Biosynthesis of rhamnolipids and their roles in bioremediation

Biosurfactants have a wide range of commercial applications, such as in food processing, pharmaceutical, and waste disposal industries. They are biocompatible, effective, and may serve as the sources for fine chemicals used in scientific and industrial settings; for example, rhamnolipids produced by Pseudomonas aeruginosa are the sources of rhamnose and β-hydroxyalkanoic acids. In this research, the production relationships between rhamnolipids and polyhydroxyalkanoic acids under aerobic condition were first investigated. It was found that the microorganisms accumulate poly-β-hydroxyalkanoic acids during the active growth and switch to form rhamnolipids once entering the stationary phase.; High biosurfactant productivity is normally unachievable due to the production difficulty. A severe foaming problem requires lowering the aeration, causing reduced cell activity and rhamnolipid productivity. To eliminate this foaming problem, the production of rhamnolipids under anaerobic denitrification has been successfully achieved in this study by using palmitic acid as a substrate under phosphorus limitation. Much higher cell concentrations can thus be used for higher rhamnolipid production without foaming and oxygen limitation. Phosphorus was also found more effective in the production of rhamnolipids than the nitrogen-limiting nutrient.; Since P. aeruginosa is commonly found in the environment, its bioremediation application was studied with the degradation of hydrocarbon (hexadecane). P. aeruginosa is capable of degrading hexadecane under aerobic condition with the carbons from hexadecane transformed to partially oxidized metabolites. Almost half of these oxidized metabolites can be further degraded under microaerobic and anaerobic denitrifying conditions. This sequential bioremediation application is useful since oxygen is generally limited in the sub-surface of soil.; The biosurfactants are also potentially significant for cleanup of the petroleum-contaminated soil. In this study, rhamnolipids (anionic) were found to be strongly adsorbed on the contaminated soil, caused by the ionic interaction mechanism. In addition, sophorolipids (nonionic) were found to be predominantly adsorbed on uncontaminated soil by the hydrogen bonding mechanism.