Fig. 4. Redox properties of cytochromes in model electroactive organisms and syntrophic consortia.

Electron transport routes and the reduction potentials of key components in (A) Geobacter, (B) Shewanella, (C) syntrophic consortia of SRB and methanogen based on interspecies hydrogen transfer, and (D) syntrophic consortia of ANME and SRB based on direct interspecies electron transport. Black lines indicate the midpoint reduction potentials. Gray bars indicate the reduction potential windows of multiheme cytochromes. The midpoint reduction potentials of cytochromes are more positive than their electron acceptors, but their broad redox-active windows allow reactions to occur. (E) Compilation of reduction potential windows show window size increase with increasing number of hemes in purified cytochromes. The large multiheme cytochrome c (MHC) in ANME/SRB consortia could have even broader redox-active windows to facilitate electron transport to electron acceptors at a large range of reduction potentials.