Unlocking the Potential of Biosurfactants in Agriculture: Novel Applications and Future Directions

Abstract

With rising environmental concerns and the urgent need for sustainable agricultural practices, biosurfactants have garnered significant attention. These naturally occurring, surface-active compounds produced by microorganisms offer eco-friendly alternatives to synthetic chemicals. This review explores the multifaceted role of biosurfactants in agriculture, highlighting their applications in soil nutrient enhancement, plant growth promotion, pest and pathogen control, and bioremediation. The inherent versatility and biodegradability of biosurfactants position them as pivotal agents in improving soil health and advancing sustainable farming. Cutting-edge biotechnological approaches, such as synthetic biology and metabolic engineering, are critical for optimizing biosurfactant production. Integrating these bioactive molecules into smart agricultural systems promises to enhance resource utilization and crop management. Despite challenges like high production costs and limited ecological impact studies, innovative production techniques and comprehensive ecological assessments are essential for broader applications. This review underscores the transformative potential of biosurfactants in driving sustainable agricultural practices and environmental remediation, paving the way for future research and innovation in this field.

1. Introduction

The rapid increase in population and urbanization, coupled with rapid industrialization, has led to advancements in living standards [1,2]. However, this progress has also given rise to the release of harmful pollutants, posing a threat to the environment’s quality [3]. Biosurfactants are biodegradable and have low toxicity, making them suitable for environmental applications [4,5]. These compounds are classified based on their chemical composition and microbial origin, including glycolipids, lipopeptides, and polymeric biosurfactants [6,7]. In contemporary agriculture, chemical pesticides are commonly employed to combat insect pests and pathogens threatening crop yields [8,9]. However, these chemicals have adverse environmental and health effects, contributing to the emergence of resistant pathogenic strains. The heightened awareness of health issues has intensified the need for safer food production methods, steering attention towards the negative consequences of chemical-based pesticides [10]. Consequently, biosurfactants, acting as biological control agents, emerge as a viable alternative to chemical pesticides due to their low toxicity, eco-friendly nature, and cost-effective production [11]. Overall, biosurfactants find applications in eliminating plant pathogens, enhancing soil nutrient bioavailability, fostering plant growth, and addressing soil health concerns by removing pollutants (Figure 1) [12].

Further research is essential to explore the potential role of biosurfactants in promoting plant growth and other agricultural applications by isolating them from diverse environmental strains [13,14]. Traditional molecular techniques have limitations in extracting biosurfactants from less diverse microbial populations, emphasizing the need to discover new biosurfactants from the uncultivated microorganisms present in soil biospheres. Advanced methodologies, such as functional metagenomics, can facilitate this discovery process by enabling the study of genetic material recovered directly from environmental samples [15].

By leveraging functional metagenomics, researchers can identify novel biosurfactants with unique properties that may enhance plant growth, improve soil health, and provide biological control against plant pathogens [16]. This approach not only expands the repertoire of available biosurfactants but also contributes to sustainable agricultural practices. The integration of biosurfactants into agricultural systems can reduce the reliance on chemical fertilizers and pesticides, mitigating their adverse environmental and health impacts [17].

Moreover, biosurfactants can play a crucial role in soil remediation by breaking down pollutants and enhancing the bioavailability of nutrients, leading to healthier and more productive soils [15]. Their application in agriculture aligns with the global trend towards environmentally friendly and sustainable farming practices. As the demand for sustainable agricultural solutions grows, biosurfactants offer a promising alternative to traditional agrochemicals, supporting the development of resilient and sustainable food production systems.

The increasing recognition of biosurfactants as eco-friendly alternatives to synthetic agrochemicals highlights their potential to enhance soil health, promote plant growth, and contribute to sustainable farming systems. Several studies have reviewed biosurfactants in agricultural systems, focusing on specific aspects, such as microbial biosurfactant production [18,19], bioremediation applications [20,21], and their role as biopesticides [22,23]. However, despite significant advancements in biosurfactant research, challenges, such as high production costs, scalability limitations, and limited field application studies, persist. To provide a structured analysis of biosurfactants’ role in agriculture and identify pathways for their broader adoption, this review addresses the following key research questions: (i) what are the major types of biosurfactants and their mechanisms of action in agricultural applications? (ii) How do biosurfactants contribute to soil health, plant growth, and pest/pathogen management? (iii) What are the current limitations and challenges in the large-scale adoption of biosurfactants in agriculture? (iv) What future directions and innovative strategies can enhance biosurfactant production and efficiency in agricultural systems? By synthesizing recent literature, this review aims to answer these research questions and provide a comprehensive understanding of biosurfactants’ role in sustainable agriculture, highlight research gaps, and explore emerging technological solutions to enhance their production and application.

2. Definition, Classification, and Structure of Microbial Surfactants

Biosurfactants are surfactants derived from biological sources, such as bacteria, fungi, and algae [5]. These amphiphilic compounds are composed of two parts: a hydrophilic (polar) section, including amino acids, cations, peptide anions, mono-, di-, or polysaccharides, and a hydrophobic (non-polar) section consisting of unsaturated and saturated fatty acids [4,6]. Recognized as metabolic products or the actual cell surface chemistry of microorganisms, biosurfactants are produced by various microorganisms, including Pseudomonas, Bacillus, Acinetobacter, and Candida lipolytica [7].

Biosurfactants possess important benefits, including biodegradability, lower toxicity, and diverse structural possibilities compared to chemically synthesized surfactants [24,25]. The term “microbial surfactants” refers to compounds secreted by microorganisms in cultivation environments, such as bacteria or fungi. Biological surfactants are compounds derived from living sources that alter the surface properties of water. The cultivation environment typically contains carbon sources required by these microorganisms for growth and the production of surfactants [26,27].

Microbial surfactants serve as surface-active agents that facilitate the solubilization of water-insoluble compounds, such as hydrocarbons, fats, and oils. By reducing the surface tension of water, they enable hydrophobic substances to dissolve and disperse more effectively in an aqueous environment. In this environment, hydrophobic compounds can serve as a source of nutrients for organisms [28]. Due to their microbial origin and access to nutrients, biosurfactants can be categorized into two groups based on their molecular weight and chemical composition [29]. There are two types of biosurfactants: molecules with low molecular weights and molecules with higher molecular weights. Those with low molecular weights are primarily composed of hydrophobic lipid segments, which facilitate their solubility in aqueous environments [30]. Biosurfactants with a low molecular weight are produced by bacteria, such as Bacillus and Pseudomonas [31]. On the other hand, biosurfactants with higher molecular weights can form stable emulsions when combined with proteins, polysaccharides, lipopolysaccharides, or mixed biopolymer complexes [32].

The primary criterion for classifying biosurfactants is their chemical structure [33]. Consequently, fatty acids, neutral lipids, lipopeptides, phospholipids, polymeric surfactants, and glycolipids are among the types of biosurfactants available (Figure 2).

2.1. Glycolipids

Glycolipids consist of two main components: carbohydrates and hydroxy fatty acids. Carbohydrates, such as rhamnose, sophorose, and trehalose, serve as the polar part of glycolipids, while hydroxy fatty acids provide the hydrophobic properties [33]. Researchers have extensively studied glycolipids due to their diverse surface and physical properties resulting from the carbohydrate moieties they contain [34].

The polar carbohydrate components in glycolipid structures allow these compounds to be classified into various groups, including sulfolipids, sophorolipids, trehalolipids, rhamnolipids, lipomannans, and lipids. Each subgroup has its own specific characteristics and applications, utilized in industries, such as chemicals, biotechnology, and others [34]. These applications highlight the versatility and importance of glycolipids in various industrial processes.

