Under anoxic condition, two or more different species of microorganisms often work collaboratively to oxidize organic compounds for their growth of which the individual microorganism alone cannot accomplish (i.e., microbial syntrophy). Microbial syntrophy is widespread around world and plays crucial roles in global carbon cycling, such as methane production and oxidation. Interspecies electron transfers are essential for microbial syntrophy. However, the molecular mechanisms underlying interspecies electron transfers during microbial syntrophy remain largely unknown.
       Previous investigation showed that electron transfers from Geobacter metallireducens to Geobacter sulfurreducens during their syntrophic coculture included acetate-mediated interspecies electron transfer(AIET), H₂-mediated interspecies electron transfer (HIET) and direct interspecies electron transfers (DIET). Thus, the syntrophic coculture of G. metallireducens-G. sulfurreducens was used a model system to systematically investigate the molecular mechanisms of interspecies electron transfers.
       The results show the involvements of acetate-transporting genes aorA and aplA of G. metallireducens and aplA and aplC of G. sulfurreducens in the AIET from G. metallireducens to G. sulfurreducens and the involvements of hydrogenase gene hoxS of G. metallireducens and hybL of G. sulfurreducens in the HIET from G. metallireducens to G. sulfurreducens. Many genes are involved in the DIET from G. metallireducens to G. sulfurreducens and whose gene products may form pathways for transferring electrons from the periplasm of G. metallireducens to the cytoplasmic membrane of G. sulfurreducens. In these proposed pathways, the Pcc complexes Gmet0825-0828 and Gmet0908-0913 on the outer membrane of G. metallireducens transfer electrons from the periplasm and across the outer membrane to the extracellular pili and/or c-type cytochrome (c-Cyt) Gmet2896 of G. metallireducens. Extracellular pili and c-Cyt Gmet2896 of G. metallireducens transfer the received electrons to extracellular pili and c-Cyts OmcS, OmcT and OmcZ of G. sulfurreducens, which then relay the electrons to the Pcc complexes OmaB-OmbB-OmcB and OmaC-OmbC-OmcC on the outer membrane of G. sulfurreducens. OmaB-OmbB-OmcB and OmaC-OmbC-OmcC transfer the electron across the outer membrane inward to the periplasmic c-Cyt OmcX. Finally, OmcX transfers electrons to the c-Cyt CbcL in the cytoplasmic membrane where CbcL reduces quinone to quinol.
 

interspecies electron transfers,microbial syntrophy,Geobacter metallireducens,Geobacter sulfurreducens,coculture