On the electrical conductivity of microbial nanowires and biofilms

Long-range electron transport in Geobacter sulfurreducens biofilms is redox gradient-driven

“…Outside the cell, both Shewanella and Geobacter secrete nanometer scale diameter, micrometer scale long proteinaceous filaments, referred to as pili and microbial nanowires (7), that extend from their outer surfaces into the extracellular matrix (ECM) thought to be involved in extracellular electron transport processes, including cell-to-cell electron transfer (8) and reduction of insoluble oxidants (9). Ex situ conductivity measurements of individual filaments of Shewanella oneidensis MR-1 confirm their lengthwise conductivity when isolated from cells under specific conditions (10,11), and subsequent modeling of their current-voltage characteristics is consistent with multistep electron hopping involving redox cofactors proposed to be associated with these filaments (12)(13)(14). The ECM of both species, however, contain many proteins other than those comprising pili, including a number of c-type cytochromes, and their contributions to extracellular electron transport have not been determined under physiologically relevant conditions owing largely to the lack of information regarding their spatial organization.…”

“…g is an empirical factor invoked here to describe the degree of heterogeneity among cofactors with respect to formal potential. When g = 1, all of the cofactors have the same formal potential, and a smaller value of g indicates greater heterogeneity among cofactors, resulting in a wider potential range, over which the oxidation state of the biofilm in the vicinity of the electrode surface changes from being fully oxidized to fully reduced (14,62). In Fig.…”

“…3A, 39.3 μA), which occurs when ðX Ox Þ z=0;1 = 1. i L is dependent on the IDA geometry, growth state of the biofilm (19), concentration of redox cofactors in the biofilm, and rate constant for electron exchange between adjacent cofactors (14). If electron exchange between electrode 1 and cofactors at the electrode surface is fast (20), then ðX Ox Þ z=0;1 can be determined from the potential applied to electrode 1, E 1 , by a modified version of the Nernst Equation (Eq.…”

“…A predicted feature of long-range multistep electron hopping is the presence of a redox gradient, in which the local oxidation state of the biofilm decreases with increasing distance from the anode surface (14) (that is, the local concentration of electrons residing in biofilm cofactors is expected to be higher farther from the anode surface). This gradient is expected to result from reduction of cofactors by cells distributed across the biofilm coupled to cellular oxidation of organic matter and oxidation of cofactors at the anode surface, where the potential applied to the anode determines the local oxidation state of the biofilm in the vicinity of the anode surface.…”

Microbial nanowires: an electrifying tale

“…Though MNWs from both micro-organisms are TFP, the molecular mass of their subunits (~10 kDa for G. sulfurreducens,~22 kDa for Synechocystis) and dimensions (width/length: 3-5 nm/ (a) 10-20 µm and 4.5-7 nm/2-10 µm for G. sulfurreducens and Synechocystis, respectively) differ from each other (Lovley et al, 2009;Lovley, 2011;Sure et al, 2015). In G. sulfurreducens, cytochromes are found to be associated with MNWs and its role in electron transfer through MNWs is disputed StrycharzGlaven et al, 2011;Strycharz-Glaven & Tender, 2012). It needs to be explored whether Synechocystis MNWs are embedded with cytochromes and the potential role of the latter in electron transfer.…”