Keffer Jessica L, Zhou Nanqing, Rushworth Danielle D, Yu Yanbao, Chan Clara S
Department of Earth Sciences, University of Delaware, Newark, Delaware, USA.
School of Marine Science and Policy, University of Delaware, Newark, Delaware, USA.
Appl Environ Microbiol. 2025 Apr 23;91(4):e0186524. doi: 10.1128/aem.01865-24. Epub 2025 Mar 5.
Most of Earth's iron is mineral-bound, but it is unclear how and to what extent iron-oxidizing microbes can use solid minerals as electron donors. A prime candidate for studying mineral-oxidizing growth and pathways is ES-1, a robust, facultative iron oxidizer with multiple possible iron oxidation mechanisms. These include Cyc2 and Mto pathways plus other multiheme cytochromes and cupredoxins, and so we posit that the mechanisms may correspond to different Fe(II) sources. Here, ES-1 was grown on dissolved Fe(II)-citrate and magnetite. ES-1 oxidized all dissolved Fe released from magnetite and continued to build biomass when only solid Fe(II) remained, suggesting it can utilize magnetite as a solid electron donor. Quantitative proteomic analyses of ES-1 grown on these substrates revealed global proteome remodeling in response to electron donor and growth state and uncovered potential proteins and metabolic pathways involved in the oxidation of solid magnetite. While the Cyc2 iron oxidases were highly expressed on both dissolved and solid substrates, MtoA was only detected during growth on solid magnetite, suggesting this protein helps catalyze oxidation of solid minerals in ES-1. A set of cupredoxin domain-containing proteins were also specifically expressed during solid iron oxidation. This work demonstrated that the iron oxidizer ES-1 utilized additional extracellular electron transfer pathways when growing on solid mineral electron donors compared to dissolved Fe(II).
Mineral-bound iron could be a vast source of energy to iron-oxidizing bacteria, but there is limited physiological evidence of this metabolism, and it has been unknown whether the mechanisms of solid and dissolved Fe(II) oxidation are distinct. In iron-reducing bacteria, multiheme cytochromes can facilitate iron mineral reduction, and here, we link a multiheme cytochrome-based pathway to mineral oxidation, expanding the known functionality of multiheme cytochromes. Given the growing recognition of microbial oxidation of minerals and cathodes, increasing our understanding of these mechanisms will allow us to recognize and trace the activities of mineral-oxidizing microbes. This work shows how solid iron minerals can promote microbial growth, which, if widespread, could be a major agent of geologic weathering and mineral-fueled nutrient cycling in sediments, aquifers, and rock-hosted environments.
地球上的大部分铁都与矿物结合，但尚不清楚铁氧化微生物如何以及在何种程度上能够利用固体矿物作为电子供体。用于研究矿物氧化生长和途径的一个主要候选对象是ES-1，它是一种强大的兼性铁氧化剂，具有多种可能的铁氧化机制。这些机制包括Cyc2和Mto途径以及其他多血红素细胞色素和铜蓝蛋白，因此我们推测这些机制可能对应于不同的Fe(II)来源。在此，ES-1在溶解的柠檬酸亚铁和磁铁矿上生长。ES-1氧化了从磁铁矿释放的所有溶解铁，并且当仅剩下固体Fe(II)时仍继续积累生物量，这表明它可以利用磁铁矿作为固体电子供体。对在这些底物上生长的ES-1进行的定量蛋白质组学分析揭示了响应电子供体和生长状态的全局蛋白质组重塑，并发现了参与固体磁铁矿氧化的潜在蛋白质和代谢途径。虽然Cyc2铁氧化酶在溶解和固体底物上均高度表达，但仅在固体磁铁矿上生长期间检测到MtoA，这表明该蛋白质有助于催化ES-1中固体矿物的氧化。一组含铜蓝蛋白结构域的蛋白质在固体铁氧化过程中也特异性表达。这项工作表明，与溶解的Fe(II)相比，铁氧化剂ES-1在以固体矿物电子供体生长时利用了额外的细胞外电子转移途径。
与矿物结合的铁可能是铁氧化细菌的巨大能量来源，但这种代谢的生理证据有限，并且固体和溶解的Fe(II)氧化机制是否不同尚不清楚。在铁还原细菌中，多血红素细胞色素可以促进铁矿物还原，在此，我们将基于多血红素细胞色素的途径与矿物氧化联系起来，扩展了多血红素细胞色素的已知功能。鉴于对矿物和阴极微生物氧化的认识不断增加，加深我们对这些机制的理解将使我们能够识别和追踪矿物氧化微生物的活动。这项工作展示了固体铁矿物如何促进微生物生长，如果这种情况普遍存在，可能是沉积物、含水层和岩石宿主环境中地质风化和矿物驱动的养分循环的主要因素。
