1.Introduction
With the increasing global attention to environmental protection and sustainable development, traditional chemical surfactants are facing severe challenges. Most of these substances cause environmental pollution and thus are subject to increasingly strict regulations by environmental protection laws of various countries, which has prompted researchers to actively explore greener and more efficient alternatives, and biosurfactants have emerged as a result.
The unique amphiphilic structure of biosurfactants endows them with significant advantages in interfacial behavior and performance. They have a wide range of sources, covering multiple biological systems such as microorganisms, plants, and animals. Biosurfactants from different sources have distinct characteristics in structure and performance, ranging from low-molecular-weight glycolipids and lipopeptides to high-molecular-weight polysaccharides, proteins, lipopolysaccharides, lipoproteins, etc. The complexity and diversity of their structures lay a foundation for their diverse applications. In-depth research on the classification, structural characteristics, and different synthesis processes of biosurfactants is of great significance for understanding their behavior mechanisms under different environmental conditions and expanding their applications in various fields. This not only helps promote the green transformation of the chemical industry but also triggers technological innovations in many fields such as medicine, food, and environmental protection, providing new ideas and solutions for solving global environmental and health problems.
2.Classification and Structural Characteristics of Biosurfactants
2.1 Definition and Classification of Biosurfactants
Biosurfactants are a class of amphiphilic compounds, mainly produced by microorganisms, plants, or animals through metabolic or enzymatic reactions, and can also be obtained by simple chemical modification of products from these biological sources. Among them, biosurfactants from microbial sources have attracted wide attention due to their advantages of good performance, wide sources, low cost, and easy preparation.
According to the molecular weight, they can be divided into two categories: low-molecular-weight and high-molecular-weight. Low-molecular-weight biosurfactants include glycolipids and lipopeptides. The combination of fatty acids with amino acids or sugar groups greatly enhances the amphiphilicity of the molecules and effectively reduces the molecular interfacial tension. High-molecular-weight biosurfactants include polysaccharides, proteins, lipoproteins, lipopolysaccharides, and their complexes. For example, the extracellular lipopolysaccharides produced by Acinetobacter calcoaceticus have strong adsorption capacity and can be used as biological emulsifiers.
In addition, according to the core hydrophilic functional groups, biosurfactants are mainly classified into five categories: glycolipids containing sugar hydroxyl groups and glycosidic bonds; lipopeptides and lipoproteins containing peptide bonds and amino acid side chain groups; phospholipids containing phosphate ester bonds; high-molecular polymers containing hydrophilic functional groups such as polysaccharide hydroxyl groups, amino groups, and sulfonic acid groups; fatty acids and their derivatives containing carboxyl groups or hydroxyl groups. According to the source of microorganisms, biosurfactants can also be divided into molecules from bacteria, yeasts, and filamentous fungi.
Table1 Classification of Biosurfactants and Their Microbial Origins
3.Synthesis of Biosurfactants
There are various synthesis methods for biosurfactants, mainly including microbial fermentation, enzyme-catalyzed synthesis, and chemical synthesis combined with biotechnology. Each of these methods has its own advantages and disadvantages, and currently, researchers have also adopted different strategies to increase the yield of biosurfactants at the lowest possible cost.
3.1 Microbial Fermentation Method
The first step is the selection and cultivation of microorganisms. Suitable strains are usually screened using strategies such as surface tension screening, blood plate screening, crude oil plate screening, and blue gel plate screening. For example, Bacillus species can produce lipopeptide biosurfactants, and Pseudomonas species can synthesize rhamnolipids. After screening high-efficiency strains, the culture medium is prepared according to their requirements, usually including carbon sources such as glucose and glycerol, nitrogen sources such as ammonium salts and peptone, and other trace elements. The culture conditions are strictly controlled, generally with a culture temperature of 25-40°C and a pH of 6-8, and dissolved oxygen is ensured through stirring or aeration.
Fermentation methods include batch fermentation, fed-batch fermentation, and continuous fermentation. Batch fermentation is simple to operate but is prone to being affected by substrate and product inhibition, which reduces the yield; fed-batch fermentation can supplement nutrients in a timely manner to avoid substrate inhibition, but the operation is complex, multiple feedings will increase the risk of contaminating miscellaneous bacteria, and the post-treatment is complicated; continuous fermentation can maintain a stable environment and is suitable for large-scale production, but it has high requirements for equipment and processes.
