Consumers of 4-chloro-2-methylphenoxyacetic acid from agricultural soil and drilosphere harbor cadA, r/sdpA, and tfdA-like gene encoding oxygenases

Abstract

Microbial degradation of 2-methyl-4-chlorophenoxyacetic acid (MCPA) in soil is enhanced by earthworms and initiated by tfdA-like, cadA and r/sdpA gene encoding oxygenases. Copy numbers of such genes increased during MCPA degradation in soil, and MCPA stimulated transcription of tfdA-like and r/sdpA genes up to 4×. Transcription of cadA was detected in the presence of MCPA only. DNA stable isotope probing after consumption of 0.6-0.8 mg 13C-MCPA gdw -1 in oxic microcosms indicated diverse labeled oxygenase genes in bulk soil, burrow walls, and cast. 9, 6, and 3 operational taxonomic units of tfdA-like, cadA, and r/sdpA genes, respectively, were labeled and affiliated with group 2 Alphaproteobacteria including Bradyrhizobia and group 1 class III Betaproteobacteria. New genes encoding putative MCPA degrading oxygenases were identified. Diversity of labeled OTUs tended to be lower for cast than for bulk soil. The collective data indicate (1) hitherto unknown active MCPA degraders and/or oxygenase genes in soil; (2) that multiple oxygenases are associated with MCPA degradation in soil at the same time; (3) that earthworms impact the capability of MCPA degraders in soil to respond to MCPA; and (4) the collective data enable a more in-depth analysis of MCPA degrader communities in soil by future structural gene-based experimental strategies.

Keywords: DNA-stable isotope probing; cadA; drilosphere; herbicide; r/sdpA; tfdA-like.