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 (HMWS) 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 HMWS can accelerate the IET between these two species. Results showed that the average ethanol consumption rate and methanogenic rate in the HMWS-containing co-culture were 1.6-fold and 2.9-fold higher than that without HMWS, indicating that HMWS enhanced the cooperation of S. oneidensis MR-1 and M. barkeri. Further experiment suggested that except for interspecies acetate transfer, an alternative HMWS-mediated IET pathway occurred between M. barkeri and S. oneidensis MR-1. The estimated total IET rate in the HMWS-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 HMWS. CO/CO and NO/NH2 were the functional groups of HMWS contributing to HMWS-mediated IET, and flavin and cytochrome c of microorganisms participated in the reduction and oxidation of HMWS. 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.