Microbial transformation of benzothiophenes, with carbazole as the auxiliary substrate, by Sphingomonas sp. strain XLDN2-5.

Benzothiophenes are a toxic and relatively recalcitrant fraction of coal-tar creosote. We investigated the co-metabolic transformation of benzothiophene (BT) and its derivatives by the carbazole (CA) degrader Sphingomonas sp. XLDN2-5, which is not able to grow on benzothiophenes as the sole carbon source. Among the benzothiophenes tested, BT, 2-methylbenzothiophene (2-MBT) and 5-methylbenzothiophene (5-MBT) were co-metabolically converted. For 3-methylbenzothiophene, there was complete inhibition of growth on CA. The common transformation products for BT, 2-MBT and 5-MBT are the corresponding sulfoxides and sulfones. For BT, several high-molecular-mass sulfur-containing aromatic compounds, including benzo[b]naphtho[1,2-d]thiophene, benzo[b]naphtho[1,2-d]thiophene-7-oxide, 6a,11b-dihydrobenzo[b]naphtho[1,2-d]thiophene, 6a,11b-dihydrobenzo[b]naphtho[1,2-d]thiophene-7-oxide, and a new product, 6,12-epithiobenzo[b]naphtho[1,2-d]thiophene, were detected by GC-MS. These high-molecular-mass products are thought to be generated from a Diels-Alder-type reaction. Investigations with a combination of GC and flame ionization detection showed that about 17 % of BT was transformed to benzo[b]naphtho[1,2-d]thiophene. Aerobic transformation of benzothiophenes to sulfoxides and sulfones can reduce their toxicity, and facilitate their biodegradation. However, the formation of the high-molecular-mass products, such as benzo[b]naphtho[1,2-d]thiophene, should be considered in the biodegradation of benzothiophenes.