Effects Of Rhamnolipid On Biodegradation Of Hexadecane By Candida Tropicalis And Pseudomonas Aeruginosa

In soil remediation, one way of biosurfactants enhance degradation of hydrophobic organic contaminants is to increase the surface hydrophobicity of microbial cell and facilitate the direct contact of microorganisms with the contaminants trapped in soil pores. However, the manner of interaction between biosurfactants and microorganisms as well as the rules of cell surface hydrophobicity change as the function of biosurfactants were not fully understood. In this paper, the effects of different kinds surfactants on degradation of different carbon sources are systematically studied.First the rhamnolipid biosurfactant was produced by aerobic fermentation using a Pseudomonas aeruginosa strain. It was separated from the culture by acid precipitation and purified by column chromatography until monorhamnolipid and dirhamnolipid were obtained. HPLC-MS examination showed that both of the rhamnolipids contained three main components and they were used in the following studies.Then the ability of the two rhamonolipid to enhance the apparent solubility of n-hexadecane was investigated, and the effect of monorhamnolipid adsorption on degradation of glucose, aggregate-incorporated n-hexadecane and single-phased n-hexadecane was also studied. The monorhamnolipid exhibited different manner of enhancing hexadecane solubility when its aqueous concentration was lower or higher than CMC, while the capability of dirhamnolipid was the same in the whole concentration range tested. Adsorption of monorhamnolipid of low concentration to some extent restricted the cell growth on glucose and single-pahsed hexadecane, however, adsorption of monorhamnolipid of high concentration had insignificant or stimulative effect on the cell growth on glucose and on hexadecane, respectively. For the hexadecane incorporated in aggregates formed by high-concentration monorhamnolipid, they could not be degraded by the cells treated with or without the surfactant. This result indicated that the incorporated hexadecane was unavailable to the Pseudomonas aeruginosa strain.Finally the effects of three kinds of surfactans (Triton X-100,di-RL,CTAB) on hexadecane degradation by Candida tropicalis CICC 1463 were studied. Results showed that both type and concentration of the surfactants affected cell growth and hexadecane degradation. The cell growth and hexadecane degradation were little effected by TX-100 of low concentrations, but inhibited at high concentrations. Di-RL increasingly enhanced cell growth and hexadecane degradation with concentration increasing. Di-RL was also degraded in the fermentation process, and the degradation rate increased with concentration increasing. CTAB was toxic to Candida tropicalis CICC 1463.The paper disclosed the effect of adsorption of rhamnolipid biosurfactants to microorganisms changing cell surface hydrophobicity. Because this effect occurs at low biosurfactant concentrations, it implies the possibility of economical and effective application of biosurfactant in soil remediation, especially in situ remediation with exogenous bacterial amendments.