To increase the yield and reduce costs, waste materials such as molasses and waste cooking oil can be used as cheap substrates, genetic engineering technology can be applied to modify microorganisms, methods such as response surface methodology can be used to optimize fermentation conditions and medium components, and new methods such as ultrafiltration and foam separation can be used to improve downstream processing technology, thereby enhancing extraction efficiency and purity.
3.2 Enzyme-Catalyzed Synthesis Method
The synthesis of biosurfactants can be carried out in an extracellular environment using enzymes as biological catalysts, especially the key enzymes in the process of microbial synthesis of biosurfactants. The hydrophilic and hydrophobic components can be formed either completely relying on substrates or through a combination of substrate induction and de novo synthesis. For example, lipases can catalyze the reaction between fatty acids and alcohols to generate ester biosurfactants with different hydrophilic and hydrophobic properties. The reaction conditions are relatively mild, which can avoid the impact of severe conditions such as high temperature and high pressure on the reaction system.
In addition to selecting suitable enzymes, the construction of the reaction system is also crucial for the synthesis of biosurfactants. It is usually necessary to select a suitable solvent system to ensure that the substrates and enzymes can fully contact and react. At the same time, parameters such as reaction temperature and pH need to be precisely controlled. Generally, the reaction temperature is between 20-50°C, and the pH is adjusted according to the optimal activity range of the enzyme, usually between 5-8.
The selection of substrates is also diverse. Natural oils, fatty acids, carbohydrates, and other substances can be used as starting materials. Taking oils as an example, they have wide sources and relatively low prices, and can be converted into biosurfactants with good surface activity through enzyme-catalyzed modification reactions.
During the enzyme-catalyzed synthesis of biosurfactants, enzymes can realize the connection and modification between different biological molecules through specific mechanisms of action, thereby obtaining biosurfactants with special structures and properties. For example, transglutaminase can catalyze the formation of new peptide bonds between proteins and amino polysaccharides, and laccases and peroxidases can oxidize compounds containing phenol and aniline structures, promoting the cross-linking between proteins and feruloylated polysaccharides.
3.3 Chemical Synthesis Combined with Biotechnology Method
In recent years, the combination of chemical synthesis and biotechnology to synthesize biosurfactants has become an innovative method. This strategy aims to combine the advantages of chemical synthesis and biosynthesis to achieve more efficient, cost-effective, and environmentally friendly production of biosurfactants.
This method usually includes two approaches: one is to use chemical synthesis to construct specific structural units of biosurfactants, and then use microorganisms to complete the rest of the molecules, thereby improving production efficiency and reducing production costs; the other is to use chemical synthesis to simply modify certain groups in biosurfactant molecules, such as simple modification of biosurfactants through esterification, acylation, glycosylation, and other reactions to adjust their hydrophilic-lipophilic balance stability and biological activity.
An integrated chemical-biological process is used: first, a specific catalyst is used to hydrocrack polyethylene waste to generate alkanes, and then microorganisms are used for metabolic conversion, finally successfully converting polyethylene waste into biosurfactants and improving production efficiency. The glycolipid biosurfactants produced by microorganisms are chemically modified to synthesize sophoroside amines with different alkyl chains and their quaternary ammonium salt derivatives, which change the antibacterial activity of the surfactants, and the long-chain derivatives have significant effects. This method can combine the accuracy of chemical synthesis and the specificity of biotechnology to achieve effective regulation of the structure and performance of products.
3.4 Other Innovative Methods
In the research on the synthesis of biosurfactants, in addition to microbial fermentation, enzyme-catalyzed synthesis, and chemical synthesis combined with biotechnology, some innovative methods have also emerged continuously.
Gene editing technology has also brought new ideas for the synthesis of biosurfactants. Gene editing, especially the CRISPR-Cas9 system, provides a powerful tool for optimizing microbial cell factories to efficiently produce biosurfactants. The CRISPR-Cas9 system is a gene editing tool that uses guide RNA (gRNA) to guide Cas9 nuclease to specific DNA sequences in the genome, and performs knockout, knock-in, or modification of related genes, thereby significantly improving the yield and quality of biosurfactants. The genome of Starmerella bombicola yeast is precisely modified through gene editing technologies such as the CRISPR-Cas9 system to produce single acidic sophorolipids.
