Link between capacity for current production and syntrophic growth in Geobacter species.

Amelia-Elena Rotaru,Trevor L Woodard,Kelly P Nevin,Derek R Lovley

doi:10.3389/fmicb.2015.00744

Abstract

Electrodes are unnatural electron acceptors, and it is yet unknown how some Geobacter species evolved to use electrodes as terminal electron acceptors. Analysis of different Geobacter species revealed that they varied in their capacity for current production. Geobacter metallireducens and G. hydrogenophilus generated high current densities (ca. 0.2 mA/cm2), comparable to G. sulfurreducens. G. bremensis, G. chapellei, G. humireducens, and G. uraniireducens, produced much lower currents (ca. 0.05 mA/cm2) and G. bemidjiensis was previously found to not produce current. There was no correspondence between the effectiveness of current generation and Fe(III) oxide reduction rates. Some high-current-density strains (G. metallireducens and G. hydrogenophilus) reduced Fe(III)-oxides as fast as some low-current-density strains (G. bremensis, G. humireducens, and G. uraniireducens) whereas other low-current-density strains (G. bemidjiensis and G. chapellei) reduced Fe(III) oxide as slowly as G. sulfurreducens, a high-current-density strain. However, there was a correspondence between the ability to produce higher currents and the ability to grow syntrophically. G. hydrogenophilus was found to grow in co-culture with Methanosarcina barkeri, which is capable of direct interspecies electron transfer (DIET), but not with Methanospirillum hungatei capable only of H2 or formate transfer. Conductive granular activated carbon (GAC) stimulated metabolism of the G. hydrogenophilus – M. barkeri co-culture, consistent with electron exchange via DIET. These findings, coupled with the previous finding that G. metallireducens and G. sulfurreducens are also capable of DIET, suggest that evolution to optimize DIET has fortuitously conferred the capability for high-density current production to some Geobacter species.

Highlights

Geobacter species are among the most effective microorganisms for harvesting electrical current from organic compounds (Call and Logan, 2011; Lovley et al, 2011; Kumar et al, 2015)
The electrodes that serve as the electron acceptors in current production are not found in the soils and sediments that are the natural habitat of Geobacter species
For routine cultivation G. metallireducens, G. humireducens, G. hydrogenophilus, G. bremensis, G. bemidjiensis, and G. sulfurreducens were provided with 50 mM Fe(III) citrate as electron acceptor, as previously described (Lovley and Phillips, 1988; Coates et al, 2001; Straub and Buchholz-Cleven, 2001; Nevin et al, 2005; Tremblay et al, 2012), with the exception that Fe(III) citrate was added from an anaerobic, sterile stock after the medium was sterilized

Summary

Introduction

Geobacter species are among the most effective microorganisms for harvesting electrical current from organic compounds (Call and Logan, 2011; Lovley et al, 2011; Kumar et al, 2015). Electrodes and Fe(III) oxides are both extracellular electron acceptors and it is possible that the evolution of Geobacter species to excel at Fe(III) oxide reduction yielded characteristics for effective current production.

Results

Conclusion