**4. Biosurfactant-producing yeast**

species of bacteria, yeast, and mold are capable of degrading hydrocarbons, and bacteria are the best described biosurfactant producer [21]. The exact reason why some microorganism can

Bushnell and Hass (1941) were the first to demonstrate the bacterial production of biosurfac‐ tants, using a strain of *Corynebacterium simplex* and a strain of *Pseudomonas* grown in a mineral media containing kerosene, mineral oil, or paraffin. Since then, numerous studies on the structure and mechanisms involved in the production and action of biosurfactants have been reported [22]. It can be stated that biosurfactants are produced by a variety of microorganisms, and there is also a wide variety on the chemical composition and nature of the biosurfactant produced, as well as on the location (membrane-bound, extracellular) of the produced molecule [23]. The most reported genera of biosurfactant-producing bacteria include *Pseudo‐ monas* sp., *Acinetobacter* sp., *Bacillus* sp., and *Rhodococcus* sp., among others. Table 1 shows some

**Microorganism Biosurfactant Reference** *Pseudomonas aeruginosa* Rhamnolipids [24] *Pseudomonas fluorescens* Ornithine lipids [25] *Pseudomonas stutzeri* [25] *Pseudomonas cepacia* [25] *Acinetobacter calcoaceticus* Lipopolysaccharides (biodispersant) [10] *Acinetobacter radioresistens* Heteropolysaccharide protein (alasan) [10] *Bacillus subtilis* Lipopeptides and lipoproteins (surfactin) [16] *Bacillus licheniformis* Lipopeptides (lichenysin) [5] *Rhodococcus erythropolis* Trehalolipids [26] *Mycobacterium* sp. [2] *Nocardia* sp. [5] *Tsukamurella* sp. Di- and oligosaccharide lipids [27]

Microorganisms that produce biosurfactants are isolated mainly from sites that are or were contaminated with petroleum hydrocarbons: contaminated soils, effluents, and wastewater sites. Thus, these have an ability to grow on substrates considered potentially noxious for other nonbiosurfactant-producing microorganisms. Biosurfactants play a physiologic role in increasing bioavailability of hydrophobic molecules, which are involved in cellular signaling and differentiation processes, which facilitate the consumption of carbon sources present in

The physiological role of biosurfactants is not clear yet, but it might be related to an increase in the nutrient uptake of hydrophobic substrates, in enhancing the growth on hydrophobic

also produce biosurfactants is still not clear [22].

98 Advances in Bioremediation of Wastewater and Polluted Soil

**Table 1.** Biosurfactant-producing bacteria

soil [23, 28].

of the most studied bacteria and the type of biosurfactant produced.

Yeasts are unicellular cells of dimorphic fungi that are usually classified in the subdivision Ascomycotina and Basidiomycotina. They are ubiquitous in most environments, although they are more related to sites with high organic matter content and/or high water availability. They have been isolated from leaves, flowers and fruits, trees exudates, insects, soils, and other natural environments. Nowadays, approximately 100 genera and 700 species of yeast have been classified based on their morphological, physiological, and biochemical characteristics.

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 and the type of biosurfactant produced.


**Table 2.** Biosurfactant-producing yeast

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 effluents [47].

*Yarrowia lipolytica* was the first yeast used experimentally for the degradation of aliphatic hydrocarbon; this yeast also produces a highly efficient emulsifier [48]. Most of the biosurfac‐ tants produced by yeasts are better emulsifiers than biosurfactants, mainly because of the chemical structure of the molecules [49]. The widespread occurrence of yeasts with hydrocar‐ bon-degrading activities has been extensively investigated. *Candida* species, especially *Candida lipolytica*, has been isolated from diesel oil storage tanks and fuel systems. *Candida tropicalis* and *Candida maltosa* are also noted for their use of saturated hydrocarbons. *Debaryomyces hansenii* and *Candida guiliermondii* can grow on hydrocarbons and have been isolated from hydrocarbon contaminated sites. There are many reports on the metabolism of hydrocarbons from yeasts, but very few information on the metabolites is produced [50].

The influence of the carbon source in biosurfactant production has been extensively studied in some microorganisms. For the study of yeast, different types of carbon sources have been used, depending on the yeast strain. For example, Silva et al. [51] found that biosurfactants produced using vegetable and mineral oils have different stability properties when incorpo‐ rated into aqueous solutions, with better stabilization properties when vegetable oil was used. Daverey et al. [52] reported that a *C. bombicola* strain can produce sophorolipids when growth on a mixture of hydrophobic and hydrophilic substrates. Amaral et al. [53] reported that for the production of Yansan by *Y. lipolytica*, it is important to use glucose as carbon source.

Changes in yeast cell hydrophobicity have been related to the ability of the microbial strain to degrade hydrocarbons [54]. Amaral et al. [41] observed that the interaction of *Y. lipolytica* cells with hydrophobic surfaces is mediated by proteins or glycoproteins present in the cell wall. Furthermore, they suggested that van der Waals forces were involved in the interactions between the yeast cell surface and the nonpolar solvent and biosurfactant production im‐ proved these interactions. Regarding biosurfactant chemical properties, yeast biosurfactants maintained their functionality at different pH values as well as over a wide range of temper‐ atures [55]. *Pichia anomala* and other yeasts are thermophilic, and so their biosurfactants could have a wide range of industrial applications [39].