There is also an innovative method using extremophilic microbial resources. Microorganisms capable of producing special biosurfactants are screened from extreme environments. These microorganisms have formed unique metabolic pathways in the process of adapting to extreme conditions. The biosurfactants produced by them often have special structures and properties, and can still maintain good activity in extreme environments such as high temperature, high salt, strong acid, or strong alkali, which expands new fields for the application of biosurfactants.
4.Application of Biosurfactants
As a type of surfactant, biosurfactants share common properties with traditional chemical surfactants, such as reducing interfacial tension and having emulsifying, solubilizing, dispersing, and foaming capabilities. At the same time, compared with traditional chemical surfactants, biosurfactants have significant advantages such as lower toxicity, stronger biodegradability, better biocompatibility, and stronger tolerance to extreme environments. In conclusion, biosurfactants have excellent performance and are environmentally friendly, which can effectively avoid damage to reactants and pollution to the environment during use. Therefore, biosurfactants have important applications in many fields.
In the petroleum industry, they can reduce oil viscosity and repair pollution; in the environmental protection field, they can remove heavy metals and purify wastewater; in the medical field, they can exert functions such as antibacterial and anti-cancer effects; in the food industry, they can be used as emulsifiers and preservatives; they also play a role in fields such as cosmetics and agriculture, for example, in cosmetics, they have moisturizing, antibacterial, and oil emulsifying effects, and in agriculture, they can promote pesticide degradation; they can also be used in the synthesis of nanomaterials.
4.1 Application in the Petroleum Industry
Biosurfactants can reduce oil viscosity, improve fluidity and recovery rate; they can also be used for the remediation of petroleum-contaminated soil and water bodies, solubilizing and emulsifying petroleum hydrocarbons, and promoting microbial degradation. The transposon technology is used to improve the ability of strains to produce surfactants and the degradation ability of surfactants, which is applied to treat oil sludge in petroleum refineries and can greatly improve the oil sludge degradation effect. The lipopeptide biosurfactants produced by Bacillus cereus are used to degrade polycyclic aromatic hydrocarbons in contaminated soil, which can significantly increase the degradation rate of high-molecular-weight polycyclic aromatic hydrocarbons, change the microbial community structure, and promote microbial growth. Chitosan-immobilized Bacillus subtilis is used to produce lipopeptides in a stirred tank fermenter with waste glycerol and palm oil as raw materials, which can be used for cleaning petroleum-contaminated drill cuttings. Amidated rhamnolipids are prepared through chemical modification, which can effectively reduce adsorption loss, improve oil washing efficiency in high-temperature and high-salt environments, and can be used for efficient oil displacement under harsh reservoir conditions.
Fig.1 Rhamnolipid amidation reduces biosurfactant adsorption loss and improves the oil-washing efficiency
4.2 Application in the Environmental Protection Field
Biosurfactants can remove heavy metal pollutants from soil and water bodies, and recover metals through ion exchange and other effects; they can enhance the degradation of polycyclic aromatic hydrocarbons, improve their bioavailability, and contribute to the remediation of contaminated sites; in wastewater treatment, they can be used as ion collectors to treat heavy metal-containing wastewater, or purify organic wastewater through adsorption, emulsification, and other methods. The co-cultivation of urea-hydrolyzing bacteria Bacillus and biosurfactant-producing bacteria Pseudomonas is used to synergistically improve their respective performances, producing microbial dust suppressants, which helps prevent and control coal dust pollution. The electric field-driven pollutant migration experiment is used to systematically study the effects of biosurfactants such as rhamnolipids, saponins, and sophorolipids on the removal efficiency of heavy metals in sludge, and it is found that all of them can effectively improve the removal efficiency. Cyclic lipopeptide biosurfactants are used to desorb polycyclic aromatic hydrocarbons from artificially contaminated sediments. The lipopeptide biosurfactants extracted from corn steep liquor are added to lignocellulose-based bio-composites, which can improve the performance of bio-composites and enhance their adsorption capacity for pollutants. Biosurfactant cleaning technology is used to elute petroleum substances from three types of oil sludge, and the effects of components and surfactant types are discussed. It is found that the combination of rhamnolipids and sophorolipids can greatly improve the oil removal rate.
