Biosurfactants: Promising Biomolecules for Agricultural Applications

Organic and inorganic pollutants affect soil productivity and cause abiotic stress in cultivated plants. Bioremediation processes are recommended to improve the quality of soils contaminated with hydrocarbons and heavy metals. Microorganisms producing biosurfactants and/or biosurfactants themselves can be used to remove hydrocarbons and heavy metals [105]. Biosurfactants enhance the bioavailability and biodegradation of hydrophobic compounds, and soil washing and combined cleaning technologies using biosurfactants have been used for the effective removal of hydrocarbons and metals, respectively [46,60].

Soil washing has become an appealing technology with the use of surfactant agents, especially for hydrophobic contaminants that adhere to soil particles’ surfaces and typically have low solubility in water. Surfactants can be added to solubilize soil contaminants. Anionic, cationic, zwitterionic, and nonionic surfactants have been applied for soil remediation [106].

To successfully implement enhanced remediation of surfactant-contaminated soils, several factors must be considered, including surfactant adsorption behavior in soil, their capacity to solubilize/elute target contaminants, and their toxicity and biodegradability. Economic factors such as surfactant cost and the extent of contaminated soil should also be considered. Ideally, in addition to strong contaminant desorption capacity, an ideal surfactant should be efficient and effective. It should have a low CMC and function at a low dose for washing solutions to reduce remediation costs and further ensure process economy [106].

Soil washing using surfactants can be carried out ex situ and in situ. Soil washing carried out outside its original location can effectively treat a wide range of contaminant concentrations and allow clean soil fractions to be returned to the site at a relatively low cost [107]. In the ex situ washing process, the contaminated excavated soil is pretreated, mixed with surfactants, and agitated. After washing, the clay particles are deposited, and the washing solutions can be separated and regenerated for use in the next round [105].

In the in situ remediation method, surfactant-containing washing solutions are injected into the contaminated area through injection wells. This process mobilizes soil contaminants by dissolving them through the formation of micelles with the help of washing solutions or chemical reactions. The contaminated fluid is then collected and can be either disposed of, recirculated, treated, or reinjected back into the area [105].

When surfactants are introduced into a water–soil system, the soil particles tend to adsorb a certain amount of surfactants. The amount of adsorbed surfactants increases with the increase in their concentration, which leads to a reduction in their ability to solubilize pollutants. Moreover, the hydrophobicity of the soil also increases as a result of surfactant absorption, leading to the reabsorption of solubilized organic contaminants on the soil surface [71,105]. Consequently, surfactants in low concentrations accumulate mostly at the solid–liquid or liquid–liquid interface in the form of individual molecules. As the concentration increases, surfactant molecules gradually replace the interfacial solvent, such as water, leading to a lower polarity of the aqueous phase and a decrease in surface tension. Accelerated dissolution of contaminants, such as liquid non-aqueous phase contaminants, can be achieved while increasing the surfactant concentration. When the concentration of surfactants is further increased, micelles are formed. The concentration of surfactants at which micelles start to form is referred to as the critical micelle concentration (CMC) [71]. Micelles with hydrophilic surfaces and lipophilic nuclei are effective in dispersing contaminants, such as liquid non-aqueous phase contaminants. These micelles improve the solubility of contaminants in the aqueous phase, which in turn promotes the desorption of contaminants from the soil. When contaminants are dissolved in the aqueous phase, they become more mobile, making it easier to remove them through biotic routes (such as plant uptake and microbial degradation) or abiotic pathways (such as soil washing and subsequent separation) (Figure 6) [106].

