These results demonstrate that the pili of G. sulfurreducens permit increased stacking of cells on the anode surface and that there is a corresponding increase in current production. The cells not in direct contact with the anode appear to contribute to current production because they remain viable and the per-cell efficiency of current production does not decrease as the biofilm develops and the height of the pillars increases. This has important implications for the design of microbial fuel cells because it demonstrates that it is possible to enhance current production not only by increasing the surface area of the anode but also by increasing the number of cells contributing to electron flow on a given surface. This facilitates packing more electricity-producing microorganisms into a given volume.
As shown here, and consistent with results of a previous study (7
), pili are not required for the lower levels of power production which can be generated from cells in intimate contact with anode surfaces. Under these conditions electron transfer to the anode is likely to be accomplished via outer membrane c
-type cytochromes, such as OmcS (7
). Electron transfer to electrodes via outer membrane c
-type cytochromes has also been hypothesized for other organisms (8
). However, electron transfer via this mechanism would be possible only for cells intimately associated with the anode surface.
It seems likely that pili promote long-range electron transfer across the multilayer biofilms on anodes because the pili are electrically conductive (16
). Pili as long as 20 μm have been observed on G. sulfurreducens
. Therefore, in some instances the pili of cells at a distance from the anode could potentially make electrical contact either with the anode surface or with the cells coating the anode. Cell-to-cell electron transfer via intertwined pili (16
) might establish a “nano power grid” in which the pilus network transfers electrons through the 40- to 50-μm-thick anode biofilm. Furthermore, it is well known that in other organisms pili play an important role in the early stages of biofilm formation (15
), as well as cell cluster maturation (9
), on a variety of surfaces. Therefore, it is conceivable that G. sulfurreducens
pili have other, unknown functions in biofilm differentiation on anodes.
The fact that G. sulfurreducens
and other organisms can directly transfer electrons to electrodes is remarkable, since it is unlikely that there has ever been any evolutionary pressure on these organisms to make electricity in the natural environment. The ability of G. sulfurreducens
to transfer electrons to electrodes may be attributed to its capacity to transfer electrons to Fe(III) oxides, which are also insoluble, extracellular electron acceptors. However, the stacking of G. sulfurreducens
on anodes has no known counterpart in Fe(III) oxide reduction in sedimentary environments, where it is not advantageous for the cells to permanently attach to the Fe(III) oxides (12
). These considerations suggest that there may be substantial opportunities to increase long-range electron transfer to anodes with genetic engineering and/or adaptive evolution.