Glycolipids serve as surface-active agents, facilitating the solubilization of water-insoluble compounds, like hydrocarbons, fats, and oils [35,36]. This capability is crucial in environmental applications, such as bioremediation, where glycolipids help disperse and degrade pollutants in soil and water environments [28].

2.2. Lipopeptides

Lipopeptides are biosurfactants produced by certain bacteria, especially Gram-positive bacteria, such as Bacillus species. Surfactin, a key lipopeptide, consists of two primary components: a hydrophilic section and a hydrophobic section. The hydrophilic part, known as the polar or water-attracting peptide ring, is formed by a cyclic peptide of seven consecutive amino acids. The hydrophobic section comprises a fatty acid chain with 13 to 15 carbon atoms, suitable for adsorption on non-aqueous surfaces, like oils and fats. These characteristics give lipopeptides specific biological and physical properties [31,37].

2.3. Phospholipids

Phospholipids are amphipathic molecules consisting of a hydrophilic phosphate group on one end and hydrophobic fatty acids on the other. These biosurfactants are essential components of microbial plasma membranes. When microbial strains grow in the presence of hydrocarbons, the surface area of phospholipids increases [38].

2.4. Fatty Acids

The microbial oxidation of alkanes can produce fatty acids that act as surfactants. These fatty acids may have linear or complex hydrocarbon chains with alkyl branches and hydroxyl groups. The balance between the hydrophilic and hydrophobic properties of fatty acids is closely related to the length and complexity of the hydrocarbon chain [31,38].

2.5. Polymeric Biosurfactants

Polymeric biosurfactants are composed of various chemical structures, including heteropolysaccharides, exopolysaccharides, carbohydrates, lipids, proteins, and other polysaccharide–protein complexes. The most extensively studied polymeric biosurfactants include emulsan and liposan, produced by Acinetobacter radioresistens, A. calcoaceticus, and Candida lipolytica [39,40,41].

3. Screening, Extraction, Purification, and Assessment of Biosurfactant Activity

3.1. Screening Methods for Biosurfactants

Promising microbial strains or their supernatants are subjected to various tests to evaluate biosurfactant activity [7]. These tests include assays for hemolytic activity, surface tension measurements, the drop collapse test [27,42], oil displacement assessments [43], emulsification activity [44], the emulsification index [45], the CTAB/methylene blue agar test [46], and high-throughput screening for penetration assessments [47].

Identifying microorganisms that produce biosurfactants typically involves several steps. Initially, microorganisms are isolated from a culture medium suspected of biosurfactant production. These isolates are then purified to obtain individual microbial strains. Next, DNA is extracted from the isolated microorganisms, followed by polymerase chain reaction (PCR) to amplify specific DNA regions indicative of genes associated with biosurfactant production. The amplified DNA fragments are sequenced, and the sequence data are subjected to phylogenetic analysis to identify the closest relatives or classify the microorganisms [47].

3.2. Extraction of Biosurfactants

Various techniques are used to extract biosurfactants, each with its own advantages and limitations based on the nature of the biosurfactant, the complexity of the culture medium, and the desired purity and yield [48]. Common techniques include centrifugation, which separates biosurfactants based on density differences, and methods involving the addition of solvents or acids to precipitate biosurfactants from the culture broth, followed by filtration or centrifugation. Ion exchange chromatography and isoelectric focusing are also used to separate biosurfactants based on charge and isoelectric points, respectively. Ultrasonication employs high-frequency sound waves to disrupt microbial cells and release biosurfactants, while dialysis uses a semipermeable membrane to separate biosurfactants based on molecular size or charge differences [49].

3.3. Purification of Biosurfactants

Purification is crucial for the characterization and application of biosurfactants. Several techniques are notable for this purpose:

Thin-Layer Chromatography: This method separates raw biosurfactants on a silica gel plate using a solvent mixture. The type of biosurfactant is identified using a developing solvent system and color indicators, such as ninhydrin, which produces a red spot for lipopeptides [50].

Dialysis and Ultrafiltration: These cost-effective methods enhance the purity of biosurfactants. Dialysis, using cellulose bags, removes impurities and salts, resulting in a more refined product [48].

Isoelectric Focusing: A sophisticated purification technique that involves filling a column with gradient density solutions, electrolytes, and non-ionic conducting polymers. Biosurfactants migrate through the column until they reach a neutral pH, influenced by an electric field, pH, density gradient, and ampholytes. Once separation is complete, the purified biosurfactant is compared with the crude form in terms of activity [48,51].

3.4. Assessment of Biosurfactant Activity

Evaluating biosurfactant activity typically involves measuring its ability to alter the surface tension and hydrophilic–lipophilic balance. These parameters provide insights into the effectiveness of purified biosurfactants in various applications [52].

4. Factors Influencing Biosurfactant Production

Various factors, including genetic, physiological, nutritional, and environmental elements, significantly affect the production of microbial biosurfactants (Figure 3). Optimizing these factors is essential to maximize biosurfactant production, which typically occurs under specific conditions [53]. Nutritional components, such as nitrogen and carbon sources, trace elements, and vitamins greatly influence the quantity and type of biosurfactants produced. Environmental factors, such as the pH, temperature, and aeration levels, also play a critical role in biosurfactant production dynamics [28].

4.1. Nutritional Factors

4.1.1. Carbon Source

Nutritional factors significantly influence microbial biosurfactant production, with carbon sources being particularly crucial. Biosurfactant-producing microorganisms are predominantly heterotrophic, meaning they depend on external carbon sources for growth and metabolite synthesis, including biosurfactants [54]. Carbon sources used for biosurfactant production typically fall into three main categories: hydrocarbons, oils and fats, and carbohydrates [55]. The type and concentration of the carbon source can vary depending on the microbial species and the biosurfactant being produced [56]. The metabolic pathways involved in biosurfactant precursor generation are closely linked to the availability and composition of carbon sources in the production environment [28].

4.1.2. Abundant and Cost-Effective Substrates

Recently, there has been an increased focus on using abundant and cost-effective substrates as viable carbon sources for biosurfactant production [16]. When selecting these substrates, factors, such as variability, stability, raw material availability, and waste management, should be considered [16,57]. Additionally, factors like purity, storage conditions, packaging, and transportation methods significantly impact their suitability for biosurfactant production [53].

4.1.3. Nitrogen Source

In addition to carbon sources, nitrogen is vital for synthesizing primary and secondary metabolites, such as biosurfactants, proteins, and enzymes [43]. Various nitrogen sources, including nitrate, amino acids, and ammonia, are commonly used by researchers. These nitrogen sources have proven to be effective in supporting biosurfactant production, particularly in microorganisms like Pseudomonas aeruginosa [55]. Selecting suitable nitrogen sources is crucial for optimizing biosurfactant-production processes and enhancing the yield and quality of the final product.

4.1.4. Mineral Components

Mineral components, such as phosphates, calcium, and trace elements, play a significant role in facilitating microbial activity and growth, thereby influencing biosurfactant production. KH₂PO₄ and K₂HPO₄ act as buffers in biosurfactant environments, maintaining optimal pH conditions for microbial growth [55,58]. Calcium, often added as chloride salts or hydrated chlorides, supports microbial growth and biosurfactants [59,60]. Trace elements or micronutrients also play a crucial role in biosurfactant production by positively influencing microbial growth when added in small quantities to the production environment. The specific micronutrients required can vary depending on the microorganism used [55].

4.1.5. Environmental Factors

Environmental factors significantly impact the quantity and characteristics of biosurfactants, highlighting the importance of optimizing these conditions for efficient production [7]. Most microorganisms studied for biosurfactant production are mesophiles, thriving within a temperature range of 20 to 40 °C. The optimal temperature for biosurfactant production generally falls between 25 and 37 °C [61]. Additionally, while neutral or near-neutral pH levels are preferred for bacterial biosurfactant production, acidic conditions are more conducive to biosurfactant production by algae and fungi [62,63]. Understanding and controlling these environmental factors are essential for maximizing the biosurfactant yield and quality in various industrial and environmental applications.

