Humin accelerates interspecies electron transfer between Shewanella oneidensis MR-1 and Methanosarcina barkeri

Existing researches involving accelerated interspecies electron transfer (IET) by solid redox mediators focus mainly on the conductive nature of these materials. Although non-conductive solid redox mediator-humin has been reported to promote methanogenic performance of anaerobic granular sludge, likely through accelerating IET of microorganisms, this phenomenon has not been validly proven. In this study, a wetland sediment sourced HM (HM_WS) was added into a co-culture of a syntrophic bacteria Shewanella oneidensis MR-1 and an archaeal Methanosarcina barkeri with ethanol as sole electron donor to examine whether HM_WS can accelerate the IET between these two species. Results showed that the average ethanol consumption rate and methanogenic rate in the HM_WS-containing co-culture were 1.6-fold and 2.9-fold higher than that without HM_WS, indicating that HM_WS enhanced the cooperation of S. oneidensis MR-1 and M. barkeri. Further experiment suggested that except for interspecies acetate transfer, an alternative HM_WS-mediated IET pathway occurred between M. barkeri and S. oneidensis MR-1. The estimated total IET rate in the HM_WS-containing co-culture was 0.53 ± 0.1 meq/(d^-1·g cell-dry weight^-1), which was 2.2-fold higher than that without HM_WS. CO/CO and NO/NH₂ were the functional groups of HM_WS contributing to HM_WS-mediated IET, and flavin and cytochrome c of microorganisms participated in the reduction and oxidation of HM_WS. The findings are of significance for understanding the diverse IET occurred in natural environments and providing a novel strategy for renewable bioenergy processes with high efficiency.