Abstract
Studies on the mechanisms for extracellular electron transfer in Geobacter species have primarily focused on Geobacter sulfurreducens, but the poor conservation of genes for some electron transfer components within the Geobacter genus suggests that there may be a diversity of extracellular electron transport strategies among Geobacter species. Examination of the gene sequences for PilA, the type IV pilus monomer, in Geobacter species revealed that the PilA sequence of Geobacter uraniireducens was much longer than that of G. sulfurreducens. This is of interest because it has been proposed that the relatively short PilA sequence of G. sulfurreducens is an important feature conferring conductivity to G. sulfurreducens pili. In order to investigate the properties of the G. uraniireducens pili in more detail, a strain of G. sulfurreducens that expressed pili comprised the PilA of G. uraniireducens was constructed. This strain, designated strain GUP, produced abundant pili, but generated low current densities and reduced Fe(III) very poorly. At pH 7, the conductivity of the G. uraniireducens pili was 3 × 10-4 S/cm, much lower than the previously reported 5 × 10-2 S/cm conductivity of G. sulfurreducens pili at the same pH. Consideration of the likely voltage difference across pili during Fe(III) oxide reduction suggested that G. sulfurreducens pili can readily accommodate maximum reported rates of respiration, but that G. uraniireducens pili are not sufficiently conductive to be an effective mediator of long-range electron transfer. In contrast to G. sulfurreducens and G. metallireducens, which require direct contact with Fe(III) oxides in order to reduce them, G. uraniireducens reduced Fe(III) oxides occluded within microporous beads, demonstrating that G. uraniireducens produces a soluble electron shuttle to facilitate Fe(III) oxide reduction. The results demonstrate that Geobacter species may differ substantially in their mechanisms for long-range electron transport and that it is important to have information beyond a phylogenetic affiliation in order to make conclusions about the mechanisms by which Geobacter species are transferring electrons to extracellular electron acceptors.
Highlights
The presence of Geobacter species is often equated with processes in which the capacity for long-range electron transfer via electrically conductive pili (e-pili) is an advantageous feature (Lovley et al, 2011)
The concept that Geobacter species rely on e-pili for long-range electron transport is based on a rather limited dataset from studies primarily conducted with G. sulfurreducens
Unlike G. sulfurreducens and G. metallireducens, G. uraniireducens did not produce the high current densities that have been attributed to electrically conductive pili, and G. uraniireduccens could not participate in Direct interspecies electron transfer (DIET) (Rotaru et al, 2015). These results suggest that G. uraniireducens might not rely on conductive pili for extracellular electron transfer
Summary
The presence of Geobacter species is often equated with processes in which the capacity for long-range electron transfer via electrically conductive pili (e-pili) is an advantageous feature (Lovley et al, 2011). Direct interspecies electron transfer (DIET) in anaerobic digesters has been attributed to an abundance and high metabolic activity of Geobacter species (Morita et al, 2011; Rotaru et al, 2014b; Shrestha et al, 2014). The concept that Geobacter species rely on e-pili for long-range electron transport is based on a rather limited dataset from studies primarily conducted with G. sulfurreducens. This species has been the focus of most studies because it was the first Geobacter species for which a genetic system (Coppi et al, 2001) and genome sequence (Methé et al, 2003) became available and because G. sulfurreducens produces high current densities (Nevin et al, 2008; Yi et al, 2009). Efforts to genetically manipulate other Geobacter species have as yet been unsuccessful
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