4.1.6. Cultivation Strategy

Two bioprocess strategies, submerged fermentation and solid-state fermentation, are utilized at laboratory and pilot scales. These strategies are crucial for the economical production of biosurfactants and optimizing the production environment, thus playing a vital role in establishing commercial and economical biosurfactant production [28].

4.2. Molecular Characteristics of Biosurfactants

Biosurfactants, derived from bacteria, yeasts, and fungi, are recognized for their environmentally friendly nature and high biodegradability [15]. Their diverse functionalities, phase behaviors, extensive structural variations, and biodegradable properties open up numerous possibilities for innovative applications [15]. In agriculture, the microbial aspects of biosurfactants can be broadly summarized into three dimensions, as illustrated in Figure 4. The amphiphilic nature plays a pivotal role, creating two immiscible surfaces that reduce surface tension and enhance the solubility of water-repellent compounds. Biosurfactants are currently extensively utilized for soil improvement by boosting trace element concentrations. They are applied either in conjunction with pesticides or independently on plant surfaces to address plant diseases [64].

According to their molecular weight, biosurfactants can be classified as neutral or anionic molecules, depending on their chemical properties. These versatile compounds display a range of fundamental properties, including surface tension reduction, emulsification, absorption, micelle formation, and more, making them valuable in various applications. Microbial genera, such as Bacillus spp., Pseudomonas spp., Candida spp., and Pseudozyma spp., have garnered significant attention for their potential in biosurfactant production [29]. Moreover, The use of rhamnolipid at a concentration of 25 μg/mL has demonstrated significant effectiveness in mitigating diseases caused by the Phytophthora cryptogea fungus in chicory [65]. Within Verticillium, where storage compounds are fats, surfactants can infiltrate the membrane, leading to the disappearance of these storage compounds and preventing germination [66]. According to Hultberg et al. [67], Pseudomonas fluorescens produces a surfactant that has antifungal properties, particularly against potato pathogens, such as Pythium ultimum, Fusarium oxysporum, and Phytophthora cryptogea. This finding is significant considering the widespread occurrence of Verticillium wilt, a highly destructive fungal disease that has inflicted substantial damage on crops globally.

Goswami et al. [68] found that 8 out of 12 bacteria producing surfactants exhibited antifungal properties against Fusarium sp., the causative agent of Pokkah disease, with one strain showing superior efficacy. Subsequent investigations unveiled that the surfactant from this strain was a rhamnolipid. Findings from Soltani Dashtbozorg et al. [69] demonstrated that lower concentrations of rhamnolipid induced the lysis or cessation of the movement of Phytophthora sojae zoospores, with concentrations ranging from 100 to 1000 mg/L inhibiting mycelium growth by up to 30%. Furthermore, the rhamnolipid produced by Pseudomonas aeruginosa hindered the mycelial growth of the pathogenic fungus Leptosphaeria maculans in rapeseed [70]. Prolonging the interaction time between petroleum hydrocarbons and soil has been observed to diminish the biodegradability of these compounds. An increase in the organic carbon content within the soil enhances the desorption of petroleum hydrocarbons, impeding their breakdown by microorganisms [71]. Surfactants facilitate the desorption of contaminants from the soil through mineralization and solutionization mechanisms [72]. A study investigating two biosurfactants produced by C. sphaerica and Bacillus sp. revealed their high efficiency in releasing motor oil from the soil [73]. Moldes et al. [74] conducted research on the impact of glycolipopeptide surfactant and sodium dodecyl sulfate (SDS) on oil release from a soil contaminated with 7000 mg/kg octane for 30 days. The results indicated that glycolipopeptide and SDS achieved octane release rates of 65.1% and 37.2%, respectively. The maximum octane release after 30 days reached 92% (591 mg/kg) and 94% (430 mg/kg) for glycolipopeptide and sodium dodecyl sulfate, respectively.

Lai et al. [75] explored various surfactant concentrations to assess the performance of rhamnolipid and surfactin in releasing total petroleum hydrocarbon from soils containing 3000 and 9000 g TPH/kg soil. Higher surfactant concentrations (both rhamnolipid and surfactin) appeared to enhance of total petroleum hydrocarbon (TPH) release from both soils. In soil with lower contamination percentages (3000 g/kg), both rhamnolipid and surfactin achieved maximum oil-release efficiencies. For soils with higher contamination percentages (9000 g/kg), increasing the surfactant concentration from 0 to 0.2% resulted in release efficiencies exceeding 62% for both biosurfactants [75]. Costa et al. [76] highlighted that the main process contributing to oil release from contaminated soil using the surfactant produced by P. aeruginosa L2-1 is solubilization, and increasing the concentration enhances the efficiency of oil release from the soil. Ferradji et al. [77] demonstrated that elevating the concentration of the lipopeptide biosurfactant significantly improved the oil-release efficiency from the soil, increasing it from 0.005 to 0.1 g/L.

The affinity between metals and surfactants surpasses the bond between metals and the load-bearing surfaces of the soil, attributed to the reduction in the surface tension leading to the adsorption of metals from soil surfaces to the soil solution. Cationic surfactants compete with metals for negatively charged adsorption surfaces, causing metals to be adsorbed into the soil solution [72,78,79]. Soil washing with a 25 mM rhamnolipid solution resulted in desorption levels of 91.6% for cadmium and 87.2% for zinc [80]. Research on the surfactant produced by Candida indicated its capability to desorb 98.9% zinc, 89.3% iron, and 81.1% lead [81]. The surfactant produced by Starmerella bombicola CGMCC 1576 demonstrated superior abilities in the desorption of contaminants from the soil compared to synthetic surfactants and distilled water, increasing the desorption capacity of cadmium by 83.6% and 44.8% at an 8% concentration. In soil contaminated with 45 mg/g copper and 14 mg/g nickel, a 2 g/L saponin concentration exhibited a desorption capacity of 83% for copper and 85% for nickel [82]. Gusiatin and Klimiuk [83] observed a decrease in the concentration of elements, such as copper, cadmium, and zinc, during soil washing with a saponin solution. A study on rhamnolipid, sophorolipid, surfactin, and lipopeptides produced by Pseudomonas aeruginosa ATCC 9027, Torulopsis bombicola ATCC 22214, and Bacillus subtilis ATCC 21332 found that rhamnolipid could desorb 65% of copper and 18% of zinc, whereas sophorolipid achieved 25% desorption for copper and 60% for zinc [72].

As evident from

Table 1. Recent reports of microbial surfactants in agriculture.

Experiment Conditions	Surfactant Type	Results	Reference
Review study	Microbial activity derived biosurfactants	Increasing feed digestibility, improving seed protection and fertility	[84]
Microbial sources: Pseudomonas, Bacillus, and Candida	Rhamnolipid, sophorolipid, and surfactin	Oil spill management	[85]
Analyze microbial biosurfactant production, focusing on the optimization of metabolic pathways and production	Rhamnolipids surfactin	Improved yield and reduced ATP	[86]
Review study	Rhamnolipids	Potential antimicrobials, immune modulators, virulence factors, and anticancer agent	[87]
Microbial sources: Pseudomonas, Burkholderia, and Bacillus species	Rhamnolipids and lipopeptides	Plant resistance to phytopathogens	[88]
Soil application study	Rhamnolipid and surfactin	Enhanced TPH release from soil	[75]
Soil washing study	Rhamnolipid solution	Desorption of cadmium and zinc	[80]
Heavy-metal-contaminated soil study	Surfactant produced by Pseudomonas aeruginosa	Enhanced efficiency of oil release from soil	[76]
Agricultural community cohort study	Rhamnolipid and other biosurfactants	Significant alterations in microbiome composition due to pesticide exposure	[89]
Investigation of biosurfactants for sustainable agriculture	Surfactin, lipopeptides	Improvement in nutrient availability and soil revitalization	[90]
Production of biosurfactants using agricultural residues	Rhamnolipids, sophorolipids	Potential in waste management and bioremediation applications	[17]
Study on farm work tasks	Multiple surfactants	Increased microbial diversity in indoor environments	[91]

, the predominant biosurfactants employed in agriculture include rhamnolipid, sophorolipid, surfactin, and lipopeptides.

