Microbial adaptation for the degradation of metaldehyde

Abstract

Metaldehyde is used as molluscicide of choice in agriculture and horticulture, but its detection in drinking water has become a major cause of concern. Eight new metaldehyde-degrading bacterial strains were isolated from soil and comparative genomics were used to identify a highly-conserved gene cluster responsible for metaldehyde degradation in most of the isolates. Accelerated biodegradation of metaldehyde was demonstrated in multiple soils. It was confirmed that the metaldehyde-degrading population proliferates in response to the pesticide. Third generation sequencing was used to obtain a reference genome for the model degrader Acinetobacter calcoaceticus E1. IS91 and IS6-family insertion sequences were found surrounding the degrading gene cluster. They were hypothesised to be responsible for degrading gene dissemination to different taxa. A plasmid denominated pAME76 was found to harbour the degrading operon and bioinformatic analyses suggested that the genes are also located in plasmids in most of the other isolates. A sensitive probe-based qPCR assay for degrading gene cluster detection was developed and the technique was successfully used in different soils. Higher number of gene copies correlated with periods of metaldehyde removal for half of the soils. The gene cluster was not detected in the other half of the soils indicating that other biological degrading pathways are also important. The metaldehyde-degrading bacterial library was screened using batch assays to identify the best strains for removal of the compound at micropollutant concentrations from water. A. calcoaceticus E1 and Sphingobium CMET-H were used for bioaugmentation of pilot-scale slow sand filters. The former was not able to achieve metaldehyde-compliant water even though a remarkably high population was present in the sand. In contrast, the latter could swiftly bring metaldehyde concentrations to compliant levels that persisted for 14 d until reactor shutdown, even...