Cometabolic degradation of dibenzofuran and dibenzothiophene by a newly isolated carbazole-degrading Sphingomonas sp. strain

Abstract

A carbazole-utilizing bacterium was isolated by enrichment from petroleum-contaminated soil. The isolate, designated Sphingomonas sp. strain XLDN2-5, could utilize carbazole (CA) as the sole source of carbon, nitrogen, and energy. Washed cells of strain XLDN2-5 were shown to be capable of degrading dibenzofuran (DBF) and dibenzothiophene (DBT). Examination of metabolites suggested that XLDN2-5 degraded DBF to 2-hydroxy-6-(2-hydroxyphenyl)-6-oxo-2,4-hexadienic acid and subsequently to salicylic acid through the angular dioxygenation pathway. In contrast to DBF, strain XLDN2-5 could transform DBT through the ring cleavage and sulfoxidation pathways. Sphingomonas sp. strain XLDN2-5 could cometabolically degrade DBF and DBT in the growing system using CA as a substrate. After 40 h of incubation, 90% of DBT was transformed, and CA and DBF were completely removed. These results suggested that strain XLDN2-5 might be useful in the bioremediation of environments contaminated by these compounds.

FIG. 1.

Pathway proposed for the metabolism of DBF by Sphingomonas sp. strain XLDN2-5. The compound in brackets was not detected.

FIG. 2.

GC-MS analysis of the metabolites of DBT transformed by Sphingomonas sp. strain XLDN2-5. Compound I, monohydroxydibenzothiophene; compound II, dihydroxydibenzothiophene; compound III, DBTO₂; compound IV, DBTO.

FIG. 3.

Cometabolic degradation of CA, DBF, and DBT and accumulation of DBTO by growing cells of Sphingomonas sp. strain XLDN2-5. The initial concentrations of CA, DBF, and DBT were 3.0 mM, 0.2 mM, and 0.2 mM, respectively. •, OD₆₂₀ (presents on log scale in a semilog plot); ▪, CA; □, DBF; ○, DBT; ▴, DBTO. Values are means of three replicates ± standard deviations.

FIG. 4.

Pathway proposed for the metabolism of DBT by Sphingomonas sp. strain XLDN2-5. The compound in brackets was not detected, and the compound in the square dotted box was tentatively identified.