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

We used a reactive transport model to investigate the cycling of geogenic arsenic (As) in a petroleum-contaminated aquifer. We simulated As mobilization and sequestration using surface complexation reactions with Fe(OH)3 during petroleum biodegradation coupled with Fe-reduction. Model results predict that dissolved As in the plume will exceed the U.S. and EU 10 µg/L drinking water standard for ~400 years. Non-volatile dissolved organic carbon (NVDOC) in the model promotes As mobilization by exerting oxygen demand, which maintains anoxic conditions in the aquifer. After NVDOC degrades, As re-associates with Fe(OH)3 as oxygenated conditions are re-established. Over the 400-year simulation, As transport resembles a “roll front” in which: (1) arsenic sorbed to Fe(OH)3 is released during Fe-reduction coupled to petroleum biodegradation; (2) dissolved As resorbs to Fe(OH)3 at the plume’s leading edge; and (3) over time, the plume expands, and resorbed As is re-released into groundwater. This “roll front” behavior underscores the transience of sorption as an As attenuation mechanism. Over the plume’s lifespan, simulations suggest that As will contaminate more groundwater than benzene from the oil spill. At its maximum, the model simulates that ~5.7× more groundwater will be contaminated by As than benzene, suggesting that As could pose a greater long-term water quality threat than benzene in this petroleum-contaminated aquifer.

26Geobacter 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 high-to 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. 36Mutants 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, 40the 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 44 peer-reviewed)

Coupled abiotic and biotic processes in the hyporheic zone, where surface water and groundwater mix, play a critical role in the biogeochemical cycling of carbon, nutrients, and trace elements in...

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