Abstract
It remains elusive if direct interspecies electron transfer (DIET) occurs in canonical syntrophy involving short-chain fatty acids oxidation. In the present study, we determined the effects of carbon nanomaterials on syntrophic oxidation of butyrate in two lake sediment enrichments and a defined coculture comprising Syntrophomonas wolfei and Methanococcus Maripaludis. After four continuous transfers of enrichment cultivation, Syntrophomonas dominated the bacterial populations in enrichments, and the dominated methanogens comprised Methanosarcina and Methanospirillum in one enrichment (from Weiming Lake) and Methanoregula and Methanospirillum in another (from Erhai Lake). Butyrate oxidation and CH4 production was significantly accelerated by carbon nanotubes (CNTs) in both enrichments. Replacement of CNTs by magnetite caused similar stimulating effect. For the defined coculture, two carbon nanomaterials, CNTs and reduced graphene oxide (rGO), were tested, both showed consistently stimulating effects on butyrate oxidation. Addition of kaolinite, an electric nonconductive clay mineral, however, revealed no effect. The test on M. maripaludis in pure culture showed no effect by rGO and a negative effect by CNTs (especially at a high concentration). Fluorescence in situ hybridization (FISH) and scanning electron microscopy (SEM) revealed that microbial cells were interwoven by CNTs forming cell-CNT mixture aggregates, and in case of rGO, cells were attached to surface or wrapped-up by rGO thin sheets. Collectively, our data suggest that the presence of conductive nanomaterials likely induces DIET in syntrophic butyrate oxidation.