Isolation and characterization of a quinclorac-degrading Actinobacteria Streptomyces sp. strain AH-B and its implication on microecology in contaminated soil.

Quinclorac, a highly selective auxin herbicide, is widely used for controlling weeds in rice field. However, the residual quinclorac is toxic to many crops, vegetables, and aquatic animals, resulting in one of the major problems in crop rotation. Here, we investigated the degradation of quinclorac by strain AH-B, which was isolated from long-term quinclorac-contaminated soil using continuous circulating fluidized bed reactor and subjected to atmospheric and room temperature plasma mutation. Morphological examination, 16S rRNA gene sequencing, and phylogenetic analysis revealed that strain AH-B was Streptomyces sp. The quinclorac degradation efficiency of AH-B in liquid medium was 97.2% after 18 days when the initial quinclorac concentration was 20 mg L-1. The degradation products were 3-chloro-7-methoxy-8-quinoline-carboxylic, 3-chloro-7-methyl-8-quinoline-carboxylic, 3-chloro-7-oxyethyl-8-quinoline-carboxylic, and 3,7-dichloro-6-methyl-8-quinoline-carboxylic. The inoculum size, initial quinclorac concentration, pH, and temperature were found to affect quinclorac degradation efficiency of AH-B. High-performance liquid chromatography-electrospray ionization tandem mass spectrometry analysis revealed that quinclorac degradation by AH-B produced many products. In soil with initial quinclorac content of 1 mg kg-1 dry soil, addition of AH-B resulted in 87.5% quinclorac degradation after 42 days, while that in the control (without AH-B) was 22.4%. Furthermore, microecological analysis using next-generation sequencing of 16S rRNA geneshowed that some bacterial species, such as Bacterioides and Proteobacteria, could survive in quinclorac-contaminated soil, while some bacteria, such as Firmicutes, were very sensitive to quinclorac. Besides, some fungal species, such as Basidiomycota, could also survive quinclorac-contamination. After 42 days, the diversity of bacteria and fungi in soil treated with AH-B was higher than that in the control, implying that bioaugmentation with strain AH-B could reduce quinclorac toxicity to microorganisms in soil.