Trichloroethylene degradation by methane-oxidizing cultures grown with various nitrogen sources

Methane-oxidizing microorganisms exhibit great potential for vadose zone bioremediation since they grow on a gaseous substrate, are capable of cometabolically degrading a wide range of common subsurface contaminants, and fix molecular nitrogen as a nitrogen source. This paper reports the effects of supplying nitrogen as nitrate, ammonia, and molecular nitrogen on the growth, trichloroethylene (TCE) degradation capacity, and energy storage capacity of a mixed methane-oxidizing culture. Cells inoculated from a nitrate-supplied methane-oxidizing culture grew fastest while fixing atmospheric nitrogen when oxygen partial pressures were kept less than 8%. Cell growth and methane oxidation were more rapid for ammonia-supplied cells than for nitrate-supplied or nitrogen-fixing cells. However, nitrogen-fixing cells were capable of oxidizing TCE as efficiently as nitrate- or ammonia-supplied cells, and they exhibited the highest TCE transformation capacity of all three cultures both with and without formate as an exogenous reducing energy source. The TCE product toxicity was not as pronounced for the nitrogen fixing cells as for the nitrate- or ammonia-supplied cells after exposure to high (20 mg/L) or low (2 mg/L) TCE concentrations. Energy storage in the form of poly-beta-hydroxybutyrate was 20% to 30% higher for nitrogen-fixing cells; increased energy storage may be responsible for the higher transformaton capacity of nitrogen-fixing cells when no external reducing energy was available. However, because nitrogen-fixing cells also exhibited the highest transformation capacity in the presence of formate, their cometabolic activity was enhanced beyond that which can be explained by increased energy storage alone.