The Low Conductivity of Geobacter uraniireducens Pili Suggests a Diversity of Extracellular Electron Transfer Mechanisms in the Genus Geobacter

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.

The electrically conductive pili of Geobacter species are a recently evolved feature for extracellular electron transfer

“…This hypothesis is supported by the finding that G. metallireducens, which also has a short PilA, produces e-pili (Y. Tan, unpublished), whereas the pili of Geobacter uraniireducens, which has a long PilA typical of that found in many micro-organisms, produces pili of low conductivity (Tan et al, 2016).…”

“…These truncated pilA genes are predicted to encode mature pilin structural proteins with only 60-90 amino acids, compared with the >120 amino acid residues typically found in PilA subunits of most other bacteria (Table 1). In the few instances in which pili conductivity has been directly measured, truncated pilA genes code for proteins that give rise to conductive pili (Geobacter sulfurreducens and Geobacter metallireducens), whereas longer pilA genes yield pili with poor conductivity (Geobacter uraniireducens and Pseudomonas aeruginosa) (Liu et al, 2014;Tan et al, 2016). Therefore, the truncated pilA genes were designated e-pilin to distinguish them from longer type IVa pilA genes that are more commonly found in bacteria.…”

“…This gene was annotated as 'pilA-C' in the Geobacter sulfurreducens genome, with the expectation that its protein was a component of the Geobacter sulfurreducens pilin protein. However, this annotation is incorrect because denatured pili do not contain a protein of the predicted size (Reguera et al, 2005;Tan et al, 2016;Veazey et al, 2011).…”

“…Tan et al, unpublish ed). The pili of the only other species of the genus Geobacter examined, Geobacter uraniireducens, are poorly conductive (Tan et al, 2016). G. uraniireducens functions without e-pili because it utilizes a soluble electron shuttle to reduce Fe(III) oxides, does not form thick electrically conductive biofilms and does not participate in direct interspecies electron transfer (Rotaru et al 2015;Tan et al, 2016).…”

Resilience, Dynamics, and Interactions within a Model Multispecies Exoelectrogenic-Biofilm Community

“…Both Geobacter strains depend on short-range electron transfer via direct contact and/or long-range electron transfer via conductive pili. For Geobacteraceae, the latter seems to be an important or even necessary feature for thick and highly conductive biofilms and for participation in direct interspecies electron transfer (DIET), since G. uraniireducens, a strain that produces pili with low conductivity, cannot participate in DIET and produces thin and poorly conductive biofilms (35).…”