The microbial degradation of the DDT metabolite dichlorobenzophenone (DBP)

Abstract

Soil samples which had been previously exposed to chlorinated aromatics were screened for ability to degrade dichlorobenzophenone (DBP). Of the samples tested soil from the Dow Elanco Agricultural Farm and soil from the AgResearch Winchmore Research Station showed apparent degradative capabilities. Degradation was not sustainable in these soil samples and a time scale study showed DBP was stable in soil over a 24 week period. Samples from Winchmore were further used to establish enrichment cultures capable of degrading DBP and its non-chlorinated analogue benzophenone (BP) through selection pressure. BP proved to be readily degraded but DBP degradation was only achieved after fungal suppressants were used. Degradation of DBP was enhanced with the addition of yeast extract and sodium salicylate to the enrichment cultures. Degradation of DBP was confirmed by capillary gas chromatography and the detection of the metabolite p-chlorophenyl acetic acid by gas chromatography-mass spectroscopy and thin layer chromatography. The enrichment cultures established on DBP also extensively degraded BP, p-chlorobenzophenone and p-chlorobenzoic acid. When inoculated back into soil, the enrichment cultures degradative capabilities were significantly reduced due to competition from other organisms, availability of alternative carbon sources and the bioavailability of DBP due to binding to soil particles. Two organisms capable of degrading BP as sole carbon source were isolated and identified as a Rhodococcus spp. and a Streptomyces spp. BP degradative capabilities were not maintained by these organisms and lost when subculturing on nutrient media Three presumptive DBP degraders were isolated and presumptively identified as two separate Streptomyces spp. and a Pseudomonas vesicularis. Of the three none were able to degrade DBP as sole carbon source in liquid culture although the P. vesicularis was able to co-metabolise DBP with the addition of yeast extract and sodium salicylate. Preliminary genetical studies of the P. vesicularis isolated were carried out.