5. The Role of Biosurfactants in Bioremediation

The primary function of biosurfactants in bioremediation is to increase the surface area of substrates, thus facilitating the remediation process. By releasing specific compounds through various mechanisms, microorganisms that produce biosurfactants promote emulsification [40]. Biosurfactants are able to increase pseudo-solubility due to their unique properties and degradability, thereby enhancing the distribution of contaminants into the aqueous phase and increasing their bioavailability [92]. As a result of this enhanced mobilization, hydrophobic pollutants can be removed through solubilization and micellization, which can be further facilitated by subsequent soil flushing or may make contaminants more susceptible to biodegradation [93].

Furthermore, the presence of heteroatoms in biosurfactant structures enables specific chemical groups (such as hydroxyl, carbonyl, or amine) to participate in complex formations with heavy metal ions, potentially facilitating their removal [92]. This interaction enhances heavy metal ion extraction efficiency using biological methods [78]. Biosurfactants also exhibit significant biological activity, particularly at the cell membrane level, where they can modify cellular properties, such as hydrophobicity and permeability [92]. These modifications may impact the biodegradation efficiency or be advantageous in bioextraction processes [94]. However, it is worth noting that changes in cellular properties may not always correlate directly with the ability to utilize specific carbon sources and may not be easily indicative of bioremediation efficiency [92,95].

5.1. Biosurfactants and Hydrocarbons

Biosurfactants play a crucial role in emulsifying hydrocarbons in water, facilitating their solubility and creating diverse mixtures. Among the surfactants known for their efficacy in remediating oily pollution are lichenisins, rhamnolipids, and surfactin [40]. Remarkably, a bacterium isolated from a crude oil sample demonstrated the production of a biosurfactant with the emulsifying properties specific to crude oil.

Biosurfactants derived from marine bacteria exhibit remarkable capabilities in addressing oil slicks, dispersing oil in water through stable emulsion formation, and consequently enhancing the rate of biodegradation. Marine bacteria known for hydrocarbon degradation, termed hydrocarbonoclastic bacteria, actively break down hydrocarbons in polluted marine environments. The biosurfactants produced by these oil-degrading bacteria aid in the absorption of hydrocarbons and nutrients in the environment, contributing to remediation efforts [40]. For instance, surfactin from Bacillus coagulans 30 has been shown to form stable emulsions with crude oil, leading to a significant increase in oil recovery rates, from 17% to 31%. Moreover, surfactin exhibits stability across various environmental conditions, including the pH, temperature, and salinity, further enhancing its utility in oil recovery efforts. Studies have also indicated the potential of surfactants, including those produced by Bacillus species, in microbial-enhanced oil recovery, with lipopeptides reaching concentrations of 85 to 95 mg/L within oil reservoirs [40].

Petroleum Consuming Microorganisms

Research on the degradation of petroleum-derived hydrocarbons has demonstrated the significant role of microorganisms, particularly bacteria and fungi, in utilizing these hydrocarbons as their sole carbon source for energy [96]. Among these microorganisms, bacteria are the primary decomposers of petroleum compounds, actively breaking down a wide array of hydrocarbons in both water and soil environments [97]. Many of these bacteria have been isolated from soil or groundwater samples, with notable examples including Arthrobacter spp., Marinobacter spp., Mycobacterium spp., Achromobacter spp., Flavobacterium spp., Nocardia spp., Alcaligenes spp., Pseudomonas spp., and Corynebacterium spp. [98]. Additionally, various filamentous fungi have demonstrated the ability to oxidize and transform polycyclic aromatic hydrocarbons (PAHs) into less harmful metabolic byproducts, with examples including Psilocybe spp., Cyclothyrium spp., and Penicillium simplicissimum [99]. Biosurfactants, surfactants produced by microorganisms, play a significant role in plant protection against pathogens, offering a potential alternative to chemical pesticides while being environmentally friendly. These biosurfactants possess antibacterial, antifungal, and nematicidal properties, thereby reducing pathogen attacks on plants and contributing to their overall health and vigor. By harnessing the natural capabilities of biosurfactants, agriculture can move towards more sustainable and eco-friendly practices, minimizing the reliance on synthetic chemicals and their associated environmental risks [10].

5.2. Biosurfactants and Heavy Metals

Biosurfactants, typically derived from microorganisms, play a pivotal role in microbial remediation processes, with biosorption emerging as a primary mechanism [100]. Through biosorption, extracellular agents produced by microorganisms effectively immobilize heavy metals by binding to the anionic functional groups present on cell surfaces [101]. Numerous microorganisms have been identified for their ability to modify heavy metals, including B. subtilis, Lecythophora sp., Nostoc linckia, Saccharomyces cerevisiae, and Rhizopus stolonifer [102]. For instance, Sporosarcina ginsengisoli has demonstrated proficiency in reducing the exchangeable arsenic fraction in soil through the production of substantial amounts of urease [103]. In studies, Bacillus subtilis was observed to absorb 76% of Cd²⁺ from contaminated soil, highlighting its effectiveness in heavy metal remediation [104]. Similarly, Saccharomyces cerevisiae exhibited significant heavy-metal-absorption capabilities, with the capacity to absorb up to 70% of Cd²⁺. These findings underscore the potential of microorganisms, particularly those capable of biosurfactant production, in mitigating heavy metal contamination and promoting environmental remediation efforts.

5.2.1. Microbial Remediation Techniques

Bio-Stimulation

Bio-stimulation involves the enhancement of soil conditions to promote the growth and activity of indigenous microorganisms, typically achieved by supplementing nutrients to contaminated sites [105]. Among the various organic materials utilized for bio-stimulation, biochar compost stands out as one of the most widely employed due to its promising potential [102]. Studies have shown a notable reduction in the concentration of heavy metals upon amending contaminated soil with just 10% compost. Compost, rich in organic compounds, such as phenols and exopolysaccharides, acts as an effective improver capable of sequestering heavy metals and transforming them into less soluble forms, consequently limiting their mobility within the soil [102].

Bio-Augmentation

Bioaugmentation involves the introduction of pre-adapted, competent microbial strains or consortia into contaminated environments to enhance bioremediation processes [105]. This strategy is particularly valuable when the metabolic capabilities of indigenous microorganisms are insufficient to remediate soils contaminated with heavy metals or when they cannot withstand heavy metals stress [106]. However, for bioaugmentation to be effective, the inoculum must possess traits, such as heavy metal tolerance, genetic stability, survival post-introduction, and competitive ability against indigenous microbiota [102]. Studies have demonstrated the effectiveness of bioaugmentation in significantly reducing heavy metal concentrations in contaminated soils [105]. For instance, remediation efforts employing bioaugmentation with Lysinibacillus sp. and Bacillus sp. have led to decreased heavy metal concentrations in contaminated soils [107].

