Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs and PCDFs) are among the most notorious environmental pollutants. Some congeners, particularly those with lateral chlorine substitutions at positions 2, 3, 7 and 8, are extremely toxic and carcinogenic to humans. One particularly promising mechanism for the detoxification of PCDDs and PCDFs is microbial reductive dechlorination. So far only a limited number of phylogenetically diverse anaerobic bacteria have been found that couple the reductive dehalogenation of chlorinated compounds--the substitution of a chlorine for a hydrogen atom--to energy conservation and growth in a process called dehalorespiration. Microbial dechlorination of PCDDs occurs in sediments and anaerobic mixed cultures from sediments, but the responsible organisms have not yet been identified or isolated. Here we show the presence of a Dehalococcoides species in four dioxin-dechlorinating enrichment cultures from a freshwater sediment highly contaminated with PCDDs and PCDFs. We also show that the previously described chlorobenzene-dehalorespiring bacterium Dehalococcoides sp. strain CBDB1 (ref. 3) is able to reductively dechlorinate selected dioxin congeners. Reductive dechlorination of 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD) demonstrates that environmentally significant dioxins are attacked by this bacterium.
Persistent environmental contaminants may enter agricultural fields via the application of sewage sludge, by irrigation with treated municipal wastewater or by manuring. It has been shown that such contaminants can be incorporated into crop plants. The metabolism of the bacteriostatic agents triclocarban, triclosan, and its transformation product methyl triclosan was investigated after their uptake into carrot cell cultures. A fast metabolization of triclosan was observed and eight so far unknown phase II metabolites, conjugates with saccharides, disaccharides, malonic acid, and sulfate, were identified by liquid chromatography-mass spectrometry. Triclocarban and methyl triclosan lack a phenolic group and remained unaltered in the cell cultures. Phase I metabolization was not observed for any of the compounds. All eight triclosan conjugates identified in the cell cultures were also detected in extracts of intact carrot plants cultivated on triclosan contaminated soils. Their total amount in the plants was assessed to exceed the amount of the triclosan itself by a factor of 5. This study shows that a disregard of conjugates in studies on plant uptake of environmental contaminants may severely underestimates the extent of uptake into plants and, eventually, the potential human exposure to contaminants via food of plant origin.
Many xenobiotics entering wastewater treatment plants
are known
to be persistent during wastewater treatment and tend to adsorb to
sewage sludge. The application of sewage sludge as fertilizer in agriculture
may pose the risk of an incorporation of xenobiotics in the cultivated
plants and, finally, an inclusion into the food chain. This study
was performed to investigate the uptake of common sewage sludge contaminants,
galaxolide, tonalide, and triclosan, by plants used for human consumption
and livestock feeding. Barley, meadow fescue, and four carrot cultivars
were sown and grown in spiked soils under greenhouse conditions. After
harvesting the plants, roots and leaves were analyzed separately,
and the respective bioconcentration factors were calculated. In carrots,
a concentration gradient of the xenobiotics became evident that decreased
from the root peel via root core to the leaves. A significant influence
of the differing root lipid contents on the uptake rates cannot be
supported by our data, but the crucial influence of soil organic carbon
content was confirmed. Barley and meadow fescue roots incorporated
higher amounts of the target substances than carrots, but translocation
into the leaves was negligible. The results indicated that an introduction
of persistent semi- and nonpolar xenobiotics into the food chain via
edible plants like carrots could be of certain relevance when sludge
is applied as fertilizer. Due to low rates found for the translocation
of the xenobiotics into the aerial plant parts, the entrance pathway
into food products via feeding livestock is less probable.
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