Microbial biosurfactants: a review of recent environmental applications

Microbial biosurfactants are low-molecular-weight surface-active compounds of high industrial interest owing to their chemical properties and stability under several environmental conditions. The chemistry of a biosurfactant and its production cost are defined by the selection of the producer microorganism, type of substrate, and purification strategy. Recently, biosurfactants have been applied to solve or contribute to solving some environmental problems, with this being their main field of application. The most referenced studies are based on the bioremediation of contaminated soils with recalcitrant pollutants, such as hydrocarbons or heavy metals. In the case of heavy metals, biosurfactants function as chelating agents owing to their binding capacity. However, the mechanism by which biosurfactants typically act in an environmental field is focused on their ability to reduce the surface tension, thus facilitating the emulsification and solubilization of certain pollutants (in-situ biostimulation and/or bioaugmentation). Moreover, despite the low toxicity of biosurfactants, they can also act as biocidal agents at certain doses, mainly at higher concentrations than their critical micellar concentration. More recently, biosurfactant production using alternative substrates, such as several types of organic waste and solid-state fermentation, has increased its applicability and research interest in a circular economy context. In this review, the most recent research publications on the use of biosurfactants in environmental applications as an alternative to conventional chemical surfactants are summarized and analyzed. Novel strategies using biosurfactants as agricultural and biocidal agents are also presented in this paper.