Microbial oxidation of tri-halogenated phenols - Multi-element isotope fractionation

Abstract

The tri-halogenated phenols, 2,4,6-trichlorophenol (TCP) and 2,4,6-tribromophenol (TBP), are known as groundwater contaminants. Indigenous bacteria may degrade these compounds aerobically or anaerobically, with the aerobic process initiated by ring hydroxylation at the para position. Stable isotope analysis can be proposed as a tool for determining biodegradation process of these compounds in the environment. However, to date isotope fractionation in microbial oxidation of trihalogenated phenols has not been reported. This work aimed to investigate carbon and halogen (chlorine or bromine) isotope effects associated with the process by two microbial strains: Achromobacter piechaudii strain TBPZ which is capable of degrading both TCP and TBP, and Ralstonia eutropha strain JMP 134 which was extensively studied in the past to characterize the TCP oxidation mechanism. Low carbon isotope effects were observed in all degradation experiments. Opposite fractionation trends were detected for TCP and TBP by strain TBPZ (εC = −0.51 ± 0.28‰ and εC = +0.68 ± 0.26‰, respectively). Strain JMP presented a low inverse carbon isotope effect for TCP oxidation (εC = +0.26 ± 0.13‰), contrary to that of strain TBPZ. For the halogen atoms, low yet significant normal halogen isotope effects were detected in all cases (−0.49 to −1.43‰ for chlorine, and −0.42‰ for bromine). The results suggest that the observed isotope effects do not reflect the ring hydroxylation. Instead, they likely represent rate-limiting steps preceding the catalysis of hydroxylation. Dual-element isotope effects observed in this study did not enable distinguishing between oxidation and reduction of TCP. The oxidation and reduction dual-element trends were distinct for TBP. However, it is uncertain if this difference can be extrapolated from the laboratory to the field, since the isotope effects in TBP degradation are strongly influenced by slow steps prior to hydroxylation.