Engineered Microbial Remediation

In situations where indigenous microbial strains exhibit lower resistance to the removal and remediation of heavy metal contaminated soil, bioaugmentation with genetically engineered microbial strains becomes a viable option [108]. Engineered microbial remediation, a burgeoning technology, has garnered increasing attention from scientists as an effective approach for remediating heavy-metal-contaminated soils. For instance, engineered E. coli cells were modified by introducing an artificial heavy metal uptake gene, Synthetic Heavy Metal Binding protein (SynHMB), along with an artificial type VI secretion system (T6SS) cluster from P. putida, resulting in the creation of synthetic cells (SynEc2) with a high capacity to display the heavy metal scavenger SynHMB on their cell surface. The surface exposure of the six-histidine tag on the synthetic bacteria, in combination with the carboxyl groups on modified magnetic nanoparticles (MNPs), facilitated the efficient removal of heavy metals, with removal efficiencies exceeding 90%, even within a short timeframe. Moreover, these synthetic bacterial cells and MNPs could be easily recovered using artificial magnetic fields, enhancing their practical applicability [102].

Furthermore, this engineered bioremediation system demonstrates remarkable potential for continuously reducing mercury pollution across a broad range of concentrations by converting highly toxic Hg²⁺ to volatile and less harmful Hg⁰ with exceptional selectivity [102]. Such innovations in genetic engineering offer promising solutions for addressing the challenges posed by heavy-metal-contaminated environments, paving the way for more effective and sustainable remediation strategies.

6. Biosurfactants in Biodegradation and Agricultural Waste Management

6.1. Role of Biosurfactants in Biodegradation Processes

Biosurfactants are indeed fascinating compounds produced by microorganisms that play a vital role in biodegradation processes. They are amphiphilic molecules, meaning they have both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts. This unique property allows them to interact with both water and hydrophobic substances, like oil or organic pollutants. By reducing the surface tension between water and oil, biosurfactants facilitate the dispersion of hydrophobic compounds in water, making them more accessible to microbial degradation. This enhances the biodegradation of various hydrophobic pollutants, including crude oil, diesel, and aliphatic hydrocarbons, in the environment.

Research studies, such as those conducted by Elumalai et al. [109], Das et al. [39], and Koutinas et al. [110], have demonstrated the effectiveness of biosurfactants in enhancing the removal of these pollutants. Their findings underscore the potential of biosurfactants as eco-friendly and efficient agents for bioremediation processes, contributing to the restoration and preservation of environmental health. They achieve this by increasing the abundance of hydrocarbon-degrading bacteria and elevating the activity of hydrocarbon-degrading enzymes [111]. Biosurfactants also possess emulsifying, dispersing, and foaming properties, aiding in the solubilization and dispersion of pollutants, making them more accessible for microbial degradation. Moreover, compared to chemical surfactants, biosurfactants are more effective and environmentally friendly in removing hydrocarbons from contaminated soils. In general, biosurfactants serve as valuable tools in biodegradation processes, offering potential applications in environmental remediation and other biotechnological fields [111].

Biosurfactants produced by Pseudomonas citronellolis SJTE-3 reduce the surface tension of water and enhance the biodegradation of drilling fluid effluents [110]. They increase the abundance of hydrocarbon-degrading bacteria and the rate of biological hydrocarbon degradation. Biosurfactants are biodegradable compounds with wide-ranging applications in industries and environmental remediation [39]. By enhancing the solubility and bioavailability of hydrophobic compounds, such as hydrocarbons, in biodegradation processes, biosurfactants make them more accessible for microbial breakdown [111]. Biosurfactants produced by hydrocarbonoclastic bacteria can enhance the biodegradation of oil and have potential applications in bioremediation processes [112]. Biosurfactants exhibit diverse functions in sustainable agriculture, including combating plant pathogens, improving nutrient availability, and aiding in soil revitalization [90]. As microbial products, biosurfactants are biodegradable with a broad spectrum of applications and high biodegradability. Biosurfactants produced by Pseudomonas aeruginosa, especially at lower concentrations, enhance the biodegradation of herbicides [113]. By facilitating the biodegradation of hydrocarbons and reducing harm to ecosystems, biosurfactants play a crucial role in biodegradation processes [40].

6.2. Use of Biosurfactants in Bioconversion of Agricultural Waste

Biosurfactants play a crucial role in the biotransformation of agricultural residues, offering solutions to challenges associated with waste generation and environmental sustainability. Utilizing industrial agricultural by-products, such as residual fruit materials and lignocellulosic waste, like corn husks, coconut oil cake, and defatted rice bran, proves to be effective for biosurfactant production [114]. Microorganisms, including bacteria and fungi, are employed to synthesize biosurfactants using these agricultural residues [9]. Biosurfactants derived from waste streams have demonstrated potential applications across various sectors, including environmental and industrial applications [115]. These biosurfactants can be applied in agriculture, chemicals, food, and pharmaceutical industries, showcasing their versatility. In essence, incorporating biosurfactants in the biotransformation of agricultural waste presents a hopeful strategy for producing sustainable and value-added products. However, additional research is necessary to fully explore the potential of biosurfactants in addressing challenges related to organic waste [115].

6.3. Case Studies of Successful Biosurfactant Applications in Biodegradation and Agricultural Waste Management

Biosurfactants are utilized in environmental cleanup initiatives to facilitate the breakdown and removal of both water-soluble and insoluble organic pollutants [115]. Microbial-based solutions play a critical role in waste treatment, utilizing engineered microbial strains for efficient degradation and environmental sanitation [116,117]. While composting toilets rely on natural microbial activity, engineered microbial consortia could enhance fecal waste degradation by accelerating organic matter breakdown and reducing pathogen loads. These microbial solutions, though promising, require further development to optimize their application in sustainable sanitation systems. The incorporation of biostimulation, which involves introducing microorganisms with remediation capabilities, along with biosurfactants, has been employed to improve the efficiency of bioremediation processes [118]. Biotechnological studies have indicated that waste materials can serve as economic substrates for biosurfactant production, contributing to waste management and environmental preservation efforts. Notable case studies include the use of Pseudomonas aeruginosa and other oil-degrading bacteria to produce biosurfactants for the effective remediation of oil spills, significantly improving the biodegradation rates of crude oil in contaminated environments [40]. In agricultural waste management, various strains, including Bacillus spp., have been utilized to convert agricultural residues, like corn husks and coconut oil cake, into valuable biosurfactants, with applications in environmental remediation and industrial processes [17]. Additionally, genetically engineered bacteria, such as Pseudomonas putida, have been employed for the bioremediation of heavy metals, enhancing heavy metal uptake and removal from contaminated soils, thereby improving soil health [102].

7. Biosurfactants in Soil Nutrient Availability and Soil Quality Improvement

7.1. Effects of Biosurfactants on Soil Nutrient Availability

Biosurfactants produced by bacteria in the soil’s rhizosphere enhance the availability of water-repellent molecules, potentially assisting in plant growth through various methods [15]. These methods include acting as nutrient supplements, enhancing soil wettability, and facilitating the proper distribution of chemical fertilizers in the soil. The properties of biosurfactants indirectly influence nutrient availability. When combined with chemical compounds, biosurfactants serve as plant protectants, contributing to increased solubility and the powdered form of these compounds. Additionally, these substances exhibit valuable antimicrobial properties, aiding in the absorption of biogenic materials and promoting seed germination [119].

