Characterization and antifungal properties of rhamnolipids produced by mangrove sediment bacterium Pseudomonas aeruginosa strain KVD-HM52

A Pseudomonas aeruginosa strain KVD-HM52 exhibiting good emulsification activity (E24%) and oil spreading capacity was isolated from oil contaminated mangrove sediments. Biochemical characteristics and sequencing of 16S rRNA gene of the selected strain confirmed the isolate as P. aeruginosa. Highest biosurfactant production of 5.26 g/L was obtained in MSM supplemented with 2% molasses. TLC analysis of crude biosurfactant showed two characteristic spots of di-rhamnolipid and mono-rhamnolipid. FTIR spectroscopy revealed the presence of aliphatic chain, hydroxyl groups, ester and glycosidic bonds. Presence of mono and di-rhamnolipidic structural analogues with molecular compositions of Rha-C10-C10 and Rha-Rha-C10-C10 were revealed from LC–MS coupled with ESI. The surface tension was reduced from 74.12 m Nm−1 to 33.03 m Nm−1 at 25 °C, at a CMC of 120 mg/mL. The purified RL biosurfactant of strain KVD-HM52 showed significant antifungal activity against Fusarium oxysporum wilt disease in tomato plants. Application of 200 µg/L of purified rhamnolipid biosurfactant completely inhibited disease severity in tomato plants after 14 days. Biosurfactant being cost effective, low toxic and highly biodegradable can be efficiently used as a potential alternative to replace harmful agrochemicals.