Title : Biosurfactants a multifunctional molecules for bioremediation of oil contaminated soils biocontrol agent for plant pathogen elimination and insect pest control An Alternative strategy
Biosurfactants (BS) are amphiphilic compounds produced by microorganisms mainly bacteria are known to contain hydrophobic and hydrophilic moieties that play a major role in reducing surface tension (ST) and interfacial tensions of air/water, water/oil, and between individual molecules. Increasing environmental awareness and emphasis on justifiable society in coordination with the universal environment, during the current years, has led to serious consideration of biosurfactants as possible alternative to synthetic surfactants as they cause environmental problems due to their resistance to biodegradability as well as toxicity to ecosystems.
Chemically, BS is a complex molecules consisting of lipopeptides, glycolipids, polysaccharide protein complex, fatty acids and phospholipids. They are known for their biodegradability, low toxicity, and renewable nature. They are produced from cheaper raw materials, including agro-industrial wastes. Rhamnolipids (RLs) that belong to glycolipid type biosurfactant is one of the most intensively studied biosurfactant produced extensively by Pseudomonas aeruginosa. Biosurfactant usage finds application in environmental and agricultural sectors such as i) bioremediation of hydrocarbons, and heavy metals ii) enhanced oil recovery and iii) pesticide biodegradation and biopesticides production in agriculture.
Rhamnolipid-type biosurfactants belong to a group of anionic surfactants comprising of l-(+)-rhamnose and β-hydroxyalkanoic acid units. Pseudomonas aeruginosa is the most widely studied rhamnolipid-producing bacterium known for its ability to metabolize a variety of hydrophobic substrates, including n-alkanes, hexadecane, and oils. Di-rhamnolipid synthesis in P. aeruginosa occurs in three consecutive enzymatic reactions: 3-(3-hydroxyalkanoyloxy) alkanoic acid (HAA) synthesis catalyzed by rhamnosyltransferase I (RhlA), a glycosyltransfer reaction catalyzed by glycosyltransferase (RhlB) to form mono-rhamnolipids, and the reaction catalyzed by rhamnosyltransferase II (RhlC) that yields di-rhamnolipids . The first two enzymes encoded by RhlA and RhlB genes are both located in the RhlAB operon but are independently involved in rhamnolipid biosynthesis. Studies on the combined use of rhamnolipids and rhamnolipid-producing bacterial strains for the bioremediation of hydrocarbon-contaminated soils are scarce; at the same time, their role in the remediation of crude oil-contaminated sites has been documented by many researchers. Though much work has been conducted on rhamnolipid production focused on P. aeruginosa and other Pseudomonas strains, rhamnolipid production has also been reported for bacteria belonging to other genera, such as Acinetobacter, Enterobacter, Pantoea , and Serratia.
Rhamnolipids were mainly targeted to fungi and oomycetes including Botrytis sp., Rhizoctonia sp., Fusarium sp., Alternaria sp., Pythium sp., Phytophthora sp. Rhamnolipids affect mycelial cells resulting in lysis. Further, they have also studied to prevent the development of resistance by the target pest, insecticidal activity against Lepidopteran, Dipteran, Coleopteran pest were studied. Bacillus amyloliquefaciens AG1 biosurfactant found highly effective against Spodoptera littoralis and Pseudomonas sp producing biosurfactant were found highly effective against dipteran and coleopteran insect pests. The histopathological examination of larval midgut showed vacuolization, necrosis and disintegration of the basement membrane due to the biosurfactant. This review paper deals with possible role of biosurfactants as an alternative strategy for the remediation of crude oil-contaminated sites and to reduce or even replace fungicide and pesticide in Agriculture.