Oxygen effects on rhamnolipids production by Pseudomonas aeruginosa

Abstract

BackgroundRhamnolipids 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.ResultsQuantitative 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-C12:1 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.ConclusionsOxygen 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.

Glycerol promoted the anaerobic production of rhamnolipids by different Pseudomonas aeruginosa strains

“…Anaerobically produced rhamnolipids was con rmed by FTIR spectra analysis using rhamnolipids produced by strain P. aeruginosa SG as positive control. Brie y, 10 mg of rhamnolipids extract and 90 mg of KBr was mixed to make the translucent pellet with pressure of 25 Mpa for 25 s. A NICOLET 380 FTIR spectrometer was used to record the FTIR spectra of the translucent pellet with the wave number from 400 cm −1 to 4000 cm −1 [10,18].…”

“…The anaerobic biosynthesis of rhamnolipids has important research and practical signi cance. Anaerobic production of rhamnolipids can meet the in situ production of biosurfactant in anoxic environments, such as deep soil, oil reservoirs, sediments [8][9][10]. Based on anaerobic production of rhamnolipids, fermentation without aeration can avoid the foam problem in production of rhamnolipids.…”

“…The anaerobic biosynthesis of rhamnolipids has important research and practical signi cance, such as meeting the in situ production of biosurfactant in anoxic environments and the foamless fermentation of biosurfactants [8]. Aerobic production of rhamnolipids fails to meet the in situ applications in the anoxic environments, such as deep soil, oil reservoirs, sediments [8][9][10]. The foam problem in biosurfactant fermentation has been perplexing the production of biosurfactants [11].…”

“…Although the mainly rhamnolipids producers, Pseudomonas aeruginosa, are facultative bacteria [8,13,14], only few studies have reported the anaerobic biosynthesis of rhamnolipids by rare P. aeruginosa strains [9,10,15,16]. P. aeruginosa can grow and metabolize at both aerobic and anaerobic conditions.…”

Enhanced rhamnolipids production using a novel bioreactor system based on integrated foam-control and repeated fed-batch fermentation strategy

“…However, it cannot effectively break rhamnolipid foam, and even may aggravate more serious problem by producing more stable secondary foam in the top space of bioreactor [13]. Moreover, anaerobic fermentation, solid state fermentation and membrane assisted bioreactor have also been applied to decrease the air flow to reduce the foam formation during the fermentation [15,16,17]. In these cases, reduction of the aeration could control severe foaming, but it impaired the rhamnolipids productivity because of oxygen limitation [17].…”