**5. Role of biosurfactants in bioremediation**

The most frequently isolated yeast genera from soils are *Candida*, *Cryptococcus*, *Debaryomyces*, *Hansenula*, *Lipomyces*, *Pichia*, *Rhodotorula*, *Schizoblastosporion*, *Sporobolomyces*, *Torula*, and *Torulopsis* [32, 33]. Yeasts are involved in the production of a wide variety of foods, including fermented foods, alcoholic beverages, and bread. Yeasts are also involved in industrial fermentations for the production of antibiotics and vitamins among other commodities [34]. There are only few studies on biosurfactants synthesized by yeasts because most reports are related to bacteria and marine microorganisms, but the number of reports has increased, especially for *Candida* sp., *Pseudozima* sp., and *Yarrowia* sp. [35]. Table 2 shows yeast strains

**Microorganism Biosurfactant Reference** *Candida bombicola* (most studied system) Sophorolipid [36] *Candida apicola* [37] *Candida rugosa* [15] *Candida mucilaginosa* [15] *Rhodotorula bogoriensis* [38] *Pichia anomala* [39] *Candida lipolytica* Carbohydrate–protein (Liposan) [40]

(Yansan)

Yeasts can be preferred to bacteria as sources for biosurfactants because of their GRAS (generally regarded as safe) status, that is, they do not present risk of inducing toxicity or pathogenic reactions. Yeasts are also known for producing biosurfactants in higher concen‐ trations than bacteria, which is an advantage for the development of production schemes [28, 46]. On the other hand, when comparing bacteria and filamentous fungi to yeast, the latter has many advantages, including faster growth rate than filamentous fungi; still, they can resist unfavorable environments such as filamentous fungi, being useful in biological treatment of

*Saccharomyces cerevisiae* 2031 Mannoprotein [42] *Pseudozyma* (*Candida antarctica*) Mannosylerythritol lipids [43] *Pseudozyma rugulosa* NBRC 10877 [44] *Pseudozyma churashimaensis* [45] *Schizonella malanogramma* Erythritol and mannose lipid [10] *Ustilago maydis* [10]

[41]

*Yarrowia lipolytica* Carbohydrate–protein–lipid complex

and the type of biosurfactant produced.

100 Advances in Bioremediation of Wastewater and Polluted Soil

**Table 2.** Biosurfactant-producing yeast

effluents [47].

Waste or used lubricating oils have caused a serious environmental problem because once in the environment, it can bind to organic matter, mineral particles, and organisms, with the consequent persistence and toxicity of oil components in the environment. Research on the interaction between hydrocarbon and microorganisms has supported the hypothesis that petroleum and its derivates are subjected to microbial degradation. In the environment, with the presence of emulsifying agents, hydrocarbons are more bioavailable for degradation; it has been observed that the greater the oil–water interface of hydrocarbons, the faster the rate of decomposition by the microbial community present [56].

Bioremediation can be done in two different ways: *in situ* and *ex situ*. The *ex situ* process can be carried out in a prepared bed or in a slurry reactor system. *In situ* processes are usually accomplished by the addition of microbial nutrients to the soil, which allows considerable growth of soil microbial indigenous population [16]. Biodegradation efficiency depends on the ratio of hydrocarbon-degrading microorganisms in soil, the composition and physical state of hydrocarbon mixture and oxygen availability, and the condition of water, temperature, pH, and inorganic nutrients. The physical state of the hydrocarbon can also affect biodegradation. In addition, the biodegradation of hydrocarbon in bioremediation might be enhanced by the addition of surfactant. For use in bioremediation procedures, biosurfactants are more prom‐ ising than synthetic surfactant because they are produced by microorganisms in soil and are commonly considered as low- or nontoxic compounds [57, 49].

