Environmental pollution by toxic chemicals has become a major issue throughout the world. In particular, volatile organochlorine compounds (VOCs), heavy metals, PCBs, and oil have been detected in soil and groundwater. Various technologies for cleaning up the environment are now being developed; bioremediation is one of the most promising, because of its low cost and essentially complete destruction of pollutants.

1. Biodegradation of VOCs

　VOCs, such as trichloroethylene (TCE) and 1,1,1-trichloroethane (TCA), have been widely used as solvents; inappropriate disposal of these compounds has frequently resulted in contamination of groundwater. We isolated a methanotroph, Methylocystis sp. M, and studied its characteristics. We purified methane monooxygenase, which is the TCE-degrading enzyme, from strain M, and determined its nucleotide sequence. Recently, we isolated Mycobacterium sp. TA27, which can degrade both TCE and TCA with ethane as its energy source, and determined its TCE and TCA degradation pathways. The conversions to 2,2,2-trichloroethanol (TCAol), trichloroacetic acid (TCAA), chloral, and dichloroacetic acid (DCAA) derived from degraded TCE were 4.4%, 3.0%, 0.5%, and 0.4%, respectively. The production of TCAol, DCAA, and TCAA from the degradation of TCA suggested a novel pathway.

The methane monooxygenase (TCE-degrading enzyme) of Methylocystis sp. M.

The metabolic pathways of TCA and TCE degradation by Mycobacterium sp. TA27.

2. Bacterial removal of mercury from contaminated water and soil

　Mercury is one of the most toxic metals because Hg2+ has a strong affinity for the thiol groups in proteins. Using a mercury removal-recovery system able to recover mercury volatilized by biological reduction, we tested the removal of mercuric chloride from water and soil by resting cells of the genetically engineered mercury-volatilizing bacterium, Pseudomonas putida PpY101/pSR134. Under optimum conditions, nearly 100% of the 40 mg L-1 of mercuric chloride was removed from contaminated water and 70% was removed from soil slurry. The P. putida cells were immobilized on various carriers to stabilize the mercury removal activity and to prevent the release of genetically engineered cells into the environment. After the mercury removal experiments, bacterial cells and mercury droplets were clearly visible in the matrix of the immobilizing calcium alginate beads. It was found that some of the mercury volatilized by P. putida　accumulated in the gel beads.

Scanning electron micrograph of a Ca-alginate bead used to immobilize

the bacterial cells in the mercury removal experiments.