Title : Redox potential as a master variable controlling pathways of metal reduction by Geobacter 1 sulfurreducens 2 3 Running Title : Fe ( III ) reduction by G . sulfurreducens

26 Geobacter sulfurreducens uses at least two different pathways to transport electrons out of the 27 inner membrane quinone pool before reducing acceptors beyond the outer membrane. When growing 28 on electrodes poised at oxidizing potentials, the CbcL-dependent pathway operates at or below redox 29 potentials of -0.10 V vs. the Standard Hydrogen Electrode (SHE), while the ImcH-dependent pathway 30 operates only above this value. Here, we provide evidence that G. sulfurreducens also requires different 31 electron transfer proteins for reduction of a wide range of Fe(III)and Mn(IV)(oxyhydr)oxides, and must 32 transition from a highto low-potential pathway during reduction of commonly studied soluble and 33 insoluble metal electron acceptors. Freshly precipitated Fe(III)-(oxyhydr)oxides could not be reduced by 34 mutants lacking the high potential pathway. Aging these minerals by autoclaving did not change their 35 powder X-ray diffraction pattern, but restored reduction by mutants lacking the high-potential pathway. 36 Mutants lacking the low-potential, CbcL-dependent pathway had higher growth yields with both soluble 37 and insoluble Fe(III). Together, these data suggest that the ImcH-dependent pathway exists to harvest 38 additional energy when conditions permit, and CbcL switches on to allow respiration closer to 39 thermodynamic equilibrium conditions. With evidence of multiple pathways within a single organism, 40 the study of extracellular respiration should consider not only the crystal structure or solubility of a 41 mineral electron acceptor, but rather the redox potential, as this variable determines the energetic 42 reward affecting reduction rates, extents, and final microbial growth yields in the environment. 43