Electron transfer mechanism of extracellular respiration:a review

It has been well known that microbes can generate energy utilizing various strategies.However,only recently,it has become clear that a growing number of microbes involve electron to or from substrates.This novel microbial metabolism is so termed as extracellular which microbes oxidize organic matters to carbon dioxide and conserve energy for microbial growth coupling with of electrons and protons along the respiratory chain(a series of enzymes that function to transport protons and electrons inside the cells) to reduce electron acceptors.Different from the conventional aerobic and anaerobic respirations,the unique characteristic of respiration is its capability to use substrates as terminal electron acceptors.The well-known examples are Fe(Ⅲ)/Mn(Ⅳ) respiration,humus respiration and electricigenic respiration,in which iron/manganese(hydr)oxides,humic substances and solid electrodes are served as the electron acceptors.Extracellular electron transfer is defined as the process in which electrons derived from the oxidation of electron donors are transferred to the outer surface of the cell to reduce an terminal electron acceptor.In the first step for electrons travelling between cells and substrates,electrons are required to be transferred through electron carriers in the periplasm and come across the non-conductive cell walls,which involves various functional genes and protein complexes(such as multi-hemes c-type cytochromes inside the cell).Afterwards,electrons can be directly or indirectly transferred from the outer membrane to the electron acceptors via different mechanisms.In general,three main electron mechanisms have been proposed:(1) direct electron to electron acceptor via membrane-bounded cytochromes;(2) direct electron to electron acceptor through conductive bacterial pili(nanowires);(3) indirect electron to electron acceptor by redox mediator that can shuttle between cells and substrates.The finding of respiration provides a new perspective for understanding of the diversity and evolution of microbial respiration.Due to their environmental significance and practical application,it has received a great deal of attention at present.Oxidation of organic matters coupled with the reduction of Fe(Ⅲ) and Mn(IV) oxides plays an important role in the carbon,iron,and manganese cycles in sedimentary environments,and also influences the fate of a diversity of trace metals and phosphate.Anaerobic oxidation of organic contaminants with the reduction of Fe(Ⅲ) is important in groundwater bioremediation and stimulating dissimilatory metal reduction has shown promise as a method for immobilizing toxic metals in the subsurface.In addition,oxidation of organic matters coupled with electron to electrodes is also a potential strategy for harvesting electricity from the environmental and organic wastes.To date,many efforts have been made to identify how electrons are transferred to the electron acceptors and the factors controlling the rate and extent of this process.An improved understanding of electron mechanism in respiration is still needed to optimize practical applications and to better model natural processes.In this review,we summarized the types of respirations and the diversity of respiratory bacteria.In particular,we mainly focused on the electron processes and the molecular mechanisms underlying the whole pathway involved.