How the electrons released by organic carbon oxida­tion in the bacterial anaerobic cytoplasm are transferred by the biofilm to the anode surface is an important factor in MFC performance (Neto et al., 2010). Major advances

have been made between 2000 and 2010 in understand­ing the electron transfer mechanisms by electrogens. There are two primary mechanisms: one is the direct electron transfer (DET) and the other, mediated electron transfer (MET).

DET for Anodic Biofilms

DET occurs via a direct physical contact between the microbial cell wall and the anode surface, or via a pilus that links the two. Gene expression studies (Holmes et al., 2008) and electrochemical analysis (Busalmen et al., 2008) have demonstrated that there are active sites for cytochrome proteins on the outer cell surface (Franks and Nevin, 2010; Zhou et al., 2012). When the microbes contact the anode surface, the cytochromes can transfer the electrons from the inside of the microbial cell wall to the outer cell wall and then to the anode surface (Rinaldi et al., 2008) (Figure 9.2(a)). Shewanella putrefaciens (Kim et al., 2002), R. ferrireducens (Chaudhuri and Lovley, 2003) and G. sulfurreducens (Bond and Lovley, 2003) use such cytochromes to achieve electron transfer. One major disadvantage for this mechanism is that only a monolayer of sessile cells in a biofilm can transfer elec­trons to the anode surface. This explains why the power and current densities of MFCs relying on this kind of DET are lower, sometimes by several orders of magni­tude, than that of MFCs with MET (Schroder, 2007) because MET can utilize more than one monolayer.

Recently, some researchers have observed that some microbial strains (such as Shewanella oneidensis and G. sulfurreducens; Logan and Regan, 2006; Torres et al.,

2010) can produce pili (conductive nanowires) to form physical conductive connections between the cell wall and the anode surface while the microbial cell wall is at a short distance from the anode (Reguera et al., 2005; Rinaldi et al., 2008). An extensive pilus network would allow several layers of sessile cells to donate elec­trons to the anode, thus multiplying the MFC power output. Summers et al. found a whole cell aggregate

(b)

FIGURE 9.2 Different DET methods: (a) direct cell wall-electrode contact (b) conductive pili or filament linkage. (For color version of this figure, the reader is referred to the online version of this book.)
consisting of G. sulfurreducens and Geobacter metalliredu — cens is conductive when the coculture was grown on ethanol. The c-type cytochrome OmcS of G. sulfurredu — cens was suspected to play a key role in accepting elec­trons from G. metallireducens (Summers et al., 2010). This overcomes the inability of G. sulfurreducens to use H2 for interspecies electron transfer. The ability of inter­species electron exchange suggests that a nonelectro­genic species in a synergistic biofilm consortium may contribute to electricity generation as long as its elec­trons can be taken up by an electrogenic species through interspecies electron transfer. In addition to H2, other molecules such as formate may act as an electron shuttle for biofilm communities (Morita et al., 2011).

An exciting new discovery by Pfeffer et al. (2012) provides new hope for greatly enhancing electron trans­fer in microorganisms. They found that some filamen­tous bacteria in marine sediments are capable of transferring electrons over centimeter-long distances via conductive filaments that are 200 nM or wider in diameter. This distance is far greater than the much thinner pili could achieve. This indicates that potentially many more layers of sessile cells could be networked via the conductive filaments than pili could achieve. Figure 9.2 is a schematic illustration of DET via direct cell wall—electrode contact and via a pili or conductive filament linkage.