Edinburgh Napier University

  Biodesulphurisation (BDS) is an emerging technology that utilises microorganisms for the removal of sulphur from fossil fuels. Commercial-scale BDS needs the development of highly active bacterial strains which allow easier downstream processing. In this research, a collection of actinobacteria that originated from oil-contaminated soils in Russia were investigated to establish their phylogenetic positions and biodesulphurisation capabilities.
The eleven test strains were confirmed as members of the genus Rhodococcus based on 16S rRNA and gyrB gene sequence analysis. Two organisms namely strain F and IEGM 248, confirmed as members of the species R. qingshengii and R. opacus, respectively based on the wholegenome sequence based OrthoANIu values, exhibited robust biodesulphurisation of dibenzothiophene (DBT) and benzothiophene (BT), respectively. R. qingshengii strain F was found to convert DBT to hydroxybiphenyl (2-HBP) with DBTO and DBTO2 as intermediates. The DBT desulphurisation genes of strain F occur as a cluster and share high sequence similarity with the dsz operon of R. erythropolis IGTS8. Rhodococcus opacus IEGM 248 could convert BT into benzofuran. The BDS reaction of both strains follows the well-known 4S pathway of desulphurisation of DBT and BT.
When cultured directly in a biphasic growth medium containing 10% (v/v) oil (n-hexadecane or diesel) containing 300 ppm sulphur, strain F formed a stable oil-liquid emulsion, making it unsuitable for direct industrial application despite the strong desulphurisation activity. Whereas the strain 248 formed distinct oil, biomass and aqueous phases which enabled easy extraction of the desulphurised oil with more than 80% reduction in total sulphur content, making it a potential candidate strain for the development of a robust BDS biocatalyst to upgrade crude oils and refinery streams at industrial scale.
Whole-genome analysis of these strains also revealed the presence of a high copy number of various monooxygenases and sulphur metabolism related genes that occurred in clusters. These genes offer potential target sites for future mutation strategies to forestall sulphate induced repression of the desulphurisation genes which warrants future investigation.