Geobacter strains expressing poorly conductive pili reveal constraints on direct interspecies electron transfer mechanisms

Cytochrome-to-cytochrome electron transfer and electron transfer
along conduits of multiple extracellular magnetite grains are often proposed as
strategies for direct interspecies electron transfer (DIET) that do not require electrically
conductive pili (e-pili). However, physical evidence for these proposed
DIET mechanisms has been lacking. To investigate these possibilities further, we
constructed Geobacter metallireducens strain Aro-5, in which the wild-type pilin
gene was replaced with the aro-5 pilin gene that was previously shown to yield
poorly conductive pili in Geobacter sulfurreducens strain Aro-5. G. metallireducens
strain Aro-5 did not reduce Fe(III) oxide and produced only low current densities,
phenotypes consistent with expression of poorly conductive pili. Like G. sulfurreducens
strain Aro-5, G. metallireducens strain Aro-5 displayed abundant outer surface
cytochromes. Cocultures initiated with wild-type G. metallireducens as the
electron-donating strain and G. sulfurreducens strain Aro-5 as the electronaccepting
strain grew via DIET. However, G. metallireducens Aro-5/G. sulfurreducens
wild-type cocultures did not. Cocultures initiated with the Aro-5 strains of
both species grew only when amended with granular activated carbon (GAC), a
conductive material known to be a conduit for DIET. Magnetite could not substitute
for GAC. The inability of the two Aro-5 strains to adapt for DIET in the absence
of GAC suggests that there are physical constraints on establishing DIET
solely through cytochrome-to-cytochrome electron transfer or along chains of
magnetite. The finding that DIET is possible with electron-accepting partners
that lack highly conductive pili greatly expands the range of potential electronaccepting
partners that might participate in DIET.