Carbon isotope effects in cometabolic oxidation of halogenated organics by a methanotroph

Cometabolic oxidation is a process in which compounds, including groundwater pollutants such as halogenated aliphatics, are fortuitously oxidized by enzymes with broad substrate specificity. Assessing the magnitude of cometabolic oxidation in contaminated environments is challenging; however, it may be facilitated using Compound-Specific Isotope Analysis. Our former work on the cometabolic oxidation of trichloroethene (TCE) revealed a unique isotope pattern when the process was catalyzed by several methanotrophs rather than by toluene and ammonia oxidizers. In the current work, we aimed to study isotope effects in the cometabolic oxidation of other halogenated compounds of environmental interest by the methanotroph Methylosinus trichosporium OB3b. Results showed relatively small carbon isotope fractionation for cis-dichloroethene (cDCE; AKIE = 1.0017 ± 0.0016) and bromoform (BF; AKIE = 1.0020 ± 0.0014), which are similar to formerly determined value for TCE. Larger carbon isotope effects were recorded for chloroform (CF; AKIE = 1.0038 ± 0.0007) and dichloromethane (DCM; AKIE = 1.0043 ± 0.0009), possibly implying a decrease in the role of enzyme binding as a bottleneck for these two. Nevertheless, while DCM presented both relatively large carbon isotope effects and rapid degradation, CF presented relatively large carbon isotope effects but degraded slower than cDCE and TCE. This is possibly related to the reported toxicity of CF's byproducts, which reduces the overall degradation rate.