Methanol suppression of trichloroethylene degradation by Methylosinus trichosporium (OB3b) and methane-oxidizing mixed cultures

Trichloroethylene mineralization in a fixed-film bioreactor using a pure culture expressing constitutively toluene ortho -monooxygenase

An aerobic, single-pass, fixed-film bioreactor was designed for the continuous degradation and mineralization of gas-phase trichloroethylene (TCE). A pure culture of Burkholderia cepacia PR1(23)(TOM(23C)), a Tn5transposon mutant of B. cepacia G4 that constitutively expresses the TCE-degrading enzyme, toluene ortho-monooxygenase (TOM), was immobilized on sintered glass (SIRANtrade mark carriers) and activated carbon. The inert open-pore structures of the sintered glass and the strongly, TCE-absorbing activated carbon provide a large surface area for biofilm development (2-8 mg total cellular protein/mL carrier with glucose minimal medium that lacks chloride ions). At gas-phase TCE concentrations ranging from 0.04 to 2.42 mg/L of air and 0.1 L/min of air flow, initial maximum TCE degradation rates of 0.007-0.715 nmol/(min mg protein) (equivalent to 8.6-392.3 mg TCE/L of reactor/day) were obtained. Using chloride ion generation as the indicator of TCE mineralization, the bioreactor with activated carbon mineralized an average of 6.9-10.3 mg TCE/L of reactor/day at 0.242 mg/L TCE concentration with 0.1 L/min of air flow for 38-40 days. Although these rates of TCE degradation and mineralization are two- to 200-fold higher than reported values, TOM was inactivated in the sintered-glass bioreactor at a rate that increased with increasing TCE concentration (e.g., in approximately 2 days at 0.242 mg/L and <1 day at 2.42 mg/L), although the biofilter could be operated for longer periods at lower TCE concentrations. Using an oxygen probe and phenol as the substrate, the activity of TOM in the effluent cells of the bioreactor was monitored; the loss of TOM activity of the effluent cells corroborated the decrease in the TCE degradation and mineralization rates in the bioreactor. Repeated starving of the cells was found to restore TOM activity in the bioreactor with activated carbon and extended TCE mineralization by approximately 34%. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 55: 674-685, 1997.

Methane and Trichloroethylene Degradation by Methylosinus trichosporium OB3b Expressing Particulate Methane Monooxygenase

Whole-cell assays of methane and trichloroethylene (TCE) consumption have been performed on Methylosinus trichosporium OB3b expressing particulate methane monooxygenase (pMMO). From these assays it is apparent that varying the growth concentration of copper causes a change in the kinetics of methane and TCE degradation. For M. trichosporium OB3b, increasing the copper growth concentration from 2.5 to 20 μM caused the maximal degradation rate of methane ( V max ) to decrease from 300 to 82 nmol of methane/min/mg of protein. The methane concentration at half the maximal degradation rate ( K s ) also decreased from 62 to 8.3 μM. The pseudo-first-order rate constant for methane, V max / K s , doubled from 4.9 × 10 −3 to 9.9 × 10 −3 liters/min/mg of protein, however, as the growth concentration of copper increased from 2.5 to 20 μM. TCE degradation by M. trichosporium OB3b was also examined with varying copper and formate concentrations. M. trichosporium OB3b grown with 2.5 μM copper was unable to degrade TCE in both the absence and presence of an exogenous source of reducing equivalents in the form of formate. Cells grown with 20 μM copper, however, were able to degrade TCE regardless of whether formate was provided. Without formate the V max for TCE was 2.5 nmol/min/mg of protein, while providing formate increased the V max to 4.1 nmol/min/mg of protein. The affinity for TCE also increased with increasing copper, as seen by a change in K s from 36 to 7.9 μM. V max / K s for TCE degradation by pMMO also increased from 6.9 × 10 −5 to 5.2 × 10 −4 liters/min/mg of protein with the addition of formate. From these whole-cell studies it is apparent that the amount of copper available is critical in determining the oxidation of substrates in methanotrophs that are expressing only pMMO.

Retaining and recovering enzyme activity during degradation of TCE by methanotrophs

To determine if compounds added during trichloroethylene (TCE) degradation could reduce the loss of enzyme activity or increase enzyme recovery, different compounds serving as energy and carbon sources, pH buffers, or free radical scavengers were tested. Formate and formic acid (reducing power and a carbon source), as well as ascorbic acid and citric acid (free radical scavengers) were added during TCE degradation at a concentration of 2 mM. A saturated solution of calcium carbonate was also tested to address pH concerns. In the presence of formate and methane, only calcium carbonate and formic acid had a beneficial effect on enzyme recovery. The calcium carbonate and formic acid both reduced the loss of enzyme activity and resulted in the highest levels of enzyme activity after recovery.