Substrate interactions during aerobic biodegradation of methane, ethene, vinyl chloride and 1,2-dichloroethenes

Intrinsic biodegradation of trichloroethene and 1,1,1-trichioroethane in gr oundwater at a Superfund site in California has been observed. An anaerobic zone exists in the area closest to the source location, yielding the expec ted complement of reductive dechlorination daughter products, including cis -1,1-dichloroethene (cis-DCE) and vinyl chloride (VC). Significant levels o f methane and ethene were also generated in the anaerobic zone. The groundw ater returns to aerobic conditions downgradient of the source, with methane , ethene, VC, and several other compounds still present. Attenuation of VC in the aerobic zone suggests that it is being biodegraded. In this study mi crocosms were used to evaluate the role of methane and ethene as primary su bstrates for aerobic biodegradation of VC. Biodegradation of VC was fastest in the bottles containing ethene, with 40 mu mol of VC consumed over a 150 day period, compared to approximately 15-20 mu mol with methane or a mixtu re of methane and ethene. VC did not noticeably inhibit ethene biodegradati on but did slow the rate of methane use. Methane inhibited ethene metabolis m, which apparently caused a reduction in VC biodegradation when methane wa s present with ethene. These results suggest that ethene plays an important role during in situ natural attenuation of VC under aerobic conditions. Mi crocosms were also set up with VC alone. Following a 75 day lag period, VC consumption began and subsequent additions were consumed without a lag, sug gesting the presence of organisms capable of using VC as a growth substrate . After providing VC alone for nearly 400 days, aliquots of the enrichment culture were used to evaluate its ability to biodegrade cis- and trans-DCE. Both compounds were readily consumed, although addition of VC as the prima ry substrate was needed to sustain biodegradation of repeated additions. Th is result suggests that organisms capable of using VC as a sole substrate m ay play an active role in aerobic natural attenuation of DCEs.