Banded Iron Formations (BIFs) are ancient marine chemical sediments that contain various Fe-bearing minerals such as hematite (Fe₂O₃), magnetite (Fe₃O₄), siderite (FeCO₃) and a variety of Fe^II-/Fe^III-silicates. The prevailing opinion is that primary Fe(III) (oxyhydr)oxides, such as ferrihydrite (simplified formula of Fe(OH)₃), were precipitated from the ocean’s photic zone by marine plankton, and a fraction of these minerals was subsequently transformed into secondary magnetite and siderite by dissimilatory Fe(III)-reducing bacteria (DIRB). However, aside from broad estimates, it is currently unknown what fraction of the primary Fe(III) minerals was sedimented to the seafloor where it was eventually lithified, and what fraction was reduced by DIRB in the water column, thus forming a microbial Fe cycle in the water column. To test this, we conducted Fe cycling experiments with marine phototrophic Fe(II)-oxidizing bacteria and DIRB under conditions mimicking the Precambrian ocean water column with elevated Fe(II) and Si concentrations. We followed secondary mineral formation over three consecutive redox cycles (oxidation followed by reduction) over a time interval of up to 58 days to determine which mineral phases would ultimately have settled as BIF forming sediments. We used wet geochemical methods to follow Fe speciation, measured dissolved silica and volatile fatty acid (VFA) concentrations, determined cell-mineral associations using fluorescence and electron microscopy, and characterized the mineralogy of the precipitates using ⁵⁷Fe-Moessbauer spectroscopy and X-ray diffraction (XRD). Our results showed that both the absence of silica and an increasing number of Fe cycles favored the formation of more crystalline minerals, such as goethite (α-FeOOH). However, in the presence of high concentrations of monomeric silica, as suggested for ancient oceans (2.2 mM), only short-range ordered (SRO) Fe(III) minerals such as ferrihydrite were observed. These did not transform into the more thermodynamically stable goethite during repeated Fe cycling. Interestingly, no magnetite formed in any of the setups. Instead, increasing Si concentrations favored the formation of increasing quantities of Fe(II) minerals. Microscopy revealed a tight association between microbial biomass and minerals formed. Dissolved silica analysis showed the removal of Si from solution congruent with Fe(II) oxidation and a release of Si during Fe(III) reduction. Together, these results suggest an important role of co-precipitated biomass as well as silica for secondary mineral formation by either constraining crystal growth and/or inhibiting Fe(II)-induced mineral transformation. Overall, our results imply that microbial Fe cycling during settling of primary ferrihydrite through the photic zone in a Precambrian ocean would have resulted in the partial transformation of ferrihydrite into secondary Fe(II) mineral phases in the water column. This would have resulted in the accumulation of mixtures of a ferrihydrite-silica composite and Fe(II) minerals in the initial BIF forming sediments.

带状铁地层 (BIF) 是古老的海洋化学沉积物，含有各种含铁矿物，如赤铁矿 (Fe ₂ O ₃ )、磁铁矿 (Fe ₃ O ₄ )、菱铁矿 (FeCO ₃ ) 和各种 Fe ^II-/Fe ^III -硅酸盐。普遍的观点是，初级 Fe(III)（羟基）氧化物，如水铁矿（Fe(OH) _{3的简化式）})，由海洋浮游生物从海洋的透光区沉淀出来，这些矿物的一部分随后通过异化 Fe(III) 还原细菌 (DIRB) 转化为次生磁铁矿和菱铁矿。然而，除了广泛的估计之外，目前尚不清楚主要 Fe(III) 矿物的哪一部分沉积到海底并最终石化，以及哪一部分在水柱中被 DIRB 还原，从而形成微生物 Fe 循环在水柱中。为了测试这一点，我们在模拟前寒武纪海洋水柱的条件下，使用海洋光养 Fe(II) 氧化细菌和 DIRB 进行了 Fe 循环实验，Fe(II) 和 Si 浓度升高。我们在长达 58 天的时间间隔内跟踪了三个连续氧化还原循环（氧化后还原）的次生矿物形成，以确定哪些矿物相最终会沉淀为 BIF 形成沉积物。我们使用湿地球化学方法来跟踪 Fe 形态，测量溶解的二氧化硅和挥发性脂肪酸 (VFA) 浓度，使用荧光和电子显微镜确定细胞-矿物结合，并使用表征沉淀物的矿物学⁵⁷Fe-Moessbauer 光谱和 X 射线衍射 (XRD)。我们的结果表明，二氧化硅的缺乏和铁循环次数的增加都有利于形成更多的结晶矿物，例如针铁矿（α-FeO​​OH）。然而，在存在高浓度单体二氧化硅的情况下，如古代海洋 (2.2 mM) 所建议的那样，仅观察到短程有序 (SRO) Fe(III) 矿物，如水铁矿。在重复的铁循环过程中，这些并没有转化为热力学更稳定的针铁矿。有趣的是，在任何设置中都没有形成磁铁矿。相反，增加 Si 浓度有利于形成数量增加的 Fe(II) 矿物。显微镜显示微生物生物质和形成的矿物质之间存在紧密联系。溶解二氧化硅分析表明，从溶液中去除 Si 与 Fe(II) 氧化一致，并在 Fe(III) 还原过程中释放出 Si。总之，这些结果表明共沉淀生物质和二氧化硅通过限制晶体生长和/或抑制 Fe(II) 诱导的矿物转化对次生矿物形成具有重要作用。总体而言，我们的研究结果表明，在初级水铁矿沉降通过前寒武纪海洋中的透光区期间，微生物 Fe 循环将导致水铁矿部分转化为水柱中的次生 Fe(II) 矿物相。这将导致在初始 BIF 形成沉积物中积累水铁矿-二氧化硅复合物和 Fe(II) 矿物的混合物。这些结果表明，通过限制晶体生长和/或抑制 Fe(II) 诱导的矿物转化，共沉淀生物质和二氧化硅对次生矿物形成具有重要作用。总体而言，我们的研究结果表明，在初级水铁矿沉降通过前寒武纪海洋中的透光区期间，微生物 Fe 循环将导致水铁矿部分转化为水柱中的次生 Fe(II) 矿物相。这将导致在初始 BIF 形成沉积物中积累水铁矿-二氧化硅复合物和 Fe(II) 矿物的混合物。这些结果表明，通过限制晶体生长和/或抑制 Fe(II) 诱导的矿物转化，共沉淀生物质和二氧化硅对次生矿物形成具有重要作用。总体而言，我们的研究结果表明，在初级水铁矿沉降通过前寒武纪海洋中的透光区期间，微生物 Fe 循环将导致水铁矿部分转化为水柱中的次生 Fe(II) 矿物相。这将导致在初始 BIF 形成沉积物中积累水铁矿-二氧化硅复合物和 Fe(II) 矿物的混合物。我们的研究结果表明，在初级水铁矿通过前寒武纪海洋中的光带沉降过程中的微生物 Fe 循环将导致水铁矿部分转化为水柱中的次生 Fe(II) 矿物相。这将导致在初始 BIF 形成沉积物中积累水铁矿-二氧化硅复合物和 Fe(II) 矿物的混合物。我们的研究结果表明，在初级水铁矿通过前寒武纪海洋中的光带沉降过程中的微生物 Fe 循环将导致水铁矿部分转化为水柱中的次生 Fe(II) 矿物相。这将导致在初始 BIF 形成沉积物中积累水铁矿-二氧化硅复合物和 Fe(II) 矿物的混合物。