Fig.2 Biosurfactants enhance the oil removal rate
4.3 Application in the Medical Field
Biosurfactants have antibacterial activity; for example, lipopeptides can inhibit a variety of pathogenic bacteria; some can have anti-cancer effects by inducing differentiation and apoptosis of cancer cells; they can be used as immunomodulators to regulate the function of immune cells; they can also be used for drug delivery to improve the solubility and stability of drugs and achieve targeted drug delivery. A membrane-fused drug delivery system modified with coiled-coil lipopeptides is constructed, which can effectively deliver cisplatin to drug-resistant cancer cells, improve the drug delivery efficiency, and thus effectively inhibit tumor growth. A virus mimic based on dendritic lipopeptides is designed to enhance the accumulation of drugs in tumor sites and cell uptake, thereby achieving effective tumor treatment. A lipopeptide is prepared using pyridyl disulfide bond reaction, and when incorporated into liposome formulations, it can effectively promote cell uptake and cytoplasmic release, improving the drug delivery effect. Four types of lipopeptides containing cycloalkane chains are synthesized by solid-phase peptide synthesis using Rink amide resin as a solid-phase carrier, which are used for wound healing, have good biocompatibility, and can stimulate the production of collagen and blood vessels.
Fig.3 Cycloalkane-based lipopeptides promote wound healing
4.4 Application in the Food Industry
Biosurfactants can be used as emulsifiers to disperse oil and water evenly and stabilize emulsions such as beverages and dairy products; they can be used as preservatives to inhibit microbial growth and extend the shelf life of food; they can improve food texture and enhance taste and quality in food. For example, adding yeast mannoproteins during winemaking can improve the quality of wine; hydrophobin HFBII can be used to prepare air-filled emulsions, which can effectively reduce the fat content in emulsions.
4.5 Application in Other Fields
In cosmetics, biosurfactants are beneficial for moisturizing, antibacterial, and emulsifying; in agriculture, biosurfactants have a wide range of application scenarios. They can promote pesticide degradation, reduce pesticide residues in soil and crops, reduce harm to the ecological environment and human health, and at the same time improve the utilization rate of pesticides and enhance the control effect of pesticides. In terms of preventing and controlling plant diseases, some biosurfactants have antibacterial activity, which can directly inhibit the growth and reproduction of plant pathogenic bacteria, and can also induce plants to produce disease resistance, enhance the plant’s own resistance to diseases, and reduce the use of chemical fungicides. In addition, biosurfactants can improve soil structure, enhance soil aeration and water retention, promote the growth of beneficial microorganisms in the soil, enhance soil fertility, and provide a good soil environment for crop growth. At the same time, they can also be used as plant growth regulators to promote crop seed germination, root growth, and nutrient absorption, and improve crop yield and quality; they can also be used in the synthesis of nanomaterials to control the growth and dispersion of nanoparticles. A glycolipid ohmline with an asymmetric hydrophobic core is studied, which can self-assemble to form nanotubes below the melting point. The hydrogen bond interaction between trehalose glycolipids and polar molecules is used to replace half of the ionizable lipids to form a stable lipid nanoparticle structure, which can significantly reduce the toxicity to cells and organs.
Fig.4 Trehalose glycolipids replace ionizable lipids to reduce the toxicity of lipid nanoparticles
5.Conclusion and Prospect
Biosurfactants are amphiphilic compounds produced during the metabolism of microorganisms, plants, or animals. According to their molecular weight, they can be divided into two categories: low-molecular-weight and high-molecular-weight. Low-molecular-weight biosurfactants can effectively reduce interfacial tension, while high-molecular-weight ones have strong adsorption capacity. The synthesis methods of biosurfactants mainly include microbial fermentation, enzyme-catalyzed synthesis, and chemical synthesis combined with biotechnology, each with its own advantages and characteristics. Biosurfactants are widely used in fields such as petroleum, environmental protection, medicine, food, cosmetics, and agriculture, ranging from improving oil recovery to treating pollutants, delivering drugs, and preserving food, and have broad application prospects.
Advances in chemical biotechnology and chemical processes will bring new opportunities and challenges to the research of biosurfactants. In terms of synthesis methods, it is expected to improve the yield and quality of biosurfactants by optimizing methods and reducing costs, such as using gene editing technology to regulate microbial metabolism, developing high-performance biological catalysts, and exploring new synthesis pathways. Its applications are expected to expand to fields in the future.
For additional insights into agricultural surfactant options, refer to the Dora surfactant series.