Degradation of 1,2,3,4-Tetrachlorobenzene by <i>Pseudomonas chlororaphis</i> RW71

ABSTRACT Pseudomonas chlororaphis RW71 mineralized 1,2,3,4-tetrachlorobenzene, a highly recalcitrant pollutant hitherto not known to be degraded by pure cultures, as a sole source of carbon and energy, thereby releasing stoichiometric amounts of chloride. The transient excretion of tetrachlorocatechol in the early growth phase suggests an initial attack by a dioxygenase to form the corresponding dihydrodiol which rearomatizes to the catechol. The activity of chlorocatechol 1,2-dioxygenase in crude cell extracts was found to be extraordinarily high towards 3-chlorocatechol (ratio of 2.6 compared to catechol) and other chlorocatechols, including tetrachlorocatechol, which was transformed at a low but significant rate. Further identification of tetrachloromuconic acid, 2,3,5-trichlorodienelactone, 2,3,5-trichloromaleyl acetic acid, and 2,4-dichloro-3-oxoadipic acid as their methyl esters, together with high specific enzyme activities for chlorinated substrates, implicated a functioning chlorocatechol pathway to be induced during growth. </jats:p

Chlorocatechols Substituted at Positions 4 and 5 Are Substrates of the Broad-Spectrum Chlorocatechol 1,2-Dioxygenase of Pseudomonas chlororaphis RW71

The nucleotide sequence of a 10,528-bp region comprising the chlorocatechol pathway gene cluster tetRtetCDEF of the 1,2,3,4-tetrachlorobenzene via the tetrachlorocatechol-mineralizing bacterium Pseudomonas chlororaphis RW71 (T. Potrawfke, K. N. Timmis, and R.-M. Wittich, Appl. Environ. Microbiol. 64:3798–3806, 1998) was analyzed. The chlorocatechol 1,2-dioxygenase gene tetC was cloned and overexpressed in Escherichia coli. The recombinant gene product was purified, and the α,α-homodimeric TetC was characterized. Electron paramagnetic resonance measurements confirmed the presence of a high-spin-state Fe(III) atom per monomer in the holoprotein. The productive transformation by purified TetC of chlorocatechols bearing chlorine atoms in positions 4 and 5 provided strong evidence for a significantly broadened substrate spectrum of this dioxygenase compared with other chlorocatechol dioxygenases. The conversion of 4,5-dichloro- or tetrachlorocatechol, in the presence of catechol, displayed strong competitive inhibition of catechol turnover. 3-Chlorocatechol, however, was simultaneously transformed, with a rate similar to that of the 4,5-halogenated catechols, indicating similar specificity constants. These novel characteristics of TetC thus differ significantly from results obtained from hitherto analyzed catechol 1,2-dioxygenases and chlorocatechol 1,2-dioxygenases

Microbial Growth on Dichlorobiphenyls Chlorinated on Both Rings as a Sole Carbon and Energy Source

We have isolated bacterial strains capable of aerobic growth on ortho-substituted dichlorobiphenyls as sole carbon and energy sources. During growth on 2,2′-dichlorobiphenyl and 2,4′-dichlorobiphenyl strain SK-4 produced stoichiometric amounts of 2-chlorobenzoate and 4-chlorobenzoate, respectively. Chlorobenzoates were not produced when strain SK-3 was grown on 2,4′-dichlorobiphenyl

Effects of Solvent Composition and Hydrogen Pressure on the Catalytic Conversion of 1,2,4,5-Tetrachlorobenzene to Cyclohexane

Toward the development of a “green” technology for cleaning soil contaminated by halogenated hydrophobic organic contaminants, here we demonstrate that combined use of palladium (Pd) and rhodium (Rh) catalysts enables the conversion of 1,2,4,5-tetrachlorobenzene (TeCB) to cyclohexane in mixtures of water and ethanol. We tested the hypotheses that, in batch reactors, (1) an increased ratio of water to ethanol in water/ethanol solvents would increase the reaction rates of both Pd-catalyzed hydrodehalogenation (HDH) and Rh-catalyzed hydrogenation, and (2) catalytic reaction rate coefficients would be constant above a hydrogen (H2) pressure threshold, but would decrease with decreasing H2 pressure below that threshold. These hypotheses were derived from a Langmuir–Hinshelwood model for the heterogeneous catalytic reactions. Complete conversion of TeCB to cyclohexane was achieved at all experimental conditions tested, suggesting that the proposed technology may be technically viable. Concentration data were consistent with an apparent first-order kinetic model in which Pd-catalyzed HDH and Rh-catalyzed hydrogenation occur in series. As expected, HDH and hydrogenation rate coefficients increased as the fraction of water in the solvent increased. However, contrary to expectations, HDH rate coefficients decreased when H2 pressure increased from 69 to 207 to 345 kPa. We attributed this to the displacement of TeCB by H2 on the catalyst surface at higher H2 pressures. No statistically significant effect of H2 pressure on hydrogenation rate coefficients was observed. The findings suggest that the proposed technology should be operated with at least 50% water in the solvent and a H2 pressure as low as 30–70 kPa