Rhamnolipids produced under aerobic/anaerobic conditions: Comparative analysis and their promising applications

This research comprises a comparative study of the properties, rhl genes expression, and structural difference in rhamnolipids produced under different oxygen conditions via Pseudomonas sp. CH1. The critical micelle concentration (CMC) of rhamnolipids produced under aerobic conditions (RAO) was 100 mg/L. In contrast, rhamnolipids produced under anaerobic conditions (RNO) had a low CMC of 40 mg/L. RNO comprised six rhamnolipids homologs, and the proportion of mono-rhamnolipids was up to 87.83%; meanwhile, the percent ratio of di-rhamnolipids and mono-rhamnolipids in RAO was 63.1:36.9. Additionally, diversified applications for solubilization of hydrophobic pollutants and reduction in heavy oil viscosity were investigated. The addition of RNO greatly enhanced the solubility of phenanthrene in water, from 1.29 mg/L to 193.14 mg/L, a 148.7-fold increase. Moreover, the viscosity of heavy oil decreased by over 90% for both kinds of rhamnolipids, whereas RAO effectively reduced the viscosity even at a low temperature (10 °C). The findings of this study provide insights into the versatile potential applications of rhamnolipids produced under different oxygen conditions.

Enhanced production of mono-rhamnolipid in Pseudomonas aeruginosa and application potential in agriculture and petroleum industry

Differences in the rhamnolipid structures must result in its different activities, thus affecting its application effect. The rhlC gene in Pseudomonas aeruginosa SG was knocked out to construct strain P. aeruginosa SGΔrhlC. Rhamnolipid production was enhanced by 23.3% through knocking out rhlC gene. P. aeruginosa SGΔrhlC produced 14.22 g/L of rhamnolipid using glycerol and nitrate. Five kinds of mono-rhamnolipid but no di-rhamnolipid were produced by strain SGΔrhlC. The main rhamnolipid homologues were Rha-C₁₀-C₁₀, Rha-C₁₀-C_12:1 and Rha-C₁₀-C₁₂. Mono-rhamnolipid exhibited better antimicrobial activity to Escherichia coli, Staphylococcus aureus, Aspergillus niger and Penicillium chrysogenum. Rhamnolipid produced from strain SGΔrhlC showed greater emulsifying activity to crude oil with EI₂₄ of 84.73%. Rhamnolipid produced from strain SGΔrhlC efficiently washed oily sludge at 35 °C. High-producing strain P. aeruginosa SGΔrhlC and its produced mono-rhamnolipid are more promising in agriculture and petroleum industry. This study is a step forward to the tailor-made biosynthesis and application of rhamnolipid.

Bioprospecting of rhamnolipids production and optimization by an oil-degrading Pseudomonas sp. S2WE isolated from freshwater lake

The present study revealed biosurfactants production by a novel oil-degrading Pseudomonas sp. S2WE isolated from hydrocarbon enriched water sample, where the genus Pseudomonas (48.65%) was dominated amongst several other genera. Biosurfactants produced by this strain showed the great potential for surface tension reduction (SFT) and emulsification. The extracted crude biosurfactants were characterized using ultra-high-performance liquid chromatography-Mass Spectrometry (UHPLC-MS) and identified various mono-and di-rhamnolipids homologs from the mixture. Moreover, the lowest SFT 33.05 ± 0.1 mN/m and highest emulsification of 60.65 ± 0.64% were achieved from rhamnolipids produced from glycerol with urea. Compared to initial screening, almost (>87%) higher emulsification was observed. In addition, the biosurfactants were found highly stable at different environmental factors i.e. temperature (4 °C-121 °C), pH (3-10) and NaCl conc. (1-9%). The high stable rhamnolipids produced by new Pseudomonas sp. S2WE in this study could widely be used in enormous industrial as well as environmental applications.

Production, functional stability, and effect of rhamnolipid biosurfactant from Klebsiella sp. on phenanthrene degradation in various medium systems

The present study investigated the stability and efficacy of a biosurfactant produced by Klebsiella sp. KOD36 under extreme conditions and its potential for enhancing the solubilization and degradation of phenanthrene in various environmental matrices. Klebsiella sp. KOD36 produced a mono-rhamnolipids biosurfactant with a low critical micelle concentration (CMC) value. The biosurfactant was stable under extreme conditions (60 °C, pH 10 and 10% salinity) and could lower surface tension by 30% and maintained an emulsification index of > 40%. The emulsion index was also higher (17-43%) in the presence of petroleum hydrocarbons compared to synthetic surfactant Triton X-100. Investigation on phenanthrene degradation in three different environmental matrices (aqueous, soil-slurry and soil) confirmed that the biosurfactant enhanced the solubilization and biodegradation of phenanthrene in all matrices. The high functional stability and performance of the biosurfactant under extreme conditions on phenanthrene degradation show the great potential of the biosurfactant for remediation applications under harsh environmental conditions.

