Effect of fatty acid substrate chain length on Pseudomonas aeruginosa ATCC 9027 monorhamnolipid yield and congener distribution

Rhamnolipids are surface-active molecules produced by Pseudomonas aeruginosa as congener mixtures. They are considered "green" alternatives to synthetic surfactants used in industrial, remediation and pharmaceutical applications. Optimizing yield as well as controlling congener distribution are necessary steps for successful commercialization of rhamnolipids. This study used a mixture of glucose and fatty acids of different chain length (C₁₂-C₂₂) and saturation (C_18:1 and C_18:2) to produce monorhamnolipids and determine the effect of fatty acid substrates on rhamnolipid yield, percent carbon conversion and congener distribution. Results show that 1% glucose + 0.25% stearic acid (C ₁₈) produced the greatest yield (2.1 g L^-1) compared to other glucose-fatty acid combinations (0.8-1.8 g L^-1). Various glucose + C₁₈ ratios were then tested to optimize yield and percent substrate carbon conversion to monorhamnolipid. Results revealed a positive linear correlation between the mass percent of C₁₈ used and the percent carbon conversion. A mass percent of 67% C₁₈ was optimal resulting in a 44% carbon conversion and a yield of 13.7 g L^-1 monorhamnolipid. For all fatty acid substrates tested, the RhaC ₁₀C₁₀ was the most abundant and RhaC₁₀C _12:1 was the least abundant of the four major congeners produced. However, the relative amount of RhaC₁₀C₈ and RhaC ₁₀C₁₂ congeners was dependent on several factors: in general, fatty acid substrates with relatively short chain length (C ₁₂ and C₁₄), unsaturated fatty acid substrate ( _C18:2), and longer cultivation time resulted in a higher RhaC ₁₀C₈/RhaC₁₀C₁₂ ratio. These findings will assist in mass production of monorhamnolipids and controlling the specific congeners produced.