Metabolism of pentachlorophenol by Saccharomonospora viridis strains isolated from mushroom compost

Abstract

A number of actinomycetes were isolated from mushroom compost and screened for the ability to degrade pentachlorophenol (PCP) in liquid medium containing 10 mg PCP l −1. Three apparent PCP-degrading strains were obtained in this way and all were identified as members of the species Saccharomonospora viridis. The ability to remove PCP from liquid cultures was identified also in all five culture collection strains of S. viridis examined, and therefore appears to be a characteristic of the species. PCP removal was confirmed by gas chromatography with electron capture detection (GC–ECD) and complete removal of 10 mg PCP l −1 from shake flask cultures within 8 days was recorded for both a S. viridis isolate (3:7) and a reference strain (BD120). However, these strains were found to be unable to metabolise other chlorophenols (2-, 3-, and 4-chlorophenol; 2,4-dichlorophenol; 2,4,6-trichlorophenol and 2,3,4,6-tetrachlorophenol). PCP removal by S. viridis strains was not due to adsorption to biomass but was found to be growth dependent and required de novo protein synthesis. Neither dechlorinated intermediates nor transformation products such as chloroanisoles could be detected by GC–ECD. Furthermore, S. viridis was shown to be unable to mineralise PCP in experiments conducted with uniformly labelled 14C-PCP, in which all of the 14C was recovered from the culture supernatant. During incubation, the 14C from radiolabelled PCP became progressively less soluble in the n-hexane solvent used for GC analysis. When supernatants from S. viridis cultures grown in the presence of PCP were treated with NaOH, a compound was released which had the same GC retention time as tetrachlorohydroquinone (TCHQ), and its formation was coincident with PCP disappearance. This suggests that S. viridis metabolises PCP by conjugation to form a more polar transformation product, but unlike other PCP-degrading bacteria, is incapable of effecting total degradation of the xenobiotic. Microorganisms such as S. viridis may therefore contribute to PCP removal by microbial communities in situ, despite being unable to completely mineralise chlorophenols in pure culture