The process of washing soil with biosurfactants to remove hydrophobic organic pollutants can occur through two mechanisms. The first mechanism, called displacement, occurs below the CMC. The second mechanism, called solubilization, occurs above the CMC (Figure 7). Surfactant monomers below the CMC cause the soil to roll. They accumulate at the interfaces between the soil contaminant and soil water, changing the wettability of the system by increasing the contact angle between the soil and hydrophobic contaminants. When biosurfactant molecules are adsorbed on the contaminant surface, they cause repulsion between the main groups of surfactant molecules and soil particles. This further promotes the separation of contaminants from soil particles [107,108]. When the concentration is above the CMC, the biosurfactant can increase the solubilization of hydrophobic organic pollutants in the micelles and the partition of pollutants in the aqueous phase increases notably. Contaminants that are found in the micellar phase during the soil washing process can be further separated and treated using methods such as adsorption with activated carbon, electrochemical treatment, and demulsification. The washing solution or surfactant can be recycled or disposed of finally. It is desirable to recycle surfactants to reduce the cost of the remediation process.

Biosurfactants can improve the degradation of chemical insecticides in agricultural soils [47]. Reports suggest the role of biosurfactants in improving the health of agricultural soil through soil remediation processes. Examples include surfactin-supported pesticide biodegradation [109] and hydrocarbon degradation supported by glycolipids [110]. Burkholderia species isolated from oil-contaminated soil produce biosurfactants that could potentially remediate pesticide contamination [111]. Thus, biosurfactants have the potential to enhance soil quality, making them a valuable addition to agriculture. Soil pollution caused by metal salt-based fungicides, sewage, and sludge reduction techniques in agricultural fields can lead to the presence of heavy metals. While these metals are essential micronutrients for plant growth and physiological processes, high concentrations can cause harm to plants, damaging their roots and foliage. In contrast to organic contaminants in soil, heavy metals are mainly removed from the soil through complexation associated with surfactants and ion exchange [107]. The usefulness of surfactants in remediating heavy metal-contaminated soils is primarily based on their ability to form complexes with metals. Anionic surfactants, through ionic bonds, form complexes that are usually stronger than the metal’s bonds with soil complexes, leading to the desorption of the metal–surfactant complex from the soil matrix into the solution due to reduced interfacial tension. Cationic surfactants, on the other hand, can compete with charged ions on negatively charged surfaces through ion exchange. Metallic ions can also be removed from the soil surface by surfactant micelles [105,112].

In more detail, ionic surfactants remove heavy metal by the following sequence: (1) biosurfactant complexation with the metal through sorption of the biosurfactant to the soil surface, (2) desorption of the metal from the soil into the solution, and (3) association of the heavy metal with surfactant micelles, i.e., heavy metals are trapped in the micelles through electrostatic interactions and can be recovered with membrane separation techniques [107] (Figure 8). Several studies have highlighted the abilities of biosurfactants produced by Bacillus sp., Pseudomonas sp., and Acinetobacter sp. in removing heavy metals from soil and accelerating pesticide biodegradation [112]. Rhamnolipids and surfactin can remove metals such as Mg, Ca, Cd, Ni, Mn, Ba, Cu, Li, and Zn from the soil [113]. Synthetic surfactants are also used to remove nonpolar organic compounds from the soil. However, these surfactants are required in high concentrations and can affect microbial biodegradation [114].

Micronutrients present in the soil are essential for plant physiological processes, contributing to hormonal metabolism, protein synthesis, improvement in plant defense mechanisms, and maintenance of biological membranes, among others [115,116,117]. Many chemical fertilizers have been administered to maintain nutrient supply for plant growth, but they often become unavailable due to complexation with soil particles. Nevertheless, these chemical fertilizers can damage the physical structure, chemical balance, and biological activities of soils, and their activities are influenced by soil ionic charge and pH [18,118].

Therefore, biosurfactants can enhance the availability of metals to plants grown in soil by reducing interfacial tension and increasing the solubility and mobility of ionic nutrients, leading to increased uptake by plants [19,105]. When anionic biosurfactants form stabilizing forces with complexes, they become stronger than metal complexes with soil particles. This results in the desorption of metals from the soil matrix, allowing mixing at the soil–water interface and making them more available to soil microflora and plant roots. In contrast, cationic biosurfactants follow the ion exchange mechanism and replace charged metal ions that are bound to soil particles due to their higher affinity for them. [71].