Classifying biosurfactants based on their molecular weight is indeed a common approach, dividing them into two main categories: high-molecular-weight biosurfactants and low-molecular-weight biosurfactants. Each category exhibits distinct properties and functions in various applications, particularly in biodegradation processes and emulsion stabilization. Low-molecular-weight biosurfactants typically have molecular weights below 1 kilodalton (kDa) and are known for their ability to reduce surface and interfacial tension between immiscible fluids, such as oil and water. This property allows them to enhance the solubility and dispersion of hydrophobic compounds, facilitating their degradation by microorganisms. On the other hand, high-molecular-weight biosurfactants, with molecular weights typically exceeding 1 kDa, are more effective at stabilizing emulsions, particularly oil-in-water emulsions. They form a protective layer around oil droplets, preventing their coalescence and promoting the formation of stable emulsions. This stabilization mechanism is valuable in various industrial processes, such as the production of food, cosmetics, and pharmaceuticals [120]. Research has highlighted that the adoption of nutrient-management practices can lead to improved productivity and the reduced environmental impact of farming. However, the adoption of key practices remains below expectations globally. Nutrient management planning can significantly enhance soil nutrient availability, influencing farmers’ intentions to apply fertilizers based on soil test results [121]. In urban gardening, the quality of soil can be influenced by the cultivation techniques, highlighting the role of biosurfactants in maintaining the soil quality [122]. Overall, biosurfactants serve as valuable tools in enhancing soil nutrient availability and improving soil quality, contributing to sustainable agricultural practices.

7.2. Use of Biosurfactants to Improve Soil Quality

In the realm of agriculture, biosurfactants offer a versatile range of applications. They can be employed to combat plant pathogens, enhance the accessibility of nutrients for beneficial plant-related microorganisms, and improve soil quality through amendments. As eco-friendly alternatives, these bio-based molecules have the potential to replace industrial surfactants, contributing significantly to environmental pollution control [15]. The amphiphilic nature of biosurfactants offers a significant advantage, as they spontaneously segregate into two immiscible phases by both reducing the surface tension and enhancing the solubility of hydrophobic compounds. Their environmentally friendly and non-toxic characteristics, coupled with resilience to elevated temperatures and resistance to varying pH levels, highlight their importance [64].

Biosurfactants play a crucial role in breaking down hydrocarbon compounds and providing bio-available substrates for microorganisms, thereby enhancing microbial activity—a pivotal factor in soils contaminated with hydrocarbon compounds [123]. For instance, urban cultivation for food production has highlighted the role of biosurfactants in protecting the soil quality through the integration of organic matter and appropriate cultivation techniques, which can prevent the degradation caused by chemical treatments [122]. Additionally, research has shown that soil quality significantly impacts population mobility in agricultural regions, with improved soil conditions leading to reduced migration, thereby underlining the importance of maintaining soil health.

8. Biosurfactants as Pesticides and Soil Hydrophilization Agents

8.1. Use of Biosurfactants as Pesticides

For many years, microbial bioinsecticides have demonstrated their potential in the management of agricultural pests [31]. Bacillus species are renowned for their biosurfactant-producing capabilities, generating a diverse array of lipopeptides, like surfactin, itorin, bacillomycin, fengycin, and lichenisin [124]. These lipopeptides exhibit robust larvicidal activity by inducing hemolysis. The efficacy of biological pesticides derived from Bacillus and Pseudomonas bacterial species in pest control has been explored (Figure 3). Specifically, Bacillus amyloliquefaciens G1 produces a surfactant that influences aphid cuticles, leading to severe dehydration and mortality in peach aphids, like Myzus persicae, displaying insecticidal effects [125]. The biosurfactant produced by B. amyloliquefaciens AG1 has demonstrated potential in controlling Tuta absoluta larvae and is composed of lipopeptides and polyketides. This biosurfactant operates by binding to receptors on larval brush border membrane vesicles. Furthermore, biosurfactants from Bacillus thuringiensis Vip3Aa16 and B. amyloliquefaciens AG1 have exhibited insecticidal activity against Spodoptera littoralis [31]. Histopathological examinations of treated larvae have revealed vacuolization, necrosis, and disruption of the basement membrane. Bacillus subtilis, historically utilized in human disease treatment since the 1900s, is now employed for pest control through its biosurfactants [31]. For instance, B. subtilis SPB1, a lipopeptide biosurfactant producer, displays insecticidal activity against the carob moth, Ectomyelois ceratoniae [33]. Histopathological investigations have demonstrated the formation of vesicles in the apical area of cells, as well as the lysis and vacuolation of columnar cells in the midgut of E. ceratoniae treated with the B. subtilis SPB1 biosurfactant. Similar histopathological effects were observed in other pests, such as Ephestia kuehniella, Spodoptera littoralis, and Prays oleae [31]. Additionally, rhamnolipid biosurfactants, particularly those produced by Pseudomonas aeruginosa LBI 2A1, have been explored for their insecticidal properties, demonstrating larvicidal effects against Aedes aegypti larvae [10]. Furthermore, glycolipid biosurfactants have proven to be highly effective in the biological control of various pests, including arachnids, larvae, eggs, boxelder bugs, and grasshoppers.

8.2. Biosurfactants and Biological Control Agents Against Nematodes

Nematodes, residing underground in habitats, pose a concealed threat to crop growers, leading to crop losses that often go unnoticed by farmers [126]. The damage inflicted by plant parasitic nematodes is frequently misattributed to other factors, such as water stress, physiological disorders, or fungal attacks [127]. Several studies have highlighted the lethal impact of bacterial biosurfactants on plant parasitic nematodes [10]. For instance, the pathogenicity of a lipoprotein from Bacillus species against plant nematodes was demonstrated [128]. The biosurfactant produced by Bacillus halotolerans strain LYSX1 has shown systemic resistance against Meloidogyne javanica, a plant-parasitic nematode commonly affecting tomato. Additionally, bacterial strains, such as Pseudomonas fluorescens, Pasteuria spp., and various Panibacillus strains, have displayed inhibitory activities against plant nematodes [10].

8.3. Effects of Biosurfactants on Plant Pathogen Elimination

8.3.1. Biosurfactants and Biological Control Agents Against Fungi

Fungi, being the predominant plant pathogens, pose a significant threat to global crop health. Historical famines, such as the Irish Potato Famine caused by Phytophthora infestans and the Bengal Famine attributed to Helminthosporium oryzae, underscore the severe impact of fungal pathogens on crops. Biosurfactants have emerged as effective biological control agents against fungal plant pathogens, playing a crucial role in managing these destructive organisms [10]. A comprehensive overview of recent studies on various types of biosurfactants produced by fungal strains is presented in

Table 2. Some different types of biosurfactants produced by fungal strains.

Fungal Strains	Biosurfactants	Properties	References
Aspergillus ustus	Glycolipoprotein	Antimicrobial activity	[129]
Cunninghamella echinulata	Complex Carbohydrate/Protein/Lipid	Reducing and increasing the viscosity of hydrophobic substrates and their molecules	[130]
Penicillium chrysogenum SNP5	Lipopeptide	Role in pharmaceuticals, as well as in the oil and petroleum industry	[131]
Candida utilis	Emulsifiers	Effective emulsifiers for various applications	[132]
Microsphaeropsis sp.	Eremophilane derivative	Antimicrobial properties	[133]
Candida bombicola	Sophorolipids	Emulsification, detergency, and potential therapeutic applications	[134]

.

