It's Evolutionary: Selective Pressure Yields Microbe Strain Capable of Enhanced Electricity Production

The wild type Geobacter sulfurreducens, strain DL-1, produces current densities in microbial fuel cells that are among the highest known for pure cultures. However, as far as is known, this capacity for current production is fortuitous because there has not been any previous selective pressure on this microorganism to produce current. In order to determine if selective pressure might enhance the capacity for current production DL-1 was cultured in a system in which a graphite anode was poised at -400 mV (versus Ag/AgCl). After five months of selective pressure a strain of Geobacter sulfurreducens, designated KN400 was recovered. KN400 was much more effective in current production than strain DL-1. This was apparent with anodes poised at -400 mV, as well as in systems run in true fuel cell mode. KN400 had current (7.6 A/m2) and power (3.9 W/m2) densities that respectively were substantially higher than those of DL1 (1.4 A/m2 and 0.5 W/m2). On a per cell basis KN400 was more effective in current production than DL1, requiring thinner biofilms to make equivalent current. The enhanced capacity for current production in KN400 was associated with a greater abundance of electrically conductive microbial nanowires than DL1 and lower internal resistance (0.015 vs. 0.130 ohms/m2) and mass transfer limitation in KN400 fuel cells. KN400 produced flagella, whereas DL1 does not. Surprisingly, KN400 had much less outer-surface c-type cytochromes than DL1. KN400 also had a greater propensity to form biofilms on glass or graphite than DL1, even when growing with the soluble electron acceptor, fumarate. These results demonstrate that it is possible to enhance the ability of microorganisms to electrochemically interact with electrodes with the appropriate selective pressure and that improved current production is associated with clear differences in the properties of the outer surface of the cell that may provide insights into the mechanisms for microbe-electrode interactions.

Reference

| PubMed: 19487117 | PDF | Press info and photos |

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UMass Press Release

Photos (click on image for high resolution version)

Dr. Hana Yi takes a reading from fuel cells

Dr. Kelly Nevin isolates strains in anaerobic chamber

Co-authors Dr. Hana Yi (left), and Dr. Kelly Nevin

Co-authors Dr. Anna Klimes (left), and Dr. Ashley Franks

Transmission electron micrograph illustrating abundant pili of strain KN400

Transmission electron micrograph illustrating flagella of KN400 grown in an anode biofilm

Thin sectioning TEM images from fast freeze fixation illustrating similar cell size and cell wall thickness

Heme-stain of SDS PAGE of outer-surface protein preparations from KN400 and DL1; 2 µg outer surface proteins per lane

Western blot analysis of structural pilin protein, PilA, in KN400 and DL1 harvested at 10 mA current from anodes poised at +300 mV; 10 µg proteins per lane

Confocal scanning laser microscopy images of biofilms grown in fuel cell mode of KN400 (A and C) on platinum wire (A) or graphite (C) anode and comparable biofilms of DL1 on platinum wire (B) or graphite (D) anodes. Biofilms of KN400 (E) and DL1 (F) grown on graphite sticks poised at +300 mV and imaged when current was 10 mA, 70% of maximum for DL1. Computer generated three-dimensional representation (3D-projection) of the entire biofilms across the platinum wire surface. Single CSLM sections (XYZ-projection) of horizontal biofilms sections (large panels) and vertical sections (right and bottom panels)

Confocal scanning laser micrographs of biofilms attached to graphite or glass coupons. Numbers inside image are 1010cells/m2. Images are 6.25 x 10-10 m2

Crystal violet staining of cells attached to culture tubes in cultures of KN400 and DL1 grown with acetate as the electron donor and fumarate as the electron acceptor