Reductive dechlorination of chlorinated ethenes and 1, 2-dichloroethane by "Dehalococcoides ethenogenes" 195

Abstract

"Dehalococcoides ethenogenes" 195 can reductively dechlorinate tetrachloroethene (PCE) completely to ethene (ETH). When PCE-grown strain 195 was transferred (2% [vol/vol] inoculum) into growth medium amended with trichloroethene (TCE), cis-dichloroethene (DCE), 1,1-DCE, or 1,2-dichloroethane (DCA) as an electron acceptor, these chlorinated compounds were consumed at increasing rates over time, which indicated that growth occurred. Moreover, the number of cells increased when TCE, 1,1-DCE, or DCA was present. PCE, TCE, 1,1-DCE, and cis-DCE were converted mainly to vinyl chloride (VC) and then to ETH, while DCA was converted to ca. 99% ETH and 1% VC. cis-DCE was used at lower rates than PCE, TCE, 1,1-DCE, or DCA was used. When PCE-grown cultures were transferred to media containing VC or trans-DCE, products accumulated slowly, and there was no increase in the rate, which indicated that these two compounds did not support growth. When the intermediates in PCE dechlorination by strain 195 were monitored, TCE was detected first, followed by cis-DCE. After a lag, VC, 1,1-DCE, and trans-DCE accumulated, which is consistent with the hypothesis that cis-DCE is the precursor of these compounds. Both cis-DCE and 1,1-DCE were eventually consumed, and both of these compounds could be considered intermediates in PCE dechlorination, whereas the small amount of trans-DCE that was produced persisted. Cultures grown on TCE, 1,1-DCE, or DCA could immediately dechlorinate PCE, which indicated that PCE reductive dehalogenase activity was constitutive when these electron acceptors were used.

FIG. 1

(a) VC formation by cultures that were inoculated with PCE-grown strain 195 and received H₂ and one of the chloroethenes or DCA. Substrates were added in increasing incremental doses beginning with 0.3 mmol per liter. The total amounts of the substrates utilized by the cultures were as follows: PCE, 2.5 mmol/liter; TCE, 2.5 mmol/liter; 2.5 cis-DCE, 0.67 mmol/liter; trans-DCE, 0.07 mmol/liter; 1,1-DCE, 2.85 mmol/liter; VC, 0.17 mmol/liter; and DCA, 1.8 mmol/liter. (Inset) Plot with an expanded scale, showing VC formation from trans-DCE and ETH formation from VC. (b) ETH production by the cultures shown in panel a. Note the expanded scale for ETH formation from DCA.

FIG. 2

(a) Product formation by PCE-grown strain 195 inoculated into medium to which one dose of PCE (0.7 mmol/liter) was added. (b) Plot with an expanded scale, showing intermediates in PCE metabolism, including individual DCE isomers.

FIG. 3

Intermediate formation by TCE-grown strain 195 inoculated into medium to which a single dose consisting of 0.35 mmol of TCE per liter was added.

FIG. 4

Product formation from a 0.35-mmol/liter dose of trans-DCE after cultures of strain 195 were fed three consecutive doses of PCE (0.3, 0.5, and 0.7 mmol/liter). DCE utilization by a culture that was autoclaved after day 5 is also shown (dashed line).

FIG. 5

Product formation by strain 195 growing on DCA. Note the differences in the scales for VC and ETH.

FIG. 6

TCE (a) and PCE (b) utilization and VC formation by TCE-grown cultures of strain 195. The cultures received three previous doses of TCE, which was converted mainly to VC (note the different scale for VC).