Microbial utilization of L-leucine in environmental remediation

As a branched-chain amino acid, L-leucine serves as an essential nitrogen and carbon source for microbial growth and metabolism. Its microbial utilization in environmental remediation centers on regulating the growth, metabolism, and community structure of functional microorganisms to enhance their capacity for degrading and transforming pollutants. It is applicable to multiple scenarios including the remediation of heavy metal-contaminated soil, degradation of organic pollutants, and nitrogen and phosphorus removal from wastewater. The specific mechanisms of action and application practices are as follows:

I. Core Mechanisms of Action in Microbial Environmental Remediation

The enhancing effect of L-leucine on microbial remediation processes stems from its role as a nutrient substrate in regulating microbial physiological functions, mainly manifested in three aspects:

Providing Nitrogen and Carbon Nutrition to Promote Proliferation of Functional MicroorganismsMost environmental pollutants (e.g., petroleum hydrocarbons, polycyclic aromatic hydrocarbons, heavy metal chelates) are difficult for microorganisms to directly utilize as nutrient sources. Polluted environments often suffer from nitrogen and carbon deficiency, which limits the growth and reproduction of functional microorganisms. L-leucine contains amino (-NH₂) and carboxyl (-COOH) groups, and can be decomposed into ammonium nitrogen and small-molecule organic acids by proteases secreted by microorganisms. It provides readily available nitrogen and carbon sources for functional microorganisms such as degrading bacteria, nitrogen-fixing bacteria, and flocculant-producing bacteria, thereby increasing microbial biomass and activity. For example, in the remediation of petroleum-contaminated soil, the addition of L-leucine can significantly promote the growth of petroleum hydrocarbon-degrading bacteria (e.g., Pseudomonas, Bacillus), increasing the community abundance of degrading bacteria by 2–3 times and further accelerating the degradation rate of petroleum hydrocarbons. In heavy metal-contaminated water bodies, it can stimulate the proliferation of flocculant-producing microorganisms (e.g., Zoogloea), whose secreted extracellular polymeric substances (EPS) can immobilize heavy metal ions through complexation.

Inducing Microorganisms to Secrete Functional Metabolites to Enhance Pollutant TransformationL-leucine acts as an inducer to activate the expression of key enzyme genes in specific metabolic pathways of microorganisms, prompting them to secrete metabolites with environmental remediation functions:

Degradative Enzymes: In organically polluted environments, L-leucine can induce microorganisms to secrete degradative enzymes such as peroxidase, laccase, and lipase. These enzymes can break chemical bonds (e.g., benzene rings, ester bonds) of refractory organic pollutants, converting macromolecular pollutants into harmless small-molecule substances. For example, in polychlorinated biphenyl (PCB)-contaminated soil, its addition can increase microbial laccase activity by 30%–50%, accelerating the dechlorination and degradation of PCBs.

Extracellular Polymeric Substances (EPS): L-leucine can promote microorganisms to secrete EPS composed of polysaccharides, proteins, nucleic acids, etc. EPS is rich in active groups such as hydroxyl, carboxyl, and amino groups. On the one hand, it can form stable chelates with heavy metal ions (e.g., Cd²⁺, Pb²⁺, Cr⁶⁺) through complexation and adsorption, reducing their bioavailability. On the other hand, it can form a hydrophobic microenvironment to enrich hydrophobic organic pollutants (e.g., petroleum hydrocarbons), improving the contact efficiency between microorganisms and pollutants.

Signal Molecules: Metabolites of L-leucine can act as quorum-sensing signal molecules to regulate microbial group behavior and promote degrading bacteria to form biofilms. The biofilm structure can enhance the tolerance of microorganisms to extreme environments (e.g., high salinity, high heavy metal concentrations) while improving the mass transfer efficiency of pollutants within the biofilm.

Regulating Microbial Community Structure to Optimize Remediation Flora FunctionMicrobial communities in polluted environments are often structurally simple and functionally weak. The addition of L-leucine can optimize the community structure through selective enrichment: functional microorganisms that prefer to utilize it (e.g., degrading bacteria, probiotics) will proliferate rapidly, while the growth of inefficient or harmful microorganisms is inhibited, thus constructing a high-efficiency remediation flora. For example, in the denitrification treatment of aquaculture wastewater, L-leucine can enrich nitrifying bacteria (e.g., Nitrosomonas) and denitrifying bacteria (e.g., Paracoccus), improving the rate of nitrification and denitrification efficiency, and increasing the total nitrogen removal rate of wastewater by 20%–30%. In saline-alkali heavy metal-contaminated soil, it can promote the growth of salt-alkali-tolerant heavy metal remediation bacteria, improving the diversity and stability of soil microbial communities.

II. Microbial Utilization Practices in Typical Environmental Remediation Scenarios

Microbial Remediation of Heavy Metal-Contaminated Soil/Water BodiesIn heavy metal pollution remediation, L-leucine reduces the toxicity and mobility of heavy metals mainly through the synergistic effect of "microbial adsorption–complexation–transformation":

Adsorption and Complexation: L-leucine stimulates microorganisms to secrete EPS, whose carboxyl and amino groups can form stable chelates with heavy metal ions, converting free heavy metals into bound forms and reducing their leaching risk and bioavailability in soil. Experimental data show that after adding L-leucine, the proportion of exchangeable Cd²⁺ in soil decreases by 40%–60%.

Biotransformation: With nutritional support from L-leucine, some functional microorganisms (e.g., sulfate-reducing bacteria) can reduce high-valence toxic heavy metals (e.g., Cr⁶⁺, Hg²⁺) to low-valence non-toxic or low-toxic forms. For example, after Cr⁶⁺ is reduced to Cr³⁺, it can form precipitates with anions in soil, further reducing its mobility.

