Mechanisms for Electron Transfer Through Pili to Fe(III) Oxide in Geobacter
- Univ. of Massachusetts, Amherst, MA (United States)
The purpose of these studies was to aid the Department of Energy in its goal of understanding how microorganisms involved in the bioremediation of metals and radionuclides sustain their activity in the subsurface. This information is required in order to incorporate biological processes into decision making for environmental remediation and long-term stewardship of contaminated sites. The proposed research was designed to elucidate the mechanisms for electron transfer to Fe(III) oxides in Geobacter species because Geobacter species are abundant dissimilatory metal-reducing microorganisms in a diversity of sites in which uranium is undergoing natural attenuation via the reduction of soluble U(VI) to insoluble U(IV) or when this process is artificially stimulated with the addition of organic electron donors. This study investigated the novel, but highly controversial, concept that the final conduit for electron transfer to Fe(III) oxides are electrically conductive pili. The specific objectives were to: 1) further evaluate the conductivity along the pili of Geobacter sulfurreducens and related organisms; 2) determine the mechanisms for pili conductivity; and 3) investigate the role of pili in Fe(III) oxide reduction. The studies demonstrated that the pili of G. sulfurreducens are conductive along their length. Surprisingly, the pili possess a metallic-like conductivity similar to that observed in synthetic organic conducting polymers such as polyaniline. Detailed physical analysis of the pili, as well as studies in which the structure of the pili was genetically modified, demonstrated that the metallic-like conductivity of the pili could be attributed to overlapping pi-pi orbitals of aromatic amino acids. Other potential mechanisms for conductivity, such as electron hopping between cytochromes associated with the pili were definitively ruled out. Pili were also found to be essential for Fe(III) oxide reduction in G. metallireducens. Ecological studies demonstrated that electron conduction along pili is a better strategy for Fe(III) oxide reduction under conditions found in the subsurface than producing an electron shuttle. The role of pili in uranium reduction was also elucidated. Our results are the first example of metallic-like conductivity in a biological protein and represent a paradigm shift in the understanding of long-range biological electron transport. The results are of importance not only for understanding subsurface microbial processes involved in the mobility of metal contaminants and carbon cycling, but also make a basic contribution to microbiology and the emerging field of bioelectronics.
- Research Organization:
- Univ. of Massachusetts, Amherst, MA (United States). Dept. of Microbiology
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- SC0004114
- OSTI ID:
- 1172030
- Report Number(s):
- DOE-UMASS-04114
- Country of Publication:
- United States
- Language:
- English
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