Microbial conversion of agro-processing waste (peanut meal) to rhamnolipid by Pseudomonas aeruginosa: solid-state fermentation, water extraction, medium optimization and potential applications.

The high cost and severe foam in rhamnolipid fermentation are still bottlenecks for its industrial production and application. Non-foaming production of rhamnolipid by Pseudomonas aeruginosa FA1 was explored in solid-state fermentation using the agro-processing waste (peanut meal) as low-cost substrate. An environmental-friendly extraction method was developed to harvest rhamnolipid from solid-state culture. Strain FA1 produced 265.4 ± 8.2 mg rhamnolipid using 10 g peanut meal. HPLC-MS results revealed that 7 rhamnolipid homologues were produced, mainly including Rha-C8-C10 and Rha-Rha-C10-C10. Nitrate was the optimal nitrogen source. Peanut meal, MgSO4 and CaCl2 were significant factors for rhamnolipid production in solid-state fermentation. Rhamnolipid production was enhanced 31 % using the solid-state medium optimized by response surface method. The produced rhamnolipid reduced water surface tension to 28.1 ± 0.2 mN/m with a critical micelle concentration of 70 mg/L. The crude oil was emulsified with an emulsification index of 75.56 ± 1.29 %. The growth of tested bacteria and fungi was inhibited.

Glycerol or crude glycerol as substrates make Pseudomonas aeruginosa achieve anaerobic production of rhamnolipids.

BACKGROUND: The anaerobic production of rhamnolipids is significant in research and application, such as foamless fermentation and in situ production of rhamnolipids in the anoxic environments. Although a few studies reported that some rare Pseudomonas aeruginosa strains can produce rhamnolipids anaerobically, the decisive factors for anaerobic production of rhamnolipids were unknown. RESULTS: Two possible hypotheses on the decisive factors for anaerobic production of rhamnolipids by P. aeruginosa were proposed, the strains specificity of rare P. aeruginosa (hypothesis 1) and the effect of specific substrates (hypothesis 2). This study assessed the anaerobic growth and rhamnolipids synthesis of three P. aeruginosa strains using different substrates. P. aeruginosa strains anaerobically grew well using all the tested substrates, but glycerol was the only carbon source that supported anaerobic production of rhamnolipids. Other carbon sources with different concentrations still failed for anaerobic production of rhamnolipids by P. aeruginosa. Nitrate was the excellent nitrogen source for anaerobic production of rhamnolipids. FTIR spectra analysis confirmed the anaerobically produced rhamnolipids by P. aeruginosa using glycerol. The anaerobically produced rhamnolipids decreased air-water surface tension to below 29.0 mN/m and emulsified crude oil with EI24 above 65%. Crude glycerol and 1, 2-propylene glycol also supported the anaerobic production of rhamnolipids by all P. aeruginosa strains. Prospects and bottlenecks to anaerobic production of rhamnolipids were also discussed. CONCLUSIONS: Glycerol substrate was the decisive factor for anaerobic production of rhamnolipids by P. aeruginosa. Strain specificity resulted in the different anaerobic yield of rhamnolipids. Crude glycerol was one low cost substrate for anaerobic biosynthesis of rhamnolipids by P. aeruginosa. Results help advance the research on anaerobic production of rhamnolipids, deepen the biosynthesis theory of rhamnolipids and optimize the anaerobic production of rhamnolipids.