RONALD H. TARABAN,1 DUANE F. BERRY,1* DAVID A. BERRY,2 AND HUBERT L. WALKER,
The biodegradability of dicamba was investigated under anaerobic conditions with a consortium enrichedfrom wetland soil. Degradation proceeded through an initial demethylation reaction, forming 3,6-dichlorosalicylic acid, followed by reductive dechlorination, forming 6-chlorosalicylic acid. The consortium, consisting ofa sulfate reducer, three methanogens, and a fermenter, was unable to mineralize the aromatic ring.
Dicamba (3,6-dichloro-2-methoxybenzoic acid) is a chem¬ically stable postemergence herbicide commonly used tocontrol weeds in cereal grain crops and broadleaf weeds inpastures and rangelands. Because it exists primarily in adissociated anionic form-in soils, dicamba is highly mobile and susceptible to dispersal through both runoff and leaching(3, 5, 12), often entering nontarget environments such asstreams and rivers, sediments, and groundwater. Many ofthese nontarget environments are anaerobic.
Although little is known about the fate of dicamba underanaerobic conditions, its fate under aerobic conditions hasbeen extensively investigated. Dicamba is probably de¬graded in moist soils through biologically mediated pro¬cesses (15-17). Studies by Smith (15, 16) have shown thatdicamba is readily transformed to 3,6-dichlorosalicylic acid(3,6-DCSA) in soil. Krueger et al. (7) established that di¬camba could be degraded by pure cultures of bacteria underaerobic conditions. They isolated eight species of bacteriafrom soil and water samples with long histories of dicambaexposure that were capable of using dicamba as the solecarbon source. They also found that the primary metaboliteof dicamba in soil was 3,6-DCSA.
Microbially mediated reductive dehalogenation of chlori¬nated aromatic compounds is now widely recognized as aprocess that plays a crucial role in the restoration of con¬taminated anoxic environments (8, 11). Over the past 10years, several organic chemicals, including the haloben¬zoates (6, 18), chlorinated phenols (1, 2), and the polychlo¬rinated biphenyls (13), have been studied with respect toreductive dehalogenation. Kuhn and Suflita (8) reported thataryl reductive dehalogenation has been observed for a fewpesticides. Biodegradation of the phenoxyalkanoic acid her¬bicide, 2,4-dichlorophenoxy acetic acid, similar in herbicidalactivity to dicamba and equally mobile, has been evaluatedunder methanogenic conditions in environmental samplesincluding pond sediment aquifer material (4) and sewagesludge (10). Under methanogenic conditions, 2,4-dichlo¬rophenoxy acetic acid is known to undergo reductive deha¬logenation, forming products such as 2-, and 4-chlorophe¬noxyacetate (8).
Although most studies have examined degradation ofdicamba under aerobic conditions, to our knowledge nonehave determined the degradative pathway under strict anaerobic conditions. The present study was conducted to elucidate the degradative pathway of dicamba by using a metha¬nogenic dicamba-degrading consortium enriched fromwetland soil.The soilinoculumconsistedofaLawnessandyloam (fine-loamy, mixed, nonacid, thermic family of TypicSulfaquents) collected from a site located in Surry County,Va. Forty milliliters of soil slurry (containing 1.4 g of solids)was added to a 160-ml serum bottle with 60 ml of deoxygen¬ated mineral salts medium and 9.3 ,umol of dicamba. Themineral salts medium, containing 1 ml of a 0.1% resazurinsolution, was prepared as described by Boyd et al. (2).Analytical-grade dicamba (purity, 98.7%), purchased fromChem Services Inc., West Chester, Pa., was used withoutfurther purification.
The biodegradability of dicamba was investigated under anaerobic conditions with a consortium enrichedfrom wetland soil. Degradation proceeded through an initial demethylation reaction, forming 3,6-dichlorosalicylic acid, followed by reductive dechlorination, forming 6-chlorosalicylic acid. The consortium, consisting ofa sulfate reducer, three methanogens, and a fermenter, was unable to mineralize the aromatic ring.
Dicamba (3,6-dichloro-2-methoxybenzoic acid) is a chem¬ically stable postemergence herbicide commonly used tocontrol weeds in cereal grain crops and broadleaf weeds inpastures and rangelands. Because it exists primarily in adissociated anionic form-in soils, dicamba is highly mobile and susceptible to dispersal through both runoff and leaching(3, 5, 12), often entering nontarget environments such asstreams and rivers, sediments, and groundwater. Many ofthese nontarget environments are anaerobic.
Although little is known about the fate of dicamba underanaerobic conditions, its fate under aerobic conditions hasbeen extensively investigated. Dicamba is probably de¬graded in moist soils through biologically mediated pro¬cesses (15-17). Studies by Smith (15, 16) have shown thatdicamba is readily transformed to 3,6-dichlorosalicylic acid(3,6-DCSA) in soil. Krueger et al. (7) established that di¬camba could be degraded by pure cultures of bacteria underaerobic conditions. They isolated eight species of bacteriafrom soil and water samples with long histories of dicambaexposure that were capable of using dicamba as the solecarbon source. They also found that the primary metaboliteof dicamba in soil was 3,6-DCSA.
Microbially mediated reductive dehalogenation of chlori¬nated aromatic compounds is now widely recognized as aprocess that plays a crucial role in the restoration of con¬taminated anoxic environments (8, 11). Over the past 10years, several organic chemicals, including the haloben¬zoates (6, 18), chlorinated phenols (1, 2), and the polychlo¬rinated biphenyls (13), have been studied with respect toreductive dehalogenation. Kuhn and Suflita (8) reported thataryl reductive dehalogenation has been observed for a fewpesticides. Biodegradation of the phenoxyalkanoic acid her¬bicide, 2,4-dichlorophenoxy acetic acid, similar in herbicidalactivity to dicamba and equally mobile, has been evaluatedunder methanogenic conditions in environmental samplesincluding pond sediment aquifer material (4) and sewagesludge (10). Under methanogenic conditions, 2,4-dichlo¬rophenoxy acetic acid is known to undergo reductive deha¬logenation, forming products such as 2-, and 4-chlorophe¬noxyacetate (8).
Although most studies have examined degradation ofdicamba under aerobic conditions, to our knowledge nonehave determined the degradative pathway under strict anaerobic conditions. The present study was conducted to elucidate the degradative pathway of dicamba by using a metha¬nogenic dicamba-degrading consortium enriched fromwetland soil.The soilinoculumconsistedofaLawnessandyloam (fine-loamy, mixed, nonacid, thermic family of TypicSulfaquents) collected from a site located in Surry County,Va. Forty milliliters of soil slurry (containing 1.4 g of solids)was added to a 160-ml serum bottle with 60 ml of deoxygen¬ated mineral salts medium and 9.3 ,umol of dicamba. Themineral salts medium, containing 1 ml of a 0.1% resazurinsolution, was prepared as described by Boyd et al. (2).Analytical-grade dicamba (purity, 98.7%), purchased fromChem Services Inc., West Chester, Pa., was used withoutfurther purification.
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