Abstract Organic matter (OM) is often associated with Fe (hydr)oxides such as ferrihydrite (Fh) in soils and sediments, forming binary Fh-OM complexes. Microbial reduction of Fh results in destabilization of the complexes and mineral/OM transformation. However, little is known about the role of clay minerals in such processes, despite their common co-existence with Fh and OM in natural environments. Here Fh-OM complexes were synthesized in the presence of montmorillonite (SWy-2), forming ternary Fh-(SWy-2)-OM complexes. A metal-reducing bacterium Geobacter sulfurreducens was used to reduce Fh in the complexes under circumneutral pH and anoxic conditions with or without H2 as extra electron donor. Various spectroscopy and mass spectrometry methods were used to monitor the progress of Fh bio-reduction and mineral/OM transformation. Results showed that G. sulfurreducens utilized mineral-bound OM as electron donor and/or carbon source to couple with Fh reduction. Relative to Fh-OM complex, addition of SWy-2 to Fh-OM complex enhanced the bio-reduction extent of Fh by increasing the proportion of bioavailable OM that was weakly bound to SWy-2. However, its effect on the bio-reduction rate was variable. SWy-2 initially decreased the rate, because it spatially separated OM (electron donor) from Fh (electron acceptor). During later incubation, SWy-2 increased the reduction rate by sorbing biogenic Fe2+ that would otherwise passivate the Fh and cell surfaces. Bio-reduction transformed mineral-bound OM to microbial products (e.g. necromass, extracellular polymeric substances), but organic compounds with aromatic structures, carboxyl groups and large molecular weight were more resistant to desorption and oxidation. The persistence of these compounds against bio-reduction induced transformation is likely due to their stronger binding with minerals and/or lower nominal oxidation states of carbon relative to other compounds. Our results provide new insights into the role of clay minerals in regulating biogeochemical cycling of solid-phase Fe and transformation of mineral-associated OM in anoxic soil environments.

中文翻译：

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水铁矿-蒙脱石-有机物复合物的生物还原：蒙脱石的影响和有机物的归宿

摘要 有机质（OM）通常与土壤和沉积物中的水铁矿（Fh）等Fe（氢）氧化物结合，形成二元Fh-OM复合物。Fh 的微生物减少导致复合物不稳定和矿物/OM 转化。然而，尽管粘土矿物在自然环境中与 Fh 和 OM 共存，但对这些过程中粘土矿物的作用知之甚少。这里 Fh-OM 复合物是在蒙脱石 (SWy-2) 存在下合成的，形成三元 Fh-(SWy-2)-OM 复合物。在有或没有 H2 作为额外电子供体的环中性 pH 值和缺氧条件下，使用金属还原细菌 Geobacter silverreducens 来还原复合物中的 Fh。使用各种光谱和质谱方法来监测 Fh 生物还原和矿物/OM 转化的进展。结果表明，G.sulfurreducens 利用矿物结合的 OM 作为电子供体和/或碳源与 Fh 还原结合。相对于 Fh-OM 复合物，将 SWy-2 添加到 Fh-OM 复合物通过增加与 SWy-2 弱结合的生物可利用 OM 的比例来增强 Fh 的生物还原程度。然而，它对生物还原率的影响是可变的。SWy-2 最初降低了速率，因为它在空间上将 OM（电子供体）与 Fh（电子受体）分开。在后来的孵化过程中，SWy-2 通过吸附生物 Fe2+ 来提高还原率，否则会钝化 Fh 和细胞表面。生物还原将矿物质结合的 OM 转化为微生物产物（例如死尸、细胞外聚合物），但具有芳香结构的有机化合物，羧基和大分子量更耐解吸和氧化。这些化合物对生物还原诱导转化的持久性可能是由于它们与矿物质的结合更强和/或相对于其他化合物的碳标称氧化态更低。我们的研究结果为粘土矿物在调节固相 Fe 的生物地球化学循环和缺氧土壤环境中与矿物相关的 OM 转化中的作用提供了新的见解。