Studies have demonstrated that rhamnolipid biosurfactants exhibit inhibitory effects on zoospore-forming phytopathogens and enhance plant immunity against diseases [10]. Pseudomonas aeruginosa-produced rhamnolipids exhibit rapid zoospore lysis, proving lethal against Phytophthora capsica, Pythium aphanidermatum, and Plasmophora lactucae radicis within one minute [135]. Biosurfactants from Pseudomonas putida have shown efficacy in preventing cucumber damping off caused by Phytophthora capsica by inducing zoospore lysis [136]. Rhizosphere-isolated Bacillus and Pseudomonas strains producing biosurfactants have exhibited biological control against soft rot induced by Pectobacterium and Dickeya spp., while fluorescent pseudomonads producing biosurfactants have shown potential in managing potato late blight caused by Phytophthora infestans [137]. Additionally, Bacillus strains producing lipopeptide biosurfactants have demonstrated promise as biological control agents, inhibiting the growth of various plant pathogenic fungi, such as Aspergillus spp., Bipolaris sorokiniana, and Fusarium spp. [138]. A surfactin biosurfactant from Brevibacillus brevis strain HOB1 exhibits antifungal and antibacterial properties, making it a valuable tool for managing plant pathogens [10]. Additionally, biosurfactants from Pseudomonas fluorescens have antifungal properties and inhibit the growth of soil fungal pathogens, such as Phytophthora cryptogea (causing fruit rot), Pythium ultimum (causing damping off), and Fusarium oxysporum (a wilt-inducing agent) [67]. Recent studies have highlighted the various types of biosurfactants produced by fungal strains, providing insights into their potential applications [64].

8.3.2. Biosurfactants and Biological Control Agents Against Bacteria

Phytopathogenic bacteria, exceeding 100 species, with the majority being facultative saprophytes, pose a considerable challenge [10]. Given the environmental concerns associated with chemical approaches to biological control, the utilization of biosurfactants emerges as a promising strategy for bacteria management [10]. The inhibitory effects of biosurfactants from Bacillus licheniformis bacteria against various bacteria and yeasts have been demonstrated, highlighting their potential in bacterial control [139]. Another biosurfactant produced by Staphylococcus spp. has been reported for its ability to inhibit the plant pathogenic bacterium Pseudomonas aeruginosa.

Lipopeptide biosurfactants obtained from Brevibacillus brevis strain HOB1 have shown remarkable antibacterial activity, positioning them as promising candidates in biological control programs [140]. A comprehensive overview of recent studies on the various types of biosurfactants produced by bacterial strains is presented in

Table 3. Some different types of biosurfactants produced by bacterial strains.

Bacterial Strains	Biosurfactants	Properties	References
Pseudomonas aeruginosa S5	Glycolipid	Removal of aromatic hydrocarbons	[141]
Pseudomonas aeruginosa	Rhamnolipid	Enhancement of oil recovery through anaerobic production	[142]
Bacillus subtilis A21	Lipopeptide	Removal of heavy metals, petroleum hydrocarbons	[53]
Pseudomonas aeruginosa PA1	Rhamnolipid	Capacity to use carbon sources	[143]
Paracoccus sp. MJ9	Rhamnolipid	Increasing the solubility of hydrophobic compounds	[144]
Brevibacillus brevis HOB1	Lipopeptide	Antibacterial activity, potential in biological control	[140]
Staphylococcus spp.	Biosurfactant	Inhibitory effect against Pseudomonas aeruginosa	[145]
Pseudomonas putida	Rhamnolipid	Zoospore lysis, inhibition of cucumber damping off disease	[136]
Pseudomonas fluorescens	Biosurfactant	Inhibition of fungal pathogens, plant disease management	[67]

. This underscores the diverse applications and potential of biosurfactants in the realm of bacterial management.

8.4. Biosurfactant and Biological Control of Post-Harvest Diseases

Biosurfactants have demonstrated significant efficacy in managing post-harvest diseases in various fruits. Notably, the biological control agent Bacillus subtilis has been effective in treating post-harvest diseases in pears, apricots, peaches, and cherries [10]. For melons, a biosurfactant derived from Bacillus subtilis EXWB1 has been successful in addressing diseases induced by Alternaria alternata [10]. Specifically, Bacillus subtilis has been effective in mitigating Alternaria rot in musk melons. Commercial microbial products, such as Aspire and Bio-save110, which are patented for controlling post-harvest diseases in pears, have also been developed. Aspire is derived from the yeast Candida oleophila strain I-182, while Bio-save contains a bacterial strain of Pseudomonas syringae [146].

The use of iprodione fungicides in conjunction with bacterial strains of Bacillus amyloliquefaciens has proven effective against gray mold attacks [147]. Moreover, biosurfactants from B. licheniformis have been reported to inhibit post-harvest diseases of mango, including anthracnose and stem end rot [10]. Additionally, surfactants composed of B. licheniformis and B. pumilus have been shown to control gray mold in apples and pears [146]. B. subtilis is also valuable in minimizing tomato rot caused by Penicillium spp. and Rhizopus stolonifera, as well as gray mold caused by Botrytis cinerea during post-harvest handling [10]. This bacterial strain is effective against post-harvest diseases, such as Sour rot caused by Geotrichum candidum, Green mold caused by Penicillium digitatum, and stem end rot caused by Botryodiplodia theobromae [148].

8.5. Use of Biosurfactants for Soil Hydrophilization

The term “soil hydrophilization” denotes the process of treating soil with additives to enhance its hydrophilicity or ability to attract and retain water [73]. Leveraging the dual hydrophobic/hydrophilic properties of microbial-derived biosurfactants, these environmentally friendly surfactants present advantages over synthetic alternatives [15]. Biosurfactants contribute to the enhancement of agricultural soil quality by fostering the biodegradation of contaminants and indirectly promoting plant growth through their antibacterial activity (Figure 3). They are widely applied in agriculture to foster beneficial plant–microbe interactions [15].

Various reports highlight the role of biosurfactants in improving the health of agricultural soil through remediation processes. This involves the extraction of less water-soluble pollutants using surfactants, thereby improving soil health [15]. Unlike synthetic surfactants, which require higher concentrations and can negatively impact microbial biodegradation, biosurfactants are more advantageous for bioremediation purposes due to their natural origin and effectiveness at lower concentrations. Additionally, recent studies emphasize the practical application and efficiency of biosurfactants in enhancing soil properties and promoting sustainable agricultural practices [37].

8.6. Case Studies of Successful Biosurfactant Applications as Pesticides and Soil Hydrophilization Agents

There are documented instances of the surfactin-supported biodegradation of pesticides and glycolipid-assisted degradation of chlorinated hydrocarbons [15]. An illustrative example involves a biosurfactant derived from Lactobacillus pentosus, which exhibited the ability to reduce octane hydrocarbons in soil by 58.6% to 62.8%, showcasing its biodegradation-enhancing properties. Species of Burkholderia, identified in oil-contaminated soil and producing biosurfactants, stand as promising candidates for the bioremediation of various pesticides [149]. The effectiveness of biosurfactants in eliminating insoluble organic pollutants from soil surpasses that of synthetic surfactants. Rhamnolipids have proven utility in removing polyaromatic hydrocarbons and pentachlorophenol from soil. Given the detrimental effects of pesticides and their associated surfactants, there is a growing imperative to replace these harmful surfactants in the pesticide industry with environmentally safe biosurfactants, thereby mitigating the risk of contamination [15].

An alternative approach to address this environmental challenge involves the discovery of soil bacteria capable of utilizing chemical surfactants in agricultural soil as a carbon source [15]. Notably, bacteria belonging to the genera Pseudomonas and Burkholderia in paddy fields have been reported to degrade surfactants [150].

9. Issues with Biosurfactant Application

Despite the numerous environmental and agricultural benefits associated with biosurfactants, their large-scale adoption faces several challenges, including high production costs, variability in effectiveness, and regulatory hurdles. Addressing these barriers is crucial for biosurfactants to become viable alternatives to synthetic surfactants in sustainable agriculture.

9.1. Limitations in Discovery and Optimization

Current molecular methods for discovering biosurfactants are limited to a narrow range of microbial populations, restricting the diversity of biosurfactant-producing strains. Advanced functional metagenomics techniques offer promising avenues for exploring uncultivated microbial consortia in soil biospheres, which may harbor novel biosurfactants with enhanced properties [7]. However, these methods require significant technological and computational resources, making them difficult to implement at scale.