Application Method: L-leucine is mixed with microbial inoculants (e.g., Bacillus inoculants) to prepare bioremediation agents, which are applied by soil spraying or water body dosing. The dosage is usually controlled at 50–200 mg/kg soil or 10–50 mg/L water to avoid nitrogen pollution caused by excessive addition.

Microbial Degradation and Remediation of Organic PollutantsFor organic pollutants such as petroleum hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), and pesticide residues, L-leucine improves remediation efficiency by enhancing microbial degradation capacity:

Petroleum Pollution Remediation: In petroleum-contaminated soil or water bodies, the addition of L-leucine can serve as a nitrogen source to compensate for the nitrogen deficiency in petroleum hydrocarbons, promoting the growth and metabolism of petroleum-degrading bacteria. Meanwhile, lipase and hydrocarbon oxidase induced and secreted by L-leucine can accelerate the decomposition of petroleum hydrocarbons, increasing the petroleum hydrocarbon degradation rate by 25%–40% and shortening the remediation cycle by 1/3.

Pesticide Residue Remediation: For organophosphorus and pyrethroid pesticide pollution, L-leucine can enhance the mineralization capacity of microorganisms for pesticides. For example, in cypermethrin-contaminated soil, after adding L-leucine, microorganisms can increase the degradation rate of cypermethrin from 35% to more than 75% within 15 days, with degradation products being CO₂, H₂O, and harmless inorganic salts.

Key Points: Specific degrading bacteria should be matched according to pollutant types. Meanwhile, the addition rate of L-leucine should be controlled, and a "batch addition" method should be adopted to avoid microbial metabolic imbalance caused by one-time high-concentration input.

Efficiency Enhancement of Wastewater Biological Treatment SystemsIn the biological treatment of industrial wastewater and domestic sewage, L-leucine can be used as a biological growth promoter to improve the treatment efficiency of activated sludge or biofilms:

Enhancing Nitrogen and Phosphorus Removal: L-leucine provides nutrients for nitrifying bacteria, denitrifying bacteria, and phosphorus-accumulating organisms, promoting nitrification (ammonia nitrogen → nitrate nitrogen) and denitrification (nitrate nitrogen → nitrogen gas) processes. It also enhances the phosphorus release and uptake capacity of phosphorus-accumulating organisms, increasing the ammonia nitrogen removal rate of wastewater to over 90% and the total phosphorus removal rate by 15%–25%.

Improving Sludge Performance: L-leucine can reduce the bulking phenomenon of activated sludge and improve sludge settling performance. Its mechanism of action is to promote flocculant-producing microorganisms to secrete EPS, enhance the cohesion of sludge particles, and reduce sludge water content and volume.

Application Scenarios: It is suitable for the treatment of wastewater with low carbon-nitrogen ratio (e.g., coal chemical wastewater, aquaculture wastewater). The addition amount is usually 0.5%–1.0% of wastewater COD, which can effectively solve the problem of low denitrification efficiency caused by insufficient carbon sources.

III. Influencing Factors and Optimization Strategies for Microbial Remediation of Environment Using L-leucine

Key Influencing Factors

Environmental pH: The utilization efficiency of L-leucine by microorganisms is significantly affected by pH. A neutral to weakly alkaline environment (pH 6.5–7.5) is most suitable for the activity of microbial proteases, facilitating the decomposition and absorption of L-leucine. Acidic or strongly alkaline environments will inhibit enzyme activity and reduce nutrient utilization efficiency.

Temperature and Dissolved Oxygen: Aerobic degrading bacteria have the highest utilization rate of L-leucine under the conditions of 25–35℃ and dissolved oxygen > 2 mg/L. Anaerobic microorganisms (e.g., denitrifying bacteria, sulfate-reducing bacteria) need to function in anoxic/anaerobic environments.

Pollutant Concentration: High-concentration pollutants (e.g., heavy metal concentration > 100 mg/kg, petroleum hydrocarbon concentration > 10000 mg/kg) will inhibit microbial activity. Physical and chemical methods should be used first to reduce pollutant concentrations, and then L-leucine and microbial inoculants should be added for remediation.

Optimization Strategies

Compound Application: L-leucine is compounded with other nutrients (e.g., glucose, B vitamins) to form a composite growth promoter, further enhancing microbial activity. Compounding with surfactants can enhance the solubility of organic pollutants and improve the contact efficiency between microorganisms and pollutants.

Immobilization Technology: Immobilized microbial technology is adopted to co-immobilize functional microorganisms and L-leucine on carriers (e.g., sodium alginate, activated carbon), reducing microbial loss and improving the stability and reusability of remediation agents.

Precision Regulation: By monitoring the microbial community structure, enzyme activity, and pollutant concentration in the environment, dynamically adjust the addition amount and frequency of L-leucine to achieve precise control of the remediation process.

IV. Application Advantages and Limitations

Core Advantages

Environmental Friendliness: L-leucine is a natural amino acid that can be completely biodegraded without causing secondary pollution, conforming to the concept of green remediation.

High Efficiency and Low Consumption: Compared with chemical remediation methods, microbial remediation has lower costs, and the addition of L-leucine can significantly improve remediation efficiency and shorten the remediation cycle.

Strong Universality: It is applicable to the remediation of various pollution types and can be combined with physical and chemical remediation technologies to achieve synergistic enhancement.

Limitations

High Cost: The large-scale production price of L-leucine is relatively high, limiting its application in large-area pollution remediation.

Strong Specificity: It needs to be matched with specific functional microorganisms, and the remediation effect varies greatly for different pollutants.

Environmental Dependence: The remediation effect is significantly affected by environmental factors such as temperature, pH, and dissolved oxygen, and the effect is limited in extreme environments.