Evidence for involvement of an electron shuttle in electricity generation by Geothrix fermentans

Abstract

In experiments performed using graphite electrodes poised by a potentiostat (+200 mV versus Ag/AgCl) or in a microbial fuel cell (with oxygen as the electron acceptor), the Fe(III)-reducing organism Geothrix fermentans conserved energy to support growth by coupling the complete oxidation of acetate to reduction of a graphite electrode. Other organic compounds, such as lactate, malate, propionate, and succinate as well as components of peptone and yeast extract, were utilized for electricity production. However, electrical characteristics and the results of shuttling assays indicated that unlike previously described electrode-reducing microorganisms, G. fermentans produced a compound that promoted electrode reduction. This is the first report of complete oxidation of organic compounds linked to electrode reduction by an isolate outside of the Proteobacteria.

FIG. 1.

Acetate disappearance (•), electron recovery predicted on the basis of acetate disappearance (○), electron recovery on the basis of integrated amperage (solid line), and current production (inset) for a representative microbial fuel cell containing G. fermentans attached to a graphite electrode. The anode was connected to a graphite cathode (in aerated water) via a 500-Ω resistor.

FIG. 2.

SEM image of a graphite anode after growth of G. fermentans in a microbial fuel cell.

FIG. 3.

Fe(III)-oxide reduction by G. sulfurreducens resting cell suspensions incubated with supernatants from G. fermentans fuel cells. Effects of supernatants from two 7-day-old fuel cells (▵, ▴) and two 35-day-old fuel cells (□, ▪) are shown, with the effects of 25 μM (♦) and 50 μM (⋄) AQDS shown for comparison.

FIG. 4.

Overvoltages for anodes and cathodes in a representative G. fermentans fuel cell over a range of current densities. Characteristics were measured 7 days after medium replacement (▵, ▴), in the same fuel cell 35 days after medium replacement (○, •), and with 50 μM AQDS addition 1 day later (□, ▪). Values closer to zero as the rate of current flow increases indicate improvements in operating cell voltage.