Comparative transcriptomic insights into the mechanisms of electron transfer in Geobacter co-cultures with activated carbon and magnetite

Both activated carbon and magnetite have been reported to promote the syntrophic growth of Geobacter metallireducens and Geobacter sulfurreducens co-cultures, the first model to show direct interspecies electron transfer (DIET); however, differential transcriptomics of the promotion on co-cultures with these two conductive materials are unknown. Here, the comparative transcriptomic analysis of G. metallireducens and G. sulfurreducens co-cultures with granular activated carbon (GAC) and magnetite was reported. More than 2.6-fold reduced transcript abundances were determined for the uptake hydrogenase genes of G. sulfurreducens as well as other hydrogenases in those co-cultures to which conductive materials had been added. This is consistent with electron transfer in G. metallireducens-G. sulfurreducens co-cultures as evinced by direct interspecies electron transfer (DIET). Transcript abundance for the structural component of electrically conductive pili (e-pili), PilA, was 2.2-fold higher in G. metallireducens, and, in contrast, was 14.9-fold lower in G. sulfurreducens in co-cultures with GAC than in Geobacters co-cultures without GAC. However, it was 9.3-fold higher in G. sulfurreducens in co-cultures with magnetite than in Geobacters co-cultures. Mutation results showed that GAC can be substituted for the e-pili of both strains but magnetite can only compensate for that of G. sulfurreducens, indicating that the e-pili is a more important electron acceptor for the electron donor strain of G. metallireducens than for G. sulfurreducens. Transcript abundance for G. metallireducens c-type cytochrome gene GMET_RS14535, a homologue to c-type cytochrome gene omcE of G. sulfurreducens was 9.8-fold lower in co-cultures with GAC addition, while that for OmcS of G. sulfurreducens was 25.1-fold higher in co-cultures with magnetite, than in that without magnetite. Gene deletion studies showed that neither GAC nor magnetite can completely substitute the cytochrome (OmcE homologous) of G. metallireducens but compensate for the cytochrome (OmcS) of G. sulfurreducens. Moreover, some genes associated with central metabolism were up-regulated in the presence of both GAC and magnetite; however, tricarboxylic acid cycle gene transcripts in G. sulfurreducens were not highly-expressed in each of these amended co-cultures, suggesting that there was considerable redundancy in the pathways utilised by G. sulfurreducens for electron transfer to reduce fumarate with the amendment of GAC or magnetite. These results support the DIET model of G. metallireducens and G. sulfurreducens and suggest that e-pili and cytochromes of the electron donor strain are more important than that of the electron acceptor strain, indicating that comparative transcriptomics may be a promising route by which to reveal different responses of electron donor and acceptor during DIET in co-cultures