The molecular basis for the biotic degradation of metaldehyde

Metaldehyde is a molluscicide that is used to prevent crop damage by slugs and snails. It is frequently found in drinking water sources at concentrations above the legal maximum, and causes >90% of water quality failures in England. There is currently no economical, widely used treatment to remove metaldehyde pollution from water. Prior to the work presented here, it was known that metaldehyde is degraded biotically in soil, but the nature and identity of organisms responsible was unknown. Two novel metaldehyde degrading bacterial strains were isolated from domestic soil that can utilise metaldehyde as a sole carbon and energy source; Acinetobacter calcoaceticus E1 and Variovorax E3. Evidence that acetaldehyde is the primary metabolite of metaldehyde is presented: the metaldehyde dependent expression of an aldehyde dehydrogenase in A. calcoaceticus E1 was found, and the maximum metabolic flux of acetaldehyde was found to be greater than that of metaldehyde in cells grown using metaldehyde. The genomes of the isolated organisms were acquired. Comparative genomics of the two isolates showed that their novel catalytic ability was not dependent on genes shared between them, despite being isolated from the same soil sample. Comparison of the A. calcoaceticus E1 genome against other Acinetobacter that cannot utilise metaldehyde yielded a strong candidate for the primary metaldehyde degrading enzyme that has characteristics consistent with an enzyme that catalysed ether hydrolysis of isochorismate in its ancestral form and has evolved to hydrolyse ether bonds in metaldehyde.