Glycolipids, particularly sophorolipids, rhamnolipids, trehalose lipids, and MELs, are the most studied surfactants in metal complex formation [71,107]. Surfactin, for example, enhances nutrient acquisition through emulsification and supports surface colonization through biofilm formation. These biosurfactants have been reported to increase the capacity of colonizing plant roots by Bacillus amyloliquefaciens in Arabidopsis thaliana [119] and wheat by B. subtilis strains [120].

Microbial surfactants often have antimicrobial properties, measured with the minimum inhibitory concentration (MIC), which is the lowest concentration needed to prevent pathogen growth [18]. Several biosurfactants show antimicrobial activity against plant pathogens, including Gram-positive and Gram-negative bacteria and yeasts, making them promising biomolecules for sustainable agriculture [71]. The nature of the biosurfactant defines its antimicrobial activity. In a comparative study among some biosurfactants, the biosurfactant from P. aeruginosa UCP 0992 was the most efficient in inhibiting Staphylococcus aureus and Escherichia coli (MIC: 20 μg/mL), while the biosurfactants from P. aeruginosa UCP 0992 and Candida bombicola URM 3718 showed similar effects on Streptococcus mutans (MIC: 20 μg/mL). The biosurfactants from P. aeruginosa UCP 0992, Bacillus cereus UCP 1615, and C. bombicola URM 3718 exhibited the same effect against Candida albicans (MIC: 40 μg/mL) [71]. In another study, the biosurfactant from Candida sphaerica UCP 0995 did not show antimicrobial activity against other Candida species or bacteria (E. coli, P. aeruginosa, and B. subtilis). Still, it exhibited bacteriostatic activity against S. aureus and Klebsiella pneumoniae [121]. Luna et al. [122] investigated the antimicrobial activity of the same biosurfactant against different fungal and bacterial species and obtained positive results. Rufisan, a microbial surfactant obtained from C. lipolytica UCP 0988 in a refinery waste-supplemented medium, demonstrated excellent antimicrobial potential against various Streptococcus species at concentrations above its critical micelle concentration as well as anti-adhesive activity against most tested microorganisms [123].

The use of chemical surfactants and biosurfactants in agriculture helps control microbes that affect plant growth through various methods, including parasitism, antibiosis, competition, induced systemic resistance, and hypovirulence. This enhances the activities of beneficial microbes and their products [124]. The insecticidal activities of surfactants have been shown in multiple in vitro and in situ studies [46]. The combination of surfactants with the fungus Myrothecium verrucaria has been used to prevent the spread of and eradicate weed species that affect land productivity and negatively affect biodiversity [125]. They have also been used to inhibit the production of aflatoxins by Aspergillus sp. that infect cotton, peanut, and maize crops during storage [126]. Thus, both synthetic and biological surfactants play diverse roles in the elimination of phytopathogens, directly or indirectly, and in different processes related to agriculture.

Isolates of biosurfactant-producing Pseudomonas and Bacillus strains exhibited biocontrol capacity against phytopathogens [127]. It has been demonstrated that rhamnolipids can inhibit plant pathogens that have developed resistance to chemical pesticides [128], as well as insecticidal potential. For instance, Kim et al. [129] isolated a biosurfactant from a Pseudomonas strain that showed insecticidal activity against green peach aphids (Myzus persicae). Pseudomonas putida, a plant growth promoter, produces biosurfactants that cause lysis of cucumber pathogen zoospores [130]. The Bacillus strains produced a lipopeptide biosurfactant that inhibited the growth of phytopathogenic fungi from the Fusarium and Aspergillus genera [131]. The Brevibacillus brevis HOB1 strain produced a surfactin with strong antibacterial and antifungal properties that can be explored for phytopathogen control [132]. The antifungal properties of biosurfactants obtained from Pseudomonas fluorescens strains are well-described in the literature [133]. The pathogen Colletotrichum gloeosporioides, which attacks papaya leaves, was successfully controlled with the biosurfactant from Bacillus subtilis isolated from soil [134]. The above examples demonstrate that green biosurfactants are well-documented in the literature for promoting plant growth due to their effects on various pathogens. Microbial surfactants have the potential to replace chemical pesticides and insecticides in agriculture. In addition to these anti-phytopathogenic properties, biosurfactants can accelerate the composting process by providing favorable conditions for microbial growth, offering an additional advantage of using these green surfactants [135]. Biosurfactants have been shown to reduce the surface tension between liquids and solids and increase the bioremediation of organic matter, as stated by De Giani et al. [136]. Additionally, the presence of biosurfactants boosts bacterial growth, which in turn enhances organic matter decomposition. Rhamnolipids have been found to increase microbial growth in composting. The combined action of Bacillus sp. and Streptomyces sp. during composting leads to a more efficient breakdown of organic materials. The use of a consortium of bacteria that generate biosurfactants, along with a cell suspension containing biosurfactants, has been proven to increase bacterial communities in composting, indicating that biosurfactants do not hinder the development of bacteria in composting and may even have a minor stimulatory effect on their growth, as noted by Shi et al. [137].

