Dissimilatory metal-reducing bacteria can oxidize intracellular organic compounds coupled with transferring electrons to terminal electron acceptors through extracellular electron transfer. These processes play a key role in degradation of organic pollutants and migration/transformation of heavy metals. Three pathways of extracellular electron transfer by dissimilatory metal reducing bacteria have been well recognized, including direct contact between the outer membrane and electron acceptors, electron shuttling, and nanowire-mediated electron transfer. While it is well known that Shewanella species interacts with electron acceptors primarily via flavins as soluble electron shuttles, it is unclear whether there are other processes of transferring electrons to the terminal electron acceptors.
Recently, the research groups of Prof. Tongxu Liu and Prof. Fangbai Li from Institute of Eco-environmental and Soil Sciences, Guangdong Academy of Sciences have very good progress on the new processes of extracellular electron transfer. They discovered that, the outer membrane c-types cytochrome (c-Cyts) of Shewanella MR-1 can also be released into the environment, and act as an electron shuttle to facilitate extracellular electron transfer. In this study, MR-1 cells were initially incubated under anoxic conditions without any electron-acceptors, and it was found that MR-1 could release not only small molecules of riboflavin but also soluble c-Cyts in solution. By using kinetic modeling analysis, they discovered that released c-Cyts could act as electron mediators to accelerate electron transfer from cells to Cr(VI), and the relative contribution of this pathway was higher than that mediated by flavins.
This study reveals a new mechanism that Shewanella can release organic compounds to support extracellular electron transfer, and highlights the geochemical importance of this process in environments lacking available electron acceptors. This research was published in the top journal of environmental science Environmental Science & Technology (IF = 7.864), and chosen as the supplementary cover.
Tongxu Liu, Xiaobo Luo, Yundang Wu, John R. Reinfelder, Xiu Yuan, Xiaomin Li, Dandan Chen, and Fangbai Li*, Extracellular electron shuttling mediated by soluble c-type cytochromes produced by Shewanella oneidensis MR-1, Environmental Science & Technology. 2020, 54, 17, 10577–10587.