Biodegradation and Abiotic Degradation of Trifluralin: A Commonly Used Herbicide with a Poorly Understood Environmental Fate.

Trifluralin is a widely used dinitroaniline herbicide, which can persist in the environment and has substantial ecotoxicity, especially to aquatic organisms. Trifluralin is very insoluble in water (0.22 mg/L at 20 °C) and highly volatile (vapor pressure of 6.7 mPa at 20 °C); these physicochemical properties determine a large part of its environmental fate, which includes rapid loss from soils if surface-applied, strong binding to soil organic matter, and negligible leaching into water. The trifluralin structure contains a tertiary amino group, two nitro-groups and a trifluoromethyl- group. Despite the strongly xenobiotic character of some of these substituents, biodegradation of trifluralin does occur, and pure cultures of bacteria and fungi capable of partially degrading the molecule either by dealkylation or nitro-group reduction have been identified. There are many unanswered questions about the environmental fate and metabolism of this herbicide; the genes and enzymes responsible for biodegradation are largely unknown, the relative roles of abiotic processes vs growth-linked biodegradation vs cometabolism are unresolved, and the impact of different environmental factors on the rates and extents of biodegradation are not clear. Here, we summarize the relevant literature on the persistence and environmental fate of trifluralin with a focus on biodegradation pathways and mechanisms, and we identify the current major knowledge gaps for future research.

Co-occurrence of genes for aerobic and anaerobic biodegradation of dichloroethane in organochlorine-contaminated groundwater.

1,2-Dichloroethane (DCA) is a problematic groundwater pollutant. Factors influencing the distribution and activities of DCA-degrading bacteria are not well understood, which has hampered their application for bioremediation. Here, we used quantitative PCR to investigate the distribution of putative DCA-dehalogenating bacteria at a DCA-impacted site in Sydney (Australia). The dehalogenase genes dhlA, tceA and bvcA were detected in all groundwater samples (n = 15), while vcrA was found in 11/15 samples. The 16S rRNA gene sequences specific to the dehalogenating genera Dehalobacter, Desulfitobacterium and Dehalogenimonas were detected in 15/15, 13/15 and 13/15 samples, respectively, while Dehalococcoides sequences were found in 9/15 samples. The tceA, bvcA and vcrA genes occurred in the same samples as Dehalococcoides and Dehalobacter. Microcosm experiments confirmed the presence of bacteria capable of dechlorination under anoxic conditions. The abundance of the dhlA gene, which is found in hydrolytic DCA degraders, was positively correlated to the DCA concentration, and was unexpectedly most abundant in samples with low oxygen conditions. A dhlA-containing bacterium isolated from the site (Xanthobacter EL8) was capable of anaerobic growth on DCA under denitrifying conditions. The presence of diverse DCA-dehalogenating bacteria at this site indicates that natural attenuation or biostimulation could be valid approaches for site cleanup.