HUANG Qiang1,2, LI Bao1,2, LIU Xiaoling1, GUO Yuan1, WANG Yanan1, HAO

（1. State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, CAS, Guiyang 550081, China;
2. University of Chinese Academy of Sciences, Beijing 100049, China;
3. CAS Center for Excellence in Quaternary Science and Global Change, Xi’an 710061, China）

Abstract： Iron ranks fourth in abundance among crustal elements and can be taken up by microbial assimilation as a biologically essential element or as an electron source in microbial metabolism. Microbially mediated iron oxidation is an important part of the iron cycle and thus drives the biogeochemical cycling of biogenic elements such as C, N, O and S in the environment, which is strongly associated with the emission or storage of carbon and nitrogen, the greenhouse effect, and fate of nutrients and toxic metals or metalloids. The microorganisms involved in Fe(II) oxidation can be classified into four groups based on their growth environment and electron acceptor status: acidophilic Fe(II) oxidizers, neutral microaerobic Fe(II) oxidizers, neutral anaerobic photosynthetic Fe(II) oxidizers and neutral anaerobic nitrate reducing Fe(II) oxidizers. A summary of the proposed mechanisms of electron transport in microorganisms involved in iron oxidation shows that they all share a common paradigm, that is, ferrous iron is oxidised on outer membrane cytochromes and the electrons obtained from ferrous iron are transmitted from the extracellular membrane through the periplasmic electron transport proteins to various proteins in the inner cell membrane for carbon sequestration and/or reduction of electron acceptors. The study of microorganisms involved in iron oxidation in modern environments has been widely applied to the evolution of early life on Earth, the bioremediation of environmental pollution as well as the synthesis of new materials and bioleaching. This paper reviews the microbial phylogenetic types and electron transfer mechanisms that mediate iron oxidation processes and describes their applications in geology, environment, materials and metallurgy. A great deal of work remains to be done on the isolation and identification of iron-oxidizing bacteria, the detailed analysis of iron-oxidizing metabolic pathways and their geochemical effects, the establishment and improvement of the iron-oxidizing microbiome database, and the application of iron oxidation in environmental pollution.

Key words：iron oxidizers; electron transfer mechanisms; early evolution of life; bioremediation

EARTH AND ENVIRONMENT Vol.51, No.3, Tot No.353, 2023, Page 363