Biotransformations of Three Dimethyldibenzothiophenes by Pure and Mixed Bacterial Cultures

The recalcitrance of dibenzothiophenes in petroleum-contaminated environments increases with alkyl substitution. In this study, three 1-methylnaphthalene (1-MN)-utilizing Pseudomonas spp., which oxidize dibenzothiophene (DBT) to various compounds including 3-hydroxy-2-formylbenzothiophene (HFBT), and four petroleum-degrading mixed bacterial cultures were tested for the ability to oxidize 2,8-, 3,4-, and 4,6-dimethyl-DBT. The aerobic cometabolism of these thiophenes was studied with 1-MN and the aromatic fraction of Prudhoe Bay crude oil provided as the growth substrates. Culture extracts were analyzed by gas chromatography with flame photometric, mass, and Fourier transform infrared detectors to identify oxidation products. The susceptibilities of the dimethyl-DBTs depended upon the positions of the methyl groups. All the cultures tested were able to degrade the unsubstituted ring of 3,4-dimethyl-DBT to give 6,7-dimethyl-HFBT and 6,7-dimethylbenzothiophene-2,3-dione, among other products. None of the Pseudomonas spp. were able to oxidize 4,6-dimethyl-DBT, which has a methyl group on each of the benzene rings, and only slight losses were observed in the petroleum-degrading mixed cultures. However, one of the Pseudomonas spp. was able to oxidize and cleave a methyl-substituted ring of 2,8-dimethyl-DBT, yielding 5-methylbenzothiophene-2,3-dione and two other products. This 2,3-dione was also observed in extracts of two of the four mixed cultures, which were capable of moderate degradation of 2,8-dimethyl-DBT.