Pristine soils mineralize 3-chlorobenzoate and 2,4-dichlorophenoxyacetate via different microbial populations

Abstract

Biodegradation of two chlorinated aromatic compounds was found to be a common capability of the microorganisms found in the soils of undisturbed, pristine ecosystems. We used 2,4-dichlorophenoxyacetate (2,4-D) and 3-chlorobenzoate (3CBA) as enrichment substrates to compare populations of degrading bacteria from six different regions making up two ecosystems. We collected soil samples from four Mediterranean (California, central Chile, the Cape region of South Africa, and southwestern Australia) and two boreal (northern Saskatchewan and northwestern Russia) ecosystems that had no direct exposure to pesticides or to human disturbance. Between 96 and 120 samples from each of the six regions were incubated with 50 ppm of [U-14C]2,4-D or [U-14C]3CBA. Soils from all regions samples mineralized both 2,4-D and 3CBA, but 3CBA was mineralized without a lag period, while 2,4-D was generally not mineralized until the second week. 3CBA degradative capabilities were more evenly distributed spatially than those for 2,4-D. The degradative capabilities of the soils were readily transferred to fresh liquid medium. 3CBA degraders were easily isolated from most soils. We recovered 610 strains that could release carbon dioxide from ring-labeled 3CBA. Of these, 144 strains released chloride and degraded over 80% of 1 mM 3CBA in 3 weeks or less. In contrast, only five 2,4-D degraders could be isolated, although a variety of methods were used in an attempt to culture the degraders. The differences in the distribution and culturability of the bacteria responsible for 3CBA and 2,4-D degradation in these ecosystems suggest that the two substrates are degraded by different populations. We also describe a 14C-based microtiter plate method that allows efficient screening of a large number of samples for biodegradation activity.

Phylogenetic analyses indicate independent recruitment of diverse gene cassettes during assemblage of the 2,4-D catabolic pathway

“…A plasmid carrying only a part of the 2,4-D catabolic pathway has also been isolated [24], demonstrating that catabolic genes of the 2,4-D pathway may be found in fragments in individual members of a community of soil micro-organisms. This is consistent with our observations that in pristine soils [38] or soils with no history of pesticide application [35], the isolation of stable 2,4-D degrading micro-organisms is di¤cult. It is therefore possible that repeated pesticide applications lead to the transfer of catabolic genes from di¡erent bacteria carrying portions of the pathway to other bacteria carrying di¡erent portions and assembly of the entire pathway in one organism.…”

Adsorption and degradation of phenoxyalkanoic acid herbicides in soils: A review

“…In the case of mecoprop, degradation was generally slow, and the three-half-order kinetic model gave better results than the zero-order model in only 4 soils. In few pristine soils in studies by Baelum et al [93], very slow linear growth of 2,4-D and MCPA degraders was observed, as in previous studies by Fulthorpe et al [103] and Kamagata et al [104] for 2,4-D. Summarizing the results of the studies cited above, it may be concluded that the lag phase is an integral part of the microbial degradation pattern of phenoxyalkanoic acid herbicides in soils. Therefore, the duration of the lag phase should not be omitted from kinetic analyses intended to determine the time within which the concentration of the test substance is reduced by 50% (disappearance time 50, or DT50) or by 90% (DT90).…”

A soil-based microbial biofilm exposed to 2,4-D: bacterial community development and establishment of conjugative plasmid pJP4

“…Many bioremediation studies have focused on chloroaromatic compounds, including the herbicide 2,4dichlorophenoxyacetic acid (2,4-D) which is a potential pollutant of terrestrial environments [7][8][9]. A number of plasmids encoding genes for 2,4-D degradation have 0168 been isolated and characterised [10][11][12] including pJP4, a broad-host-range, self-transmissible, IncP group plasmid carrying the genes (tfdA-F) for the conversion of 2,4-D to 2-chloromaleylacetate [10].…”