Biodesulfurization of naphthothiophene and benzothiophene through selective cleavage of carbon-sulfur bonds by Rhodococcus sp. strain WU-K2R

Abstract

Naphtho[2,1-b]thiophene (NTH) is an asymmetric structural isomer of dibenzothiophene (DBT), and in addition to DBT derivatives, NTH derivatives can also be detected in diesel oil following hydrodesulfurization treatment. Rhodococcus sp. strain WU-K2R was newly isolated from soil for its ability to grow in a medium with NTH as the sole source of sulfur, and growing cells of WU-K2R degraded 0.27 mM NTH within 7 days. WU-K2R could also grow in the medium with NTH sulfone, benzothiophene (BTH), 3-methyl-BTH, or 5-methyl-BTH as the sole source of sulfur but could not utilize DBT, DBT sulfone, or 4,6-dimethyl-DBT. On the other hand, WU-K2R did not utilize NTH or BTH as the sole source of carbon. By gas chromatography-mass spectrometry analysis, desulfurized NTH metabolites were identified as NTH sulfone, 2'-hydroxynaphthylethene, and naphtho[2,1-b]furan. Moreover, since desulfurized BTH metabolites were identified as BTH sulfone, benzo[c][1,2]oxathiin S-oxide, benzo[c][1,2]oxathiin S,S-dioxide, o-hydroxystyrene, 2-(2'-hydroxyphenyl)ethan-1-al, and benzofuran, it was concluded that WU-K2R desulfurized NTH and BTH through the sulfur-specific degradation pathways with the selective cleavage of carbon-sulfur bonds. Therefore, Rhodococcus sp. strain WU-K2R, which could preferentially desulfurize asymmetric heterocyclic sulfur compounds such as NTH and BTH through the sulfur-specific degradation pathways, is a unique desulfurizing biocatalyst showing properties different from those of DBT-desulfurizing bacteria.

FIG. 1.

Time course of NTH degradation by growing cells of Rhodococcus sp. strain WU-K2R. WU-K2R was cultivated in MG medium with 0.27 mM NTH as the sole source of sulfur and 1% (vol/vol) N,N-dimethylformamide. Symbols: ▵, growth (OD₆₆₀); □, pH; ○, NTH.

FIG. 2.

GC-MS analysis of desulfurized NTH metabolites produced by Rhodococcus sp. strain WU-K2R. (A) NTH; (B) NTHO₂; (C) HNE; (D) NFU.

FIG. 3.

Proposed pathways of BTH (A) and NTH (B) desulfurization by Rhodococcus sp. strain WU-K2R. Compounds in brackets, that is, HPESi in panel A and 2-(2′-hydroxynaphthyl)ethen-1-sulfinate (HNESi) and 2-(2′-hydroxynaphthyl)ethan-1-al (HNEal) in panel B were not identified and are indicated as postulated metabolites. Ring numbering of BTH and NTH is also shown. The dotted arrow in panel A indicates that BFU might be produced via HPEal from BcOTO₂ (see text).