Moreover, biosurfactant performance varies depending on the molecular composition and environmental conditions. For instance, mono-rhamnolipid exhibits greater effectiveness than di-rhamnolipid, with performance differences observed in concentration ranges from 5 mg/L to 20 mg/L [128]. Understanding these differences is critical for optimizing biosurfactant applications in agriculture, soil remediation, and plant protection.

9.2. Toxicity and Environmental Safety Concerns

Biosurfactants are widely recognized for their biodegradability and eco-friendliness, yet their large-scale application in agriculture requires a careful evaluation of toxicity, persistence, and unintended effects on non-target organisms. The safety of biosurfactant-producing microorganisms, their interactions with soil microbial communities, and their long-term environmental impact remain critical factors that require further study [151].

One major concern is that some biosurfactant-producing microbes exhibit pathogenicity. Certain species, such as Pseudomonas aeruginosa and Burkholderia sp., are highly efficient biosurfactant producers but are also classified as opportunistic pathogens, capable of causing infections in plants, animals, and humans [152]. The introduction of such microbes into agricultural systems may pose biosafety risks, particularly when biosurfactants are applied as biopesticides or soil amendments. To mitigate these risks, strain selection and genetic modification strategies have been explored to develop non-pathogenic biosurfactant-producing microbes [153]. Regulatory frameworks should ensure that biosurfactant-producing strains undergo safety screenings before field application to prevent unintended environmental consequences [154]. Beyond microbial pathogenicity, biosurfactants themselves can alter soil microbial communities, leading to shifts in the ecosystem balance. Biosurfactants with antimicrobial properties, such as lipopeptides, are effective in controlling plant pathogens, but their indiscriminate use may suppress beneficial soil microbes, including nitrogen-fixing bacteria and mycorrhizal fungi [155]. This could disrupt nutrient cycling processes and negatively impact soil fertility over time. Optimizing application rates and delivery methods is essential to ensure that biosurfactants target harmful pathogens without disturbing beneficial microbial populations [156].

Another concern is the environmental persistence and degradation rate of biosurfactants. Although biosurfactants are generally biodegradable, their degradation rates vary significantly depending on the soil texture, temperature, moisture, pH, and microbial activity [116]. Some biosurfactants, particularly glycolipids and polymeric surfactants, have been shown to persist longer in anaerobic conditions, potentially leading to accumulation in soil and groundwater. This may alter soil hydraulic properties, impacting water retention and aeration, and may also interact with agrochemicals, affecting their efficacy and mobility [157].

To ensure the safe integration of biosurfactants into agriculture, further studies should assess their degradation pathways under different environmental conditions. Strategies, such as bioaugmentation with microbial consortia capable of accelerating biosurfactant breakdown, could help to mitigate their persistence. Additionally, regulatory guidelines should be established to define acceptable biosurfactant concentrations in agricultural soils and water systems [158]. Addressing these concerns will allow biosurfactants to be safely incorporated into sustainable farming systems while minimizing unintended environmental risks [159].

9.3. Economic and Industrial Challenges

Biosurfactants hold promise as eco-friendly alternatives to synthetic surfactants, but their high production costs, scalability issues, and regulatory challenges remain major obstacles to widespread adoption in agriculture and industry [160]. Unlike conventional surfactants, which are derived from petroleum-based sources, biosurfactant production relies on microbial fermentation, requiring specialized bioreactors, controlled growth conditions, and complex downstream processing. These factors contribute to higher production costs and lower yields, making biosurfactants less economically competitive compared to synthetic counterparts [160]. A major factor limiting biosurfactant commercialization is the cost of production. Unlike synthetic surfactants, which benefit from well-established industrial processes, biosurfactants require expensive raw materials and energy-intensive fermentation processes [161]. Key cost constraints include the need for specialized growth media, long fermentation cycles, and costly purification techniques. To mitigate these challenges, research has focused on developing cost-effective production strategies, such as using agricultural waste, industrial byproducts, and renewable feedstocks as microbial substrates [162]. Additionally, solid-state fermentation (SSF) and metabolic engineering have been explored as methods to improve biosurfactant yields and reduce operational costs. While these approaches show promise, further optimization is necessary to make large-scale biosurfactant production economically viable [163].

Beyond production costs, regulatory and market barriers further hinder the commercial adoption of biosurfactants. Unlike synthetic surfactants, which have established safety guidelines and standardized quality parameters, biosurfactants must undergo extensive toxicity assessments and environmental impact evaluations before they can be approved for agricultural and industrial applications [160]. The lack of globally harmonized regulations leads to inconsistencies in product standards, purity requirements, and performance benchmarks across different regions. Additionally, unclear classification under existing chemical and agricultural regulations creates further uncertainty for biosurfactant producers, delaying market entry. Without clear regulatory frameworks, industries may hesitate to invest in biosurfactant technology due to uncertain approval timelines and high compliance costs [162]. To facilitate biosurfactant commercialization, efforts should focus on establishing regulatory guidelines, defining purity and efficacy standards, and creating financial incentives for sustainable surfactant adoption. Government policies and industry-driven initiatives, such as subsidies for biobased products and funding for green chemistry research, could help reduce the economic risks associated with biosurfactant production [160]. Encouraging public–private partnerships will also be crucial in supporting biosurfactant research, scaling up production, and integrating biosurfactants into mainstream industrial applications [161].

Future research should prioritize optimizing biosurfactant production processes through genetic engineering, strain improvement, and advanced fermentation techniques. Additionally, developing cost-effective extraction and purification methods will be essential to improve biosurfactant recovery and reduce overall production expenses. Addressing these economic and industrial challenges is critical to making biosurfactants a financially viable and scalable alternative to synthetic surfactants, ensuring their successful integration into agriculture, bioremediation, and sustainable industries [163].

10. Conclusions and Future Prospects

Biosurfactants represent a promising and versatile solution for enhancing agricultural sustainability and addressing environmental challenges. This review highlights their extensive potential in various agricultural applications, including soil nutrient enhancement, plant growth promotion, pathogen control, and bioremediation. Their biodegradable nature, low toxicity, and multifunctionality make them attractive eco-friendly alternatives to synthetic agrochemicals. The integration of biosurfactants into precision agriculture systems, supported by advancements in biotechnology, genomics, and nanotechnology, offers new opportunities for optimizing agricultural efficiency and sustainability. Despite these advantages, the widespread adoption of biosurfactants faces significant barriers, particularly in terms of high production costs, scalability, and regulatory uncertainties. While emerging bioprocessing techniques, alternative feedstocks, and metabolic engineering strategies are improving cost-effectiveness, further interdisciplinary research is required to develop economically viable production models. Additionally, the long-term ecological impact of biosurfactant applications remains an area of ongoing investigation, necessitating comprehensive risk assessments and regulatory frameworks to ensure environmental safety.

Future research should focus on enhancing the biosurfactant production efficiency, optimizing application strategies, and developing tailored solutions for specific agricultural challenges. The advancement of synthetic biology, microbial engineering, and green chemistry approaches can significantly improve biosurfactant yields and functional diversity, facilitating their large-scale deployment. Furthermore, fostering industry–government collaborations and policy-driven incentives will be crucial in bridging the gap between laboratory research and real-world agricultural adoption. As research progresses, biosurfactants have the potential to mitigate climate change impacts, improve soil health, enhance crop resilience, and promote sustainable food production systems. Their integration into modern agricultural frameworks can contribute to the development of environmentally conscious and resilient farming practices, ensuring long-term food security for future generations. By overcoming current limitations and expanding the practical applications of biosurfactants, they can play a pivotal role in shaping the future of sustainable agriculture and ecological restoration.