Role of Pseudomonas rhamnolipid surfactants in biodegradation of slightly soluble hydrocarbons.

Abstract

Biodegradation of hydrocarbons is often limited by the low water solubility. Surfactants can increase hydrocarbon solubility, however, the effects of surfactants on hydrocarbon biodegradation are not well known. Therefore, the purpose of this research is to investigate the fundamental interactions between surfactants, hydrocarbons and microorganisms. This three-way interaction was studied in the defined experimental system consisting of Pseudomonas rhamnolipid surfactants, hydrocarbons and Pseudomonas hydrocarbon degraders. A variety of system factors affecting biodegradation were examined. These factors included surfactant structure, surfactant concentration, hydrocarbon structure and cell hydrophobicity of microorganisms. Pseudomonas sp. are able to produce several rhamnolipid types. Among three rhamnolipid types tested in this study, dirhamnolipid methyl ester had the greatest effect on alkane solubilization and biodegradation. Monorhamnolipid acid exhibited higher solubilization of alkanes than dirhamnolipid acid. Monorhamnolipid acid at high concentrations ( > 0.1 mM) enhanced alkane biodegradation more effectively than dirhamnolipid acid. In contrast, dirhamnolipid acid enhanced degradation more effectively at low concentrations ( < 0.1 mM). The biodegradation of alkanes was affected by microbial cell surface hydrophobicity. Results showed that inherently slow alkane degraders had low cell hydrophobicity while the inherently fast alkane degraders had high cell hydrophobicity. Rhamnolipids enhanced cell hydrophobicity of the slow degraders but had no effect on the cell hydrophobicity of the fast degraders. The rate at which the cells became hydrophobic depended on the rhamnolipid concentration and was directly related lo the rate of alkane biodegradation. Rhamnolipid mixtures had a different effect from single rhamnolipids on solubilization and biodegradation of alkanes. The effect can be synergistic or additive depending on the surfactant mixture. For instance, the mixture of dirhamnolipid acid and methyl ester had a synergistic effect on solubilization and additive effect on biodegradation. The effect of rhamnolipids on the hydrocarbon biodegradation varied with hydrocarbon structure. For instance, enhanced biodegradation of model alkanes was determined by both rhamnolipid structure and concentration. In contrast, enhanced phenanthrene biodegradation seemed to depend only on rhamnolipid concentration. The results of this research suggest that rhamnolipids have potential use for remediation of petroleum-contaminated sites. Further research is recommended to investigate the effect of rhamnolipids on hydrocarbon biodegradation in soil systems.