Enhanced octadecane dispersion and biodegradation by a Pseudomonas rhamnolipid surfactant (biosurfactant)

A microbial surfactant (biosurfactant) was investigated for its potential to enhance bioavailability and, hence, the biodegradation of octadecane. The rhamnolipid biosurfactant used in this study was extracted from culture supernatants after growth of Pseudomonas aeruginosa ATCC 9027 in phosphate- limited proteose peptone-glucose-ammonium salts medium. Dispersion of octadecane in aqueous solutions was dramatically enhanced by 300 mg of the rhamnolipid biosurfactant per liter, increasing by a factor of more than 4 orders of magnitude, from 0.009 to >250 mg/liter. The relative enhancement of octadecane dispersion was much greater at low rhamnolipid concentrations than at high concentrations. Rhamnolipid-enhanced octadecane dispersion was found to be dependent on pH and shaking speed. Biodegradation experiments done with an initial octadecane concentration of 1,500 mg/liter showed that 20% of the octadecane was mineralized in 84 h in the presence of 300 mg of rhamnolipid per liter, compared with only 5% octadecane mineralization when no surfactant was present. These results indicate that rhamnolipids may have potential for facilitating the bioremediation of sites contaminated with hydrocarbons having limited water solubility.