Degradation of pseudo-solubilized and mass hexadecane by a Pseudomonas aeruginosa with treatment of rhamnolipid biosurfactant

Abstract

Degradation of rhamnolipid-solubilized hexadecane and mass hexadecane as a separate phase by Pseudomonas aeruginosa CCTCC AB93066 treated with rhamnolipid biosurfactant was studied for better understanding on the roles of rhamnolipid in hydrocarbon biodegradation. The results of hexadecane solubilization experiment showed that solubility of hexadecane was linearly related to the concentration of rhamnolipid below or above its critical micelle concentration (CMC), and the ability of monorhamnolipid (monoRL) to solubilize hexadecane was stronger at concentration below CMC than above CMC. MonoRL was then used for treating cells in degradation experiment. Results showed that 75 μM (1 CMC) monoRL treatment had a small inhibitory effect on cell growth on glucose or mass hexadecane, however 750 μM (10 CMC) monoRL treatment accelerated degradation of mass hexadecane by reducing the lag phase of cell growth for 36 h; this effect was not caused by initial cell surface hydrophobicity enhancement. No degradation of hexadecane solubilized by 750 μM monoRL was observed for the cells treated with or without monoRL, indicating that the pseudo-solubilized hexadecane is not available to cells. It is inferred from the data that the effectiveness of rhamnolipid to accelerate degradation of hydrocarbons by enhancing solubilization of the hydrocarbons may not always be guaranteed, which is of importance for evaluation of rhamnolipid biosurfactant application to hydrocarbon-contaminated sites during remediation.