Rhamnolipids and nutrients boost remediation of crude oil-contaminated soil by enhancing bacterial colonization and metabolic activities

• Augmentation of rhamnolipids and nutrients in soil enhanced crude oil reduction.
• Rhamnolipids and nutrients enhanced the abundance and expression of alkB gene.
• Bacterial population and activity decreased with the elapse of time.
• Gene expression and crude oil reduction was positively correlated.

Bioaugmentation is a promising approach for the remediation of soil and water contaminated with petroleum hydrocarbons. Addition of rhamnolipids and nutrients in the soil enhance survival and metabolic activity of microbes and thus has the potential to restore the quality of soil to a greater extent. This study was carried out to evaluate the prospective role that these supplementary compounds can play in remediation of crude oil-contaminated soil by aiding bacterial strains to survive and perform better. A consortium of hydrocarbon degrading bacteria was inoculated in crude oil-contaminated soil and supplemented with different combinations of rhamnolipids and nutrients. Maximum crude oil-degradation of 77.6% was observed in the soil inoculated with hydrocarbon-degrading bacteria supplemented with rhamnolipids and nutrients. Moreover, addition of supplementary compounds enhanced bacterial survival as well as abundance and expression of alkane hydroxylase gene, alkB, in oil-contaminated soil. A strong positive relationship (r = 0.94) observed between gene expression and crude oil reduction indicates that catabolic gene expression is essential for hydrocarbon mineralization. All results from the investigation collectively confirm that addition of rhamnolipids and nutrients enhance bacterial colonization and metabolic activity thus improving remediation of crude oil-contaminated soil.