Biosurfactants with antagonistic properties against phytopathogens can also affect other flora in the system. Therefore, to obtain an attractive green surfactant with specificity against phytopathogens, the chemical structure of the biosurfactant can be varied by altering production strategies [46].

The ability of the Lactobacillus rhamnosus cell-bound-derived glycolipid surfactant to inhibit bacterial adhesion and antibiofilm activities was recently observed [138]. Studies have shown that iturin, a cyclic lipopeptide produced by B. subtilis and related bacteria, has the ability to activate a plant’s natural defense mechanisms. These substances can promote the production of defense-related compounds, enhance plant immunity against infections, and improve overall plant health. Additionally, they offer an environmentally friendly alternative to chemical fungicides [139]. The lipopeptides produced by the marine bacterium B. subtilis subsp. spizizenii MC6B-22 showed broad-spectrum activity against ten phytopathogens of tropical crops at a minimum inhibitory concentration of 400 to 25 μg/mL and with a fungicidal mode of action, demonstrating the potential of the MC6B-22 strain as a biocontrol agent for agriculture [140]. The efficacy of Bacillus species associated with plant roots as antifungal biocontrol agents was evaluated. The production of lipopeptide biosurfactants was analyzed to determine their ability to control fungal infections. The results showed that the lipopeptide biosurfactant produced by B. velezensis PW192 is stable and possesses strong antifungal properties. Therefore, it can be used as a biocontrol agent in agriculture [141]. An extract of the biosurfactant derived from corn steep water, which is a residual stream of the corn wet milling industry, is fermented by probiotic lactic acid bacteria (L. casei). This extract was tested for its effectiveness as a bactericide. The results showed that at concentrations of 1 mg/mL, the biosurfactant extract was effective against P. aeruginosa and Escherichia coli. This opens up the possibility of using the biosurfactant extract in agrifood formulations to reduce the need for chemical pesticides and preservatives [142].

Plants sensitive to hazardous substances can be used as bioindicators to measure seed germination, root growth, and seedling growth. Seed germination testing is widely used to assess the phytotoxicity of any substance. In general, in agricultural practices, biosurfactants have been shown to effectively promote seed germination [71]. Although most biosurfactants have stimulated plant growth, some studies also highlighted inhibitory actions [143].

The biosurfactant derived from C. sphaerica UCP0995 did not exhibit toxicity toward the seeds of Solanum gilo, Brassica oleracea, Lactuca sativa L., or B. oleracea L. Except for B. Oleracea L., the other species also exhibited increased root elongation and seed germination in the presence of increasing biosurfactant concentrations [144]. On the other hand, the isolated biosurfactant inhibited the germination of Cichorium intybus seeds with increasing concentration, while root growth was not affected. According to a study, Solanum gilo seeds had 100% germination when treated with biosurfactant extracts at 200 mg/L, whereas no germination occurred at the 400 or 600 mg/L concentrations. This indicates an inhibitory effect at higher concentrations [119]. Silva et al. [145], who conducted phytotoxicity experiments on B. oleracea at 175, 350, 520, and 700 mg/L of a biosurfactant from P. aeruginosa UCP 0992 cultivated on glycerol as a substrate, observed no inhibitory effect on seed germination, indicating safety regarding this plant species. A study on the influence of rhamnolipids (0.25–1.00 g/L) on the germination of sunflower, lettuce, soybean, and corn seeds demonstrated an increase of up to 75.50% in the germination rate of lettuce seeds and a stimulation of corn and sunflower seed germination at a concentration of 0.25 g/L but no influence on that of soybean [146]. Finally, the germination index was used by Santos et al. [147] to evaluate the phytotoxicity of the lipopeptide biosurfactant produced by Streptomyces sp. DPUA1566 on L. sativa L. and B. oleracea. Under all tested conditions, seed germination was stimulated, and the growth of leaves and elongation of secondary roots were observed.