Oxygen effects on rhamnolipids production by Pseudomonas aeruginosa

Background

Rhamnolipids are the most extensively studied biosurfactants and has been successfully used in various areas from bioremediation to industrial fields. Rhamnolipids structural composition decide their physicochemical properties. Different physicochemical properties influence their application potential. Rhamnolipids can be produced at both aerobic conditions and anaerobic conditions by Pseudomonas aeruginosa. This study aims to evaluate the oxygen effects on the rhamnolipids yield, structural composition, physicochemical properties and the rhl-genes expression in P. aeruginosa SG. Results will guide researchers to regulate microbial cells to synthesize rhamnolipids with different activity according to diverse application requirements.

Results

Quantitative real-time PCR analysis revealed that rhlAB genes were down-regulated under anaerobic conditions. Therefore, strain P. aeruginosa SG anaerobically produced less rhamnolipids (0.68 g/L) than that (11.65 g/L) under aerobic conditions when grown in media containing glycerol and nitrate. HPLC–MS analysis showed that aerobically produced rhamnolipids mainly contained Rha-C₈-C₁₀, Rha–Rha-C₁₀-C_12:1 and Rha–Rha-C₈-C₁₀; anaerobically produced rhamnolipids mainly contained Rha-C₁₀-C₁₂ and Rha-C₁₀-C₁₀. Anaerobically produced rhamnolipids contained more mono-rhamnolipids (94.7%) than that (54.8%) in aerobically produced rhamnolipids. rhlC gene was also down-regulated under anaerobic conditions, catalyzing less mono-rhamnolipids to form di-rhamnolipids. Aerobically produced rhamnolipids decreased air–water surface tension (ST) from 72.2 to 27.9 mN/m with critical micelle concentration (CMC) of 60 mg/L; anaerobically produced rhamnolipids reduced ST to 33.1 mN/m with CMC of 80 mg/L. Anaerobically produced rhamnolipids emulsified crude oil with EI₂₄ = 80.3%, and aerobically produced rhamnolipids emulsified crude oil with EI₂₄ = 62.3%. Both two rhamnolipids products retained surface activity (ST < 35.0 mN/m) and emulsifying activity (EI₂₄ > 60.0%) under temperatures (4–121 °C), pH values (4–10) and NaCl concentrations less than 90 g/L.

Conclusions

Oxygen affected the rhl-genes expression in P. aeruginosa, thus altering the rhamnolipids yield, structural composition and physicochemical properties. Rhamnolipids produced at aerobic or anaerobic conditions was structurally distinct. Two rhamnolipids products had different application potential in diverse biotechnologies. Although both rhamnolipids products were thermo-stable and halo-tolerant, aerobically produced rhamnolipids possessed better surface activity, implying its well wetting activity and desorption property; anaerobically produced rhamnolipids exhibited better emulsifying activity, indicating its applicability for enhanced oil recovery and bioremediation of petroleum pollution.

Mechanism-specific and whole-organism ecotoxicity of mono-rhamnolipids

Biosurfactants like rhamnolipids are promising alternatives to chemical surfactants in a range of applications. A wider use requires an analysis of their environmental fate and their ecotoxicological potential. In the present study mono-rhamnolipids produced by a recombinant Pseudomonas putida strain were analyzed using the Green Toxicology concept for acute and mechanism-specific toxicity in an ecotoxicological test battery. Acute toxicity tests with the invertebrate Daphnia magna and with zebrafish embryos (Danio rerio) were performed. In addition, microbial and fungicidal effectiveness was investigated. Mutagenicity of the sample was tested by means of the Ames fluctuation assay. A selected mono-rhamnolipid was used for model simulations regarding mutagenicity and estrogenic activity. Our results indicate that mono-rhamnolipids cause acute toxicity to daphnids and zebrafish embryos comparable to or even lower than chemical surfactants. Rhamnolipids showed very low toxicity to the germination of Aspergillus niger spores and the growth of Candida albicans. No frameshift mutation or base substitutions were observed using the Ames fluctuation assay with the two tester strains TA98 and TA100. This result was confirmed by model simulations. Likewise it was computed that rhamnolipids have no estrogenic potential. In conclusion, mono-rhamnolipids are an environmental friendly alternative to chemical surfactants as the ecotoxicological potential is low.