Microbial desulfurization of diesel oils by selected bacterial strains

Because of increasingly stringent regulations concerning the sulfur content of motor fuels, sulfur removal by biocatalytic means is often considered as a potential alternative to conventional deep hydro-desulfurization processes used in the refinery industry. The first microbial strain able to selectively oxidize sulfur of molecules such as dibenzothiophene (DBT) without altering its carbon content, Rhodococcus erythropolis IGTS8, was isolated more than ten years ago. The metabolic pathway (4S pathway) was elucidated and the genes involved characterized and sequenced. The present study aimed at exploiting microbial diversity to select new strains potentially interesting for ultradeep desulfurization of diesel oils, taking into account industrially important criteria. In a first step, 15 pure strains able to use DBT as a sole sulfur source and to convert it to 2-hydroxybiphenyl (HBP) were obtained from different soils. In a second step, 5 isolates belonging to the Rhodococcus/Gordonia cluster and exhibiting good growth characteristics and high biodesulfurization activities in both aqueous and organic media were selected. The action of resting cells from these strains towards different types of diesel oils was also determined in order to better assess the potentiality of biodesufurization, especially as a finishing step complementary to deep hydrodesulfurization (HDS). Actually, in spite of their taxonomic similarity, the 5 strains displayed different activities towards the diesels oil tested. Biodesulfurization yield was also dependent upon the diesel oil used, especially its sulfur content. Some HDS-recalcitrant compounds such as 4,6-dimethyl dibenzothiophene, could be completely removed, but highly-alkylated dibenzothiophenes were resistant to the action of the biocatalysts.