The influence of MELs on the seed germination of lettuce seeds (Lactuca sativa L.) was recently investigated for the first time. The biosurfactant at 158 mg/L showed promising results in the biostimulation of cultivated seeds. However, the responses observed in the physiological and biochemical behavior indicated that MELs at 316 and 632 mg/L influenced oxidative stress and inhibited the germination and development of the seeds [148].

Soybean plant growth promotion mechanisms were observed in bacteria cells, as well as the role of bacterial metabolites, especially lipopeptides, in the biological control of diseases and the modulation of the plant’s immune response. The treatments containing only bacterial cells were not efficient in reducing Asian rust severity, with losses of leaf area reaching 15%, while the addition of biosurfactants led to a result that was similar to the biofungicide, based on B. subtilis (Serenade^®) [149].

Bioformulations were developed using Pseudomonas putida BSP9 and its biosurfactant to evaluate their impact on promoting the growth of Brassica juncea plants. The study found that bioformulations amended with biosurfactant, either alone or in conjunction with BSP9, resulted in a significant increase in the growth parameters of B. juncea compared with the untreated control. The greatest enhancement was observed in plants inoculated with the bioformulation containing both BSP9 and biosurfactant. Furthermore, the study suggested that growth promotion peaked at a certain level of biosurfactant concentration, beyond which increasing the concentration did not result in any further enhancement in the plant’s growth parameters. These findings demonstrate that novel bioformulations that integrate plant growth-promoting rhizobacteria and their biosurfactants can be developed, and effectively utilized to increase agricultural productivity while reducing our dependence on agrochemicals [150].

For rhizobacteria to provide beneficial effects to plants, their interaction with plant surfaces is crucial. Microbial factors such as biofilm formation on the root surface, motility, and release of quorum-sensing signal molecules are necessary to establish an association with a plant. Rhizobacteria rely on quorum-sensing molecules such as N-acyl-homoserine lactone (AHL) to produce antifungal compounds. Research indicates that these molecules are more abundant in the rhizosphere, the area surrounding plant roots, emphasizing their importance in the establishment of beneficial microorganisms on the root surface. Dusane et al. [151] found that Pseudomonas spp. rhamnolipids regulate quorum sensing. Biosurfactants are also known to influence the motility of microorganisms as well as biofilm formation [152]; therefore, they play an important role for microbes to establish a beneficial association with plant roots and enhance plant growth. Moreover, these biosurfactants produced by soil microorganisms enhance the bioavailability of hydrophobic molecules that serve as nutrients, ensure soil wetting, and support the appropriate dispersal of chemical fertilizers in the soil, thereby aiding in promoting plant growth [46].

In a recent study, Chopra et al. [153] discovered a strain of plant growth-promoting rhizobacteria identified as P. aeruginosa RTE4 in the tea rhizosphere. They found that its biosurfactant has biocontrol properties against tea pathogens Corticium invisium, Xanthomonas campestris, and Fusarium solani. The researchers also found that the biofungicide properties of the rhamnolipid biosurfactant are similar to the commercial fungicide carbendazim. In another study, Khare and Arora [154] designed a bioformulation that improved the yield of sunflowers by 80.80% under laboratory and field conditions. The bioformulation contains biosurfactants that enhance the biocontrol activity of the LE3 culture by 75% against M. phaseolina. The authors found that a formulation containing LE3 cells and biosurfactants enhances the yield and biocontrol activity of sunflowers by 75.45%.