Abstract
A total of twenty nine microbial isolates capable of growing on dibenzothiophene (DBT) as a secondary substrate were isolated from six soil and sewage water samples. Out of the isolates, six were capable of utilizing dibenzothiophene as a sole sulfur, carbon and energy sources. Among these isolates two were capable of utilizing DBT as only sole source of sulfur. They were identified as Candida parapsilosis S1-Y1 and Cryptococcus humicolus S1-Y2 and subjected to different culture conditions affecting the biodesulfurization of DBT. The results indicated that the optimal conditions were: culture medium containing 4 ml/l DBT, fructose, ammonium hydrogen phosphate, histidene and folic acid at 35ºC and pH 8 under shaking conditions at 200 rpm for 7 days which inoculated with 10 ml (each ml containing 12X109CFU/ml) for C. parapsilosis S1-Y1. The best conditions for Cr. humicolus S1-Y2 were: culture medium containing 4 ml/l DBT, maltose, ammonium hydrogen phosphate, tryptophan and yeast extract at 35ºC and pH 8 and inoculated with 1 ml inoculum (each ml containing 19X109CFU/ml) under shaking conditions at 200 rpm for 7 days. Gas chromatography mass spectrometry (GC-MS) analysis of biodegradation products proved that DBT was transformed by C. parapsilosis S1-Y1 in the presence of the best carbon source to dimethyl disulphide, methylbenzene, tetrachloro-ethylene,2- methylpyrazine, ethylbenzne, 1,4-dimethylbenzene (P-xylene), 2,6-dimethyl pyrazine, tetrdecanoic acid, pentadecanoic acid and hexadecanoic acid. The DBT was degraded by C. parapsilosis S1-Y1 to dimethyl disulphide, methylbenzene, tetrachloroethylene, ethyl-benzene, 1,3-dimethylbenzene (P-xylene), 1,3- dimethylbenzene (m-xylene), biphenyl, tetradecanoic acid, oleic acid, hexadecanoic acid and heptadecanoic acid in addition to palmetic acid, eicosyl trichlorosilane,3-methyl tridecane, heptadecane and hexadecane in the absence of co substrate. The two yeast isolates could be recommended for application for removing of sulfur compounds in Egyptian petroleum crude oil.

Key words: Bioremediation, Biodesulfurization, Dibenzothiophene, Egyptian crude oil, Candida parapsilosis, Cryptococcus humicolus.