Geobacter lovleyi sp. nov. Strain SZ, a Novel Metal-Reducing and Tetrachloroethene-Dechlorinating Bacterium

Abstract

A bacterial isolate, designated strain SZ, was obtained from noncontaminated creek sediment microcosms based on its ability to derive energy from acetate oxidation coupled to tetrachloroethene (PCE)-to-cis-1,2-dichloroethene (cis-DCE) dechlorination (i.e., chlororespiration). Hydrogen and pyruvate served as alternate electron donors for strain SZ, and the range of electron acceptors included (reduced products are given in brackets) PCE , respectively, with acetate as the electron donor. Alternate electron acceptors, such as U(VI) and nitrate, did not inhibit PCE dechlorination and were consumed concomitantly. With PCE, Fe(III) (as ferric citrate), and nitrate as electron acceptors, H 2 was consumed to threshold concentrations of 0.08 ؎ 0.03 nM, 0.16 ؎ 0.07 nM, and 0.5 ؎ 0.06 nM, respectively, and acetate was consumed to 3.0 ؎ 2.1 nM, 1.2 ؎ 0.5 nM, and 3.6 ؎ 0.25 nM, respectively. Apparently, electron acceptor-specific acetate consumption threshold concentrations exist, suggesting that similar to the hydrogen threshold model, the measurement of acetate threshold concentrations offers an additional diagnostic tool to delineate terminal electronaccepting processes in anaerobic subsurface environments. Genetic and phenotypic analyses classify strain SZ as the type strain of the new species, Geobacter lovleyi sp. nov., with Geobacter (formerly Trichlorobacter) thiogenes as the closest relative. Furthermore, the analysis of 16S rRNA gene sequences recovered from PCE-dechlorinating consortia and chloroethene-contaminated subsurface environments suggests that Geobacter lovleyi belongs to a distinct, dechlorinating clade within the metal-reducing Geobacter group. Substrate versatility, consumption of electron donors to low threshold concentrations, and simultaneous reduction of electron acceptors suggest that strain SZ-type organisms have desirable characteristics for bioremediation applications.Environmental pollutants, such as chlorinated solvents, toxic metals, and radionuclides (e.g., uranium), are strictly regulated due to their negative effects on ecosystem function and human health. Tetrachloroethene (PCE) and trichloroethene (TCE) are frequently used as solvents and degreasing agents. As a result of extensive usage, improper disposal, and accidental spills, PCE and TCE became widely distributed, pervasive groundwater contaminants. Uranium was released in significant amounts from nuclear weapon complexes managed by the U.S. Department of Energy (DOE) during the Cold War arms race. It was estimated that more than 80% of DOE sites have radionuclide contamination and at least 27% have volatile organic hydrocarbons as cocontaminants (47).The considerable knowledge that has accrued on the fate of specific contaminants and the bacteria involved in the transformation and degradation pathways has led to successful bioremediation field studies. For instance, acetate additions to stimulate dissimilatory metal-reducing organisms promoted reduction of soluble U(VI) to insoluble U(IV) and contributed to plume containment (2,...

Chlorine Isotope Effects from Isotope Ratio Mass Spectrometry Suggest Intramolecular C-Cl Bond Competition in Trichloroethene (TCE) Reductive Dehalogenation

“…No mechanism is known which would convert 1,1-dichloroethene into 1,2-dichloroethene intermediates so that cis-DCE can only be formed by cleavage of a C-Cl bond in the geminal α-positions and not in the β-position (see Scheme 3). Otherwise 1,1-dichloroethene would be detected which is not the case for Geobacter lovleyi strain SZ [33]. A primary isotope effect is therefore expected in the reacting bond in α-position and a secondary isotope effect in the other germinal bond in α-position that is not cleaved.…”

“…strain Viet1, a PCE-to-TCE dechlorinator [32], were used to determine the typical magnitude of primary and secondary chlorine isotope effects. Further, cultures of Geobacter lovleyi strain SZ, a PCE/TCE-to-cis-DCE dechlorinator [33], were used to specifically investigate whether one or two C-Cl bonds are reactive in TCE as non-symmetric molecule. Mathematical equations were derived for reactant and product isotope ratios to model chlorine isotope trends and to extract primary and secondary chlorine isotope effects.…”

Overview of organohalide-respiring bacteria and a proposal for a classification system for reductive dehalogenases

“…OHRB have been identified from diverse bacterial phyla, including the Proteobacteria, Firmicutes and Chloroflexi [5][6][7][8]. The known OHRB can be grouped as either obligate or non-obligate organohalide respirers [9].…”

“…Analysis of genome sequences divides the OHRB into those whose genomes contain one or two rdhA genes and those containing several (more than two) different rdhA [5,64]. Genomes of Desulfitobacterium, Dehalococcoides, Dehalogenimonas and Dehalobacter strains contain multiple rdhA genes.…”

Advancement of Nucleic Acid-Based Tools for Monitoring in Situ Reductive Dechlorination

“…The new isolate was designated Geobacter lovleyi strain SZ (Sung et al 2006b). Strain SZ also reduced soluble and mobile hexavalent uranium, U(VI), to immobile U(IV) (uraninite).…”