Geobacter sulfurreducens utilizes extracellular electron acceptors such as Mn(IV), Fe(III), syntrophic partners, and electrodes that vary from +0.4 to −0.3 V versus standard hydrogen electrode (SHE), representing a potential energy span that should require a highly branched electron transfer chain. Here we describe CbcBA, a bc‐type cytochrome essential near the thermodynamic limit of respiration when acetate is the electron donor. Mutants‐lacking cbcBA ceased Fe(III) reduction at −0.21 V versus SHE, could not transfer electrons to electrodes between −0.21 and −0.28 V, and could not reduce the final 10%–35% of Fe(III) minerals. As redox potential decreased during Fe(III) reduction, cbcBA was induced with the aid of the regulator BccR to become one of the most highly expressed genes in G. sulfurreducens. Growth yield (CFU/mM Fe(II)) was 112% of WT in ∆cbcBA, and deletion of cbcL (an unrelated bc‐cytochrome essential near −0.15 V) in ΔcbcBA increased yield to 220%. Together with ImcH, which is required at high redox potentials, CbcBA represents a third cytoplasmic membrane oxidoreductase in G. sulfurreducens. This expanding list shows how metal‐reducing bacteria may constantly sense redox potential to adjust growth efficiency in changing environments.