Key Laboratory of Marine Genetic Resources, Third Institute of Oceanography, Ministry of Natural Resources of PR China, Xiamen 361005, China; Key Lab of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China.
Key Laboratory of Marine Genetic Resources, Third Institute of Oceanography, Ministry of Natural Resources of PR China, Xiamen 361005, China.
Sci Total Environ. 2020 Jun 20;722:137723. doi: 10.1016/j.scitotenv.2020.137723. Epub 2020 Mar 6.
Dissimilatory Fe(III)-reducing bacteria (DIRBs) could reduce extracellular Fe(III) to Fe(II) via extracellular electron transfer (EET), playing an important role in biogeochemical cycling of Fe(III). Previous studies have noted the key role of multi-heme c-type cytochromes (MHCs) involved in EET by respiratory-type DIRBs, and proposed indirect electron transfer through the use of redox electron shuttles (e.g., flavins) or Fe(III)-chelation. However, knowledge about the EET of fermentative DIRBs was vitally scarce. Here, Anoxybacter fermentans DY22613 is a typical fermentative DIRB isolated from deep-sea hydrothermal sulfides, and it could utilize soluble Fe(III)-citrate and solid Fe(III)-bearing minerals as extracellular electron acceptors. Unlike respiratory-type DIRBs that utilize MHCs, this strain lacked MHCs to mediate EET. Besides, it did not adopt Fe(III)-chelation to mediate indirect EET. Nonetheless, genes encoding biosynthesis pathway of redox molecules (e.g., flavins) were found in its genome and their gene expression was up-regulated with Fe(III) reduction, suggesting redox molecules may mediate indirect EET by this strain. Subsequent physiological and biochemical tests further demonstrated endogenous riboflavin acted as main electron shuttles to mediate indirect EET by this strain, and menaquinone, indole played an assistant role in this process. Besides, this strain could employ exogenous humic acids to facilitate indirect EET. The mode of exogenous and endogenous redox molecules to co-mediate indirect EET by fermentative A. fermentans DY22613, expands our knowledge about EET of fermentative DIRBs, and would contribute to better understand its ecological role in the biogeochemistry cycle of iron.
异化型三价铁还原菌（DIRB）可以通过细胞外电子传递（EET）将细胞外的三价铁还原为二价铁，在三价铁的生物地球化学循环中起着重要作用。先前的研究已经注意到参与 EET 的多血红素 c 型细胞色素（MHCs）在呼吸型 DIRB 中的关键作用，并提出通过使用氧化还原电子穿梭体（例如黄素）或三价铁螯合来进行间接电子传递。然而，关于发酵型 DIRB 的 EET 知识却极为缺乏。在这里，深海热液硫化物中分离出的典型发酵型 DIRB 是一株厌氧发酵菌 DY22613，它可以利用可溶的三价铁柠檬酸和固体含铁矿物作为细胞外电子受体。与利用 MHCs 进行 EET 的呼吸型 DIRB 不同，该菌株缺乏 MHCs 来介导 EET。此外，它也没有采用三价铁螯合来介导间接 EET。尽管如此，在其基因组中发现了编码氧化还原分子生物合成途径的基因（例如黄素），并且随着三价铁的还原，它们的基因表达上调，这表明氧化还原分子可能通过该菌株进行间接 EET。随后的生理生化测试进一步证明，内源性核黄素作为主要电子穿梭体，通过该菌株介导间接 EET，而menaquinone、吲哚在该过程中发挥辅助作用。此外，该菌株可以利用外源性腐殖酸来促进间接 EET。发酵型 A. fermentans DY22613 中内外源氧化还原分子协同介导间接 EET 的方式，扩展了我们对发酵型 DIRB 的 EET 知识，有助于更好地理解其在铁的生物地球化学循环中的生态作用。
