土壤微生物—腐殖质—矿物间的胞外电子传递机制研究进展

Mechanism of Extracellular Electron Transfer among Microbe–Humus–Mineral in Soil:A Review

微生物胞外电子传递是地球表层系统元素循环与能量交换的重要驱动力。近年来,以微生物—腐殖质—矿物之间电子转移为核心的生物地球化学过程得到重视,拓展了以带电的土壤胶体与离子之间的相互作用为重心的土壤界面过程的内涵,成为地球表层系统物质间相互作用新的关注点,启示我们从化学与生物两个角度重新认识地球表层系统过程。本文从微生物、腐殖质和矿物等要素入手,综述了其地球化学角色与功能,讨论了它们之间的相互关系以及胞外电子传递的途径与方式;从热力学的角度探讨了胞外电子传递过程的能量变化,从动力学的角度探讨了胞外电子传递的传质与速率;介绍了若干胞外电子传递的研究方法;并提出了今后需要重点关注的重要科学问题。

The process of microbial extracellular electron transfer（EET）is an important driving force of element cycling and energy exchange in epigeosphere. While the previous studies focused on the interaction between soil particles and ions,recently,the biogeochemical processes of the EET among microbe-humusmineral received widespread attention. The current EET studies enlightened us with new insights into the epigeosphere from the perspectives of chemistry and microbiology. Since microbes,humus and minerals are very essential factors of the biogeochemical processes on earth surface system via their interactive redox reactions,the main aim of this review is to reveal the detailed mechanism of the EET among microbe-humus-mineral and illustrate their biogeochemical significances on the earth surface system. The paper introduces,first,pathways via which electrons flow from inside to outside of a microbial cell,and then,two pathways via which electrons transfer from the surface of microbes to humus and minerals：（i）direct electron transfer,including direct contact and nanowires;（ii）indirect electron transfer mediated by humus,including＂electron shuttling processes＂and processes of bonding between humus and membrane c-type cytochromes.In this review,based on the key processes and key factors of the thermodynamics,energy transport processes of the whole EET chain of the microbe-humus-mineral system was discussed on a theoretical basis. The importance of redox state of c-type cytochromes on EET was highlighted through those discussions,which suggests that the standard redox potential（E0）and electron transfer capacity（ETC）of humus play dominant roles in the humus-mediated electron shuttling processes. Furthermore,the mass transfer and reaction rates under molecule level are also analyzed using a kinetic approach,which suggests that mediated nanowirenetwork-mediated electron transfer might be the most efficient way for facilitating EET processes.In this field,there are several new technical means available to solve the key scientific issues,including：（i）spectroelectrochemistry,combining electrochemistry and spectroscopy,is a useful approach for correlating thermodynamics and kinetics;（ii）molecular biology techniques are essential for recognizing the functional proteins responsible for EET processes;（iii）high-resolution imaging techniques are very conducive to the study on micro-structure of the nanowires;and（iv）time-resolved techniques are essential to determination of the rapid reaction occurring in the EET processes.To sum up,the future studies in this field should encompass the following four aspects：（i）studies related to extracellular respiring bacteria,which may help build a complete picture of the bacterial community,and will be helpful for the reorganization of other unknown strains;（ii）The summary on the functions of the proteins responsible for EET will help understanding their roles in this EET process;（iii）The discussion on humus and minerals,especially their structure,can improve the understanding of their functional mechanism and highlight their microbial ecological significances;（iv）The modeling of EET processes from thermodynamics and kinetics can provide a quantitative understanding of the intrinsic factors controlling EET processes.