Applications of biological surface active compounds in remediation technologies

Many microorganisms synthesize a wide range of surface active compounds (SACs), classified according to their molecular weights, properties and localizations. The low molecular weight SACs or biosurfactants lower the surface tension at the air/water interfaces and the interfacial tension at oil/water interfaces, whereas the high molecular weight SACs, also known as bioemulsifiers, are more effective in stabilizing oil-in-water emulsions. The ability to biosynthesize SACs is, often, coupled with the ability of these microorganisms to grow on immiscible carbon sources, such as hydrocarbons. Different mechanisms are involved in the SACs interactions between microbial cells and immiscible hydrocarbons including: (i) emulsification, (ii) micellarization, (iii) adhesion-deadhesion of microorganisms to and from hydrocarbons and (iv) desorption of contaminants. These naturally occurring phenomena can be exploited by adding bioemulsifiers and biosurfactants into environments where bioremediation/biodegradation rates of organic pollutants is to be enhanced. However, analysis of the current literature show some cases where the complex interactions among SACs, microbial cells, organic substrates and environmental media led to an inhibition of the biodegradation. The understanding of the different physiological roles of SACs in microbial communities is fundamental in order to develop more effective remediation technologies exploiting both synthetic surfactants and microbial SACs. The physio-chemical properties of some microbial SACs have been exploited in hydrocarbon-contaminated soils washing and in mobilisation of soil-bound metal in metal-contaminated soils. Our ability to analyse the microbial diversity in the natural environments will expand our knowledge on microbial SACs with respect to their exploitation for commercial applications and their roles in the physiology of the producing microorganisms.