Aerobic cometabolic biodegradation of chlorinated aliphatic hydrocarbons by mixed cultures grown on methane or propane and by a propane-utilizing Rhodococcus bacterial strain

In the first part of this study, the long-term aerobic cometabolic biodegradation of a mixture of chlorinated aliphatic hydrocarbons (CAHs) including vinyl chloride (VC), cis- and trans-1,2-dichloroethylene (cis-DCE and trans-DCE), trichloroethylene (TCE), 1,1,2-trichloroethane (1,1,2-TCA) and 1,1,2,2-tetrachloroethane (1,1,2,2-TeCA) was investigated at both 25 °C and 17 °C by means of slurry microcosm tests, utilizing methane or propane as growth substrates. The efficacy of 2 types of inocula was tested. The lag-phases for the onset of the CAH mixture cometabolic transformation were significantly shorter and more reproducible in the inoculated tests (0-15 days at 25 °C) than in the non-inoculated ones (36-264 days at 25 °C). Biodegradation of the 6-CAH mixture was successfully continued for up to 410 days by adding subsequent pulses of the CAH mixture in the following concentration ranges: VC, 0 – 40 microM; cis-DCE and trans-DCE, 0 – 6 microM; TCE, 0 – 2.7 microM; 1,1,2-TCA, 0 – 1.1 microM; 1,1,2,2-TeCA, 0-0.7 microM. The cometabolic transformation of the 6 CAHs generally followed, within the studied concentration ranges, a first-order kinetic, with kinetic constants ranging, at 25 °C, from 0.3-1.1 1/day for 1,1,2,2-TeCA to 7.5-30 1/day for VC. We consider of particular interest the long-term aerobic transformation of 1,1,2,2-TeCA, a compound that is generally considered non biodegradable in aerobic conditions. In addiction to the kinetic studies we also conducted a molecular analysis based on the 16S rDNA sequencing of a propane-utilizing consortium (INP3) found to be of particular interest due to its degradation abilities. The study succeeded in identifying the different bacterial species composing the microbial community. \nIn the second part of the study, we investigated the capacity of one of the four bacterial strains that were found to be prevailing in the propane-utilizing consortia to transform single CAHs in resting-cell assays performed after growth on propane. The strain, belonging to the genus Rhodococcus, proved capable to deplete VC (studied concentration range: 0 – 165 microM), cis-DCE (0 – 390 microM), trans-DCE (0 – 1.4 microM), TCE (0 – 0.7 microM), 1,1,2-TCA (0 – 190 microM) and chloroform (0 – 500 microM), but not 1,1,2,2-TeCA. The cometabolic transformation of these solvents was successfully interpolated with a Monod-type model comprising a term of biomass inactivation due to the transformation products toxicity. The estimated maximum specific rates ranged between 1 micromol / (mg protein day) (for 1,1,2-TCA) and 11 micromol / (mg protein day) (for VC), whereas the estimated half-saturation constants varied from 49 microM (CF) to 96 microM (cis-DCE). \nThe results of this study confirm the effectiveness of methane and propane as growth substrates for the aerobic cometabolic biodegradation of CAHs.