大量吸附的 Fe(II) 催化）。这可能会导致大量的 Fe 同位素交换，在平衡状态下，共存的氧化物/氢氧化物和 Fe(II)aq 之间会产生高达 4‰ 56Fe/54Fe 的分馏，具体取决于矿物学。生物学在此类系统中的作用被解释为产生 Fe(II) 而不是特定的“重要”效应，例如酶促和动力学过程。然而，在酸性非生物条件下，吸附的 Fe(II) 的缺乏几乎不会产生 Fe 同位素交换，并且，通过扩展，如果吸附的 Fe(II) 是催化同位素交换的关键成分，则预计在低 pH 值下 DIR 期间几乎不会发生交换。在这项研究中，我们探索了 Fe(II)aq 和氢氧化铁（水铁矿和针铁矿）之间 Fe 同位素交换的程度和机制，包括在 pH ∼ 3.0 和 80°下用 Acidianus 菌株 DS80 在 DIR 过程中测定 56Fe/54Fe 分馏C、超过19天的孵化期。两种矿物都发生了显着的 Fe(III) 还原，同时 Fe(II)aq 的 Fe 同位素组成发生了巨大变化，表明 Fe 同位素发生了交换。使用 HCl 的固相萃取证实没有吸附的 Fe(II)，这表明在低 pH 值下 DIR 期间需要吸附以外的机制来催化 Fe 同位素交换。从矿物表面提取的反应性 Fe(III) (Fe(III)reac) 允许计算经历同位素交换的 Fe 池。总共约 20% 的针铁矿和约 60% 的水铁矿在 19 天内进行了同位素交换。对于来自生物实验的针铁矿，我们计算出 Fe(III)reac-Fe(II)aq 分馏因子为 1.57 ± 0.52‰，大于非生物平衡分馏因子（80°C 时的 ∼0.73‰）。该结果与先前关于中性 pH 值下针铁矿 DIR 的工作一致，其中大于平衡的分馏因子被解释为反映在与 Fe(II)aq 交换过程中产生的同位素不同的针铁矿“扭曲表面层”。与针铁矿相比，来自我们生物反应器的水铁矿的 Fe(III)reac-Fe(II)aq 分馏因子之间的差异 (2.91 ± 0. 40‰）和非生物平衡分馏因子（在 80°C 下为 ∼2.28‰，在无硅条件下）较小。最终，生物和非生物实验之间铁同位素交换程度的对比强调了生物学在促进酸性系统中铁同位素交换方面的重要性。我们推测，在低 pH 值下，生物学在催化 Fe 同位素交换方面的独特作用，在等效的非生物系统中是看不到的，必须在于将电子传输到原位产生 Fe(II) 原子的氢氧化铁表面。这表明同位素交换是在逐个原子的基础上发生的，因为 Fe(III) 被还原为 Fe(II)，然后是 Fe(II) 释放到溶液中。