| Liu, YJ; Liu, SJ; Drake, HL; Horn, MA: Alphaproteobacteria Dominate Active 2-Methyl-4-Chlorophenoxyacetic Acid Herbicide Degraders in Agricultural Soil and Drilosphere, Environmental Microbiology, 13, 991-1009 (2011) | |
| Abstract: 2-Methyl-4-chlorophenoxyacetic acid (MCPA) is a widely used phenoxyalkanoic acid (PAA) herbicide and subject to aerobic microbial degradation. Earthworms stimulate both growth and activity of MCPA-degrading bacteria in soil. Thus, active MCPA degraders in soil and drilosphere (i.e., burrow walls, gut content, and cast) were assessed by 16S rRNA stable isotope probing (SIP) in soil columns under experimental conditions designed to minimize laboratory incubation biases. Agriculturally relevant concentrations of [13C]MCPA (20 µg gdw 1) were degraded in soil within 23 and 27 days in the presence and absence of earthworms, respectively. Total 16S rRNA analysis revealed 73 OTUs indicative of active Acidobacteria, Actinobacteria, Bacteroidetes, Chloroflexi, Cyanobacteria, Firmicutes, Gemmatimonadetes, Planctomycetes, Proteobacteria, and Verrucomicrobia in soil and drilosphere derived material. Seven OTUs indicative of Alpha-, Beta-, Gammaproteobacteria, and Firmicutes consumed MCPA-[13C]. Dominant consumers of MCPA-[13C] were Alphaproteobacteria (Sphingomonadaceae and Bradyrhizobiaceae) in soil and drilosphere. Beta- (Comamonadaceae) and Gammaproteobacteria (Xanthomonadaceae) were also important MCPA-[13C] consumers in burrow walls only, indicating that earthworms favor betaproteobacterial MCPA degraders. In oxic microcosms with bulk soil, burrow walls, and cast, 20 and 300 400 µg [13C]MCPA gdw-1 were consumed within 24 hours and 20 days, respectively. Gut contents did not facilitate the degradation of [13C]MCPA. Sphingomonadaceae dominated MCPA-[13C] consumers in bulk soil and burrow wall microcosms, while Beta- and Gammaproteobacteria (Burkholderiacea, Comamonadaceae, Oxalobacteraceae, and Xanthomonadaceae) dominated MCPA-[13C] consumers in microcosms of cast, indicating that the latter taxa are prone to respond to MCPA in cast. The collective data indicated that Alphaproteobacteria are major MCPA degraders in soil and drilosphere. |
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