Effects of Fatty Acid Substrates on Rhamnolipid (Biosurfactant) Biosynthesis and Congener Distribution

Abstract

Rhamnolipids are surface-active molecules produced by Pseudomonas aeruginosa as congener mixtures. They are considered “green” alternatives to synthetic surfactants used in many applications. Optimizing yield and controlling congener distribution are necessary steps for successful commercialization. Studies have noted that vegetable oils, composed of a mixture of fatty acids, increase rhamnolipid yield. The physiological explanation for this is not yet understood. Furthermore the exact effects of various fatty acid components in the oils on rhamnolipid production have not been reported. The first part of the dissertation was to investigate rhamnolipid biosynthesis when fatty acid substrates are present. A combination of stable isotope tracing and gene expression assays were used to identify rhamnolipid lipid precursors and potential lipid metabolic pathways used in rhamnolipid synthesis. Result suggests that an octanoyl-CoA intermediate of β-oxidation is diverted from β-oxidation to de novo fatty acid synthesis via a “bypass route”, and is incorporated into either a 2-carbon or a 4 carbon β-ketoacyl- ACP, which can then be recognized by the RhlA enzyme for the biosynthesis of rhamnolipid lipid moiety. The second part of the dissertation focuses on studying how fatty acid substrates of different chain length (C₁₂ to C₂₂) and saturation (C(18:1) and C(18:2)) affect rhamnolipid yield, carbon conversion rate, and congener distribution. Results showed that stearic acid significantly increased rhamnolipid yield. A positive linear correlation between the mass percent of stearic acid used and the carbon conversion rate was observed. For all treatments, the RhaC₁₀C₁₀ was the most abundant and RhaC₁₀C(12:1) was the least abundant of the major congeners produced. However, the relative amounts of RhaC₁₀C₈ and RhaC₁₀C₁₂ congeners were dependent on several factors. In general, fatty acid substrates with relatively short chain length (C₁₂ and C₁₄), the unsaturated fatty acid C(18:2), and longer cultivation times resulted in a higher RhaC₁₀C₈/ RhaC₁₀C₁₂ ratio. The studies presented here demonstrate that the medium composition, in particular the organic substrate component, can affect rhamnolipid biosynthesis, yield, and congener distribution. Furthermore, this work presents evidence that C₁₈ fatty acids as co-substrates increase rhamnolipid yield by draining rhamnolipid intermediates directly from the β-oxidation pathway.