Mineralization and co-metabolic degradation of phenoxyalkanoic acid herbicides by a pure bacterial culture isolated from an aquifer.

A mecoprop [(+/-)-2-(4-chloro-2-methylphenoxy)propionic acid; MCPP]-degrading bacterium identified as Stenotrophomonas maltophilia PM was isolated from a Danish aquifer. Besides mecoprop, the bacterium was also able to degrade MCPA [(4-chloro-2-methylphenoxy)acetic acid)], MCPB [(4-chloro-2-methylphenoxy)butyric acid], 4-CPA [(4-chlorophenoxy)acetic acid], 2, 4-D [(2, 4-dichlorophenoxy)acetic acid], 2, 4-DP [(+/-)-2-(2, 4-dichlorophenoxy)propionic acid] and 2, 4-DB [(2, 4-dichlorophenoxy)butyric acid]. The bacterium was able to grow using these individual phenoxyalkanoic acids as the sole source of carbon and energy. In addition, it was able to co-metabolically degrade the phenoxyalkanoic acid 2, 4, 5-T [(2, 4, 5-trichlorophenoxy)acetic acid)] in the presence of mecoprop. At high 2, 4, 5-T concentrations (100 and 52 mg/l), however, only partial degradation of both mecoprop and 2, 4, 5-T was obtained, thus indicating the production of toxic metabolites. Bacterial yields were highest when grown on the monochlorinated phenoxyalkanoic acids as compared to the dichlorinated analogues, an exception being growth on 4CPA, which resulted in the lowest yield at all. Using [ring-U-14C]-labeled herbicides it was shown that the lower yield on 2, 4-D than on mecoprop was accompanied by greater CO2 generation, thus indicating that less energy is available from the complete oxidation of the dichlorinated phenoxyalkanoic acids than the monochlorinated analogues.

Degradation of ethylenethiourea (ETU) in oxic and anoxic sandy aquifers.

Ethylenethiourea is an important degradation product of ethylenebisdithiocarbamate fungicides, which are widely used in different kinds of crops. The ethylenebisdithiocarbamate group includes maneb, zineb and mancozeb. The ethylenebisdithiocarbamates are not highly toxic and degrade rapidly in the presence of moisture and oxygen, forming different compounds. One of these is the polar ethylenethiourea, which is relatively stable. Thus, this compound appears to be a potential contaminant for groundwater. Batch experiments were carried out under biotic as well as abiotic conditions to study the degradation dependence of concentration, temperature and organic matter. The decomposition of ethylenethiourea under abiotic conditions was found to be less than 5% of the degradation under biotic conditions. Further, ethylenethiourea showed to be stable over a period of 150 days at 20 degrees C in tap water as well as in batch with soil sterilized with NaN3. The degradation of ethylenethiourea depends on the concentration in the water implying first order reaction kinetics. The microbial degradation of ethylenethiourea is highly temperature dependent with aerobic Q10 between 2.9 and 4.2, and an anaerobic between 2.1 and 2.5. A minor increase in degradation rates was observed by application of nitrate and manure to the batches. The experiments show extremely complete degradation of ethylenethiourea in the presence of microbial nitrate reduction with pyrite which occurs in deeper parts of the aquifers.