Microbial extracellular electron transfer (EET) occurring in natural and engineering processes is attracting increasing interests. While a meaningful question for bioenergetics, microbial physiology and microbial electrochemical systems; less is known about the lower limit of electron acceptor reduction potential for EET. It is also unclear how microbes adapt to weak electron acceptors. This study evaluated Geobacter sulfurreducens biofilms grown with an electrode poised at -0.25 V vs. SHE. This potential was found to be sufficient for microbial metabolism and proliferation. The turnover cyclic voltammetries found that these biofilms had a half-saturation potential of -0.242 +/- 0.004 V, in contrast to -0.151 +/- 0.003 V for that of the biofilms grown under 0.2 V. For the biofilms grown under 0.2 V, differential pulse voltammetry showed that the metabolic current was mediated by interfacial cofactors with mid-point potential around -0.16 V performing single-electron electron transfer (ET). The major electron conduits for the biofilms respiring under -0.25 V had mid-point potentials of -0.22 V or -0.26 V, which appeared to perform two-electron ET. Under the non-turnover condition, both biofilms showed similar patterns in voltammograms and the low-potential conduits largely disappeared for the biofilms grown under -0.25 V. Transcriptome analysis identified 17 cytochrome-c genes significantly up-regulated for the biofilms grown under -0.25 V, together with many other genes linked to the ET system. It was also noted that, lowering the poised potential from -0.25 V to -0.28 V (the fuel standard oxidation potential) did not fully inhibit microbial respiration. (C) 2016 Elsevier Ltd. All rights reserved.