These results provide insights into the evolution of Geobacteraceae species into different environmental niches and biotechnological applications. The results suggest that the last common ancestor of the Geobacteraceae was an acetate-oxidizing, respiratory species capable of extracellular electron transfer, and that specialization for fermentative/syntrophic growth evolved at least twice, allowing some Geobacteraceae to fill additional niches.
The primacy of the respiratory mode is evident from the conservation of genes for all steps in this process including acetate uptake, central metabolism, and electron transfer across the cell membranes in both of the clades of the family. The fermentative/syntrophic Pelobacter species also contain many of these genes. However, they have lost several key enzymes that leave the pathways incomplete, including several necessary for the oxidation of acetate and most of the cytochromes predicted to provide the electrical connection between the inner membrane and the outside of the cell. Instead, the Pelobacter species have appropriated genes via lateral gene transfer for fermentative/syntrophic growth. It is clear that this has happened on two separate occasions. Although both P. carbinolicus and P. propionicus have closely related dehydrogenase genes for the initial metabolism of their unique substrates, acetoin and 2,3-butanediol, the genes for the further fermentation of these substrates are unrelated in the two organisms, reflecting the separate evolution of distinct metabolic pathways. The fact that P. carbinolicus also fills a syntrophic niche, participating in interspecies hydrogen transfer with hydrogen-consuming methanogens, whereas P. propionicus does not, may be explained by the genes associated with reverse electron transfer that only P. carbinolicus has appropriated. The fact that both Pelobacter species have phylogenetically distinct hydrogenase genes that are different from each other as well as those of the Geobacter species, may also reflect the difference in syntrophic capabilities of these species.
The selective pressure to specialize in syntrophic/fermentative growth may have initially been found at the interface of redox boundaries in sedimentary environments. As respiratory Geobacteraceae
deplete the supply of the terminal electron acceptor Fe(III) oxide, their capacity for growth is greatly diminished and organisms with other respiratory processes, such as sulfate reduction or methane production, become predominant [2
]. Some Geobacter
species can oxidize acetate to carbon dioxide and hydrogen when they lack external electron acceptors [44
], but the slow rate of this metabolism and the requirement for very low hydrogen partial pressures means that they are not competitive with acetate-utilizing sulfate reducers or methanogens. Acquiring the ability to ferment novel substrates and/or to grow syntrophically could have facilitated expansion into Fe(III) oxide depleted environments. Under such conditions investing energy in the production of respiratory enzymes such as the c
-type cytochromes Geobacteraceae
require to grow under Fe(III)-reducing conditions would be maladaptive.
These findings also provide insight into the types of metabolic changes that might take place as these organisms are being adapted for modern biotechnical applications. In applications such as the in situ
bioremediation of uranium-contaminated groundwater and the conversion of organic compounds to electricity Geobacter
species must deal with a scarcity of electron acceptor because electron donor is generally provided well in excess of electron acceptor availability. During in situ
bioremediation Fe(III) oxides are rapidly depleted near the source of subsurface acetate amendments, limiting the growth and effectiveness of Geobacter
-catalyzed U(VI) reduction [8
species form thick biofilms on the electrodes of microbial fuel cells, forcing many of the cells to metabolize acetate at a significant distance from this artificial electron acceptor [45
]. Preliminary results suggest that, like the Pelobacter
species described here, Geobacteraceae
that predominate during in situ
uranium reduction or on the anodes of microbial fuel cells have fewer c
-type cytochromes (DRL, unpublished data). Enhanced ability to release excess electrons as hydrogen, in a manner similar to that of P. carbinolicus
, could also be beneficial under conditions in which electron acceptor availability is limiting. Thus, these relatively few changes appear to have allowed a respiratory ancestor to radiate out into fermentative and syntrophic niches in addition to their respiratory roles in anaerobic environments. This information serves as a guide to the history of these organisms and provides information that could aid in optimizing their biotechnological applications.