Bioremediation involves the acceleration of natural biodegradative processes in contaminated environments by improving the availability of materials (e.g., nutrients and oxygen), condi‐ tions (e.g., pH and moisture content), and prevailing microorganisms [58]. Biosurfactants can improve bioremediation effectiveness by the following two mechanisms. The first mechanism includes the increase of substrate bioavailability for microorganisms; for bacteria growing on hydrocarbons, the growth rate can be limited by the interfacial surface area between water and oil. When the surface area of microorganisms with hydrophilic solvents like water is limiting, biomass increases arithmetically rather than exponentially. The second mechanism involves interaction with the cell surface, which increases the hydrophobicity of the microbial cell wall, allowing hydrophobic substrates to associate more easily with bacteria [1, 19]. Microbial cell hydrophobicity can be described as the affinity to adhere to hydrophobic substrates, such as hydrocarbons. This capacity can give the microbial cells the ability to better degrade hydro‐ carbons, and it can be a factor to understand microbial biodegradation rate differences [54]. The increase of microbial adhesion to hydrocarbons is directly related to the ability of such microorganisms to grow in the medium where hydrocarbons or other hydrophobic substrates are present [56]. If the biosurfactant compound is bound to the microbial cell wall, the cell surface will be more hydrophobic. Microorganisms can use their biosurfactants to regulate their cell-surface properties to attach or detach from surfaces accordingly to their needs [1].

There are many research reports dealing with the degradation of hydrocarbons and production of biosurfactants by microorganisms, as stated in Section 3, and there are some *in field* reports on the use of bosurfactants for bioremediation. For example, Thavasir et al. [59] demonstrated the enhanced degradation of hydrocarbons by the addition of biosurfactants to the culture media, as well as the enhancement of degradation by the addition of mineral nutrients (fertilizers). There are also reports on the identification and characterization of biosurfactantproducing microorganisms, including some genera not usually related to bioremediation, such as *Staphylococcus*. Studies include the determination of functional characteristics of the biosurfactants produced and their potential use in bioremediation [60]. Also, there are reports on the production of biosurfactants by microorganisms isolated from particular environments, such as marine sediment, that could be helpful in the bioremediation of those particular sites [61]. Furthermore, the efficiency of different surfactant solutions in removing crude oil from contaminated soil has been tested. Urum et al. [62] demonstrated the efficiency of surfactant solutions used in a soil-washing process. The synthetic surfactant SDS (sodium dodecyl sulfate) was as efficient as a biosurfactant derived from bacteria (rhamnolipid), and both were more efficient than saponins.

There are some recent literature reviews on the production and use of biosurfactants for bioremediation [63–66], but in those revisions, there are only few cases described where biosurfactants have been used on bioremediation processes at pilot-scale or field-scale studies [64]. Calvo et al. [63] focused on the need for the optimization of biosurfactant production and the tools from molecular biology that can be used to obtain hyperproducing microbial strains. This approach leads to the strategy of producing the biosurfactant and then using it to amend contaminated sites [62]. The question remains if it is possible to inoculate biosurfactantproducing microorganisms in contaminated sites and then promote the production of tensioactive agents on site so that it can be a continuous source of biosurfactant.

Sachdev and Cameotra [66] proposed that biosurfactant-producing microorganisms might have different roles in soil, which can help on agricultural production. They described the use of biosurfactants for the recovery of organic pollutant contaminated soil, with the consequent improvement in the plant-microbiota beneficial interactions, but they also suggested that biosurfactants can be used to disperse fertilizers. Considering the antimicrobial effect of some tensioactive molecules, the authors also suggest that biosurfactants can help on the control of phytopathogens.

A recent review [65] has a more critical point of view on the efficiency of biosurfactants on bioremediation. The authors did a critical analysis of reports on the use of biosurfactants and described that there are many cases on the amendment of contaminated soil with biologically synthesized surfactants showing no differences with control experiments or even showing negative results. A question that needs to be addressed is the variability of experiments reported, as well as the actual role of biosurfactants in noncontaminated environments. As with many other biological processes where the microorganisms are taken from their natural habitats and places in restricted and controlled environments, the contribution of a particular metabolite can be misled. This has always been a major concern in environmental microbiology because there are still few methods that can help us on the understanding of the actual interactions of microbiota in their environments. Therefore, questions about the role of a particular metabolite in the microhabitat and the concentration of such compound in nature are still unanswered.
