The inhibitory effect of L-valine on cyanobacteria growth

L-valine, a naturally occurring essential amino acid, plays a crucial role in biological metabolism. However, its growth-inhibiting effects on cyanobacteria (such as Microcystis and Anabaena) and potential ecological impacts have attracted attention in recent years. Cyanobacterial blooms are a prominent problem in global freshwater ecosystems; their excessive proliferation causes water hypoxia, produces cyanotoxins, and threatens aquatic organisms and human health. Exploring environmentally friendly cyanobacterial inhibitors is a key direction for bloom control, and L-valine, due to its natural origin and biodegradability, has emerged as a potential candidate.

I. Growth-Inhibiting Effects on Cyanobacteria and Mechanisms

Manifestations of Growth Inhibition

Experiments show that L-valine significantly inhibits the growth of various cyanobacteria (e.g., Microcystis aeruginosa, Anabaena flos-aquae) in a concentration-dependent manner. When L-valine concentrations in water reach 0.5–2.0 mmol/L, cyanobacterial cell density, chlorophyll a content, and photosynthetic activity (e.g., PSⅡ reaction center efficiency) decrease significantly. At high concentrations (≥5.0 mmol/L), it can even cause cyanobacterial cell lysis. Compared to chemical algaecides (e.g., copper sulfate), L-valine’s inhibitory effect takes effect more slowly but lasts longer (up to 7–10 days). It also exhibits high selectivity for cyanobacteria, with relatively weaker impacts on other phytoplankton such as green algae and diatoms.

Potential Inhibitory Mechanisms

Interference with amino acid metabolism: Cyanobacteria can obtain valine through self-synthesis or environmental absorption. High concentrations of L-valine may competitively inhibit the activity of key enzymes in valine synthesis (e.g., acetolactate synthase), leading to intracellular valine metabolic imbalance, which further impairs protein synthesis and cell division.

Damage to photosynthetic systems: L-valine may enter cyanobacterial cells through osmosis, disrupting the structural stability of thylakoid membranes, reducing chlorophyll a synthesis efficiency, and inhibiting the photosynthetic electron transport chain (e.g., the oxygen-evolving complex of PSⅡ), resulting in impaired energy metabolism.

Induction of oxidative stress: High concentrations of L-valine can induce excessive reactive oxygen species (ROS) production in cyanobacterial cells, triggering lipid peroxidation and DNA damage, ultimately leading to cell apoptosis. For example, in Microcystis aeruginosa treated with 1.0 mmol/L L-valine, ROS content increases 2–3 times compared to the control group, and malondialdehyde (MDA) content rises by more than 50%.

II. Ecological Safety Assessment

Ecological safety evaluation requires comprehensive consideration of impacts on non-target organisms, environmental degradability, and food chain transfer risks, including:

Toxic Effects on Non-Target Organisms

Aquatic organisms: Low concentrations of L-valine (≤1.0 mmol/L) have no significant effects on the survival, growth, or reproduction of fish (e.g., zebrafish), cladocerans (e.g., Daphnia magna), or benthic organisms (e.g., Limnodrilus hoffmeisteri). At concentrations ≥5.0 mmol/L, osmotic imbalance may reduce the survival rate of some sensitive organisms (e.g., Daphnia magna), but this is far below the toxicity threshold of chemical algaecides (e.g., the 48h-EC₅₀ of copper sulfate for Daphnia magna is approximately 0.1 mmol/L).

Microbial communities: L-valine can be decomposed and utilized as a carbon and nitrogen source by heterotrophic bacteria in water (e.g., Bacillus, Pseudomonas). It does not significantly alter the structure of aquatic microbial communities; instead, it may promote the reproduction of degrading bacteria, accelerating its own degradation and avoiding long-term residues.

Environmental Degradation and Residue Risks

L-valine in natural water bodies can be decomposed into harmless substances such as carbon dioxide and ammonia nitrogen through microbial degradation (the main pathway) and photolysis, with a half-life of typically 3–7 days. Temperature, pH (degradation is fastest under neutral conditions), and microbial activity are key factors affecting degradation rates. In eutrophic water, rich in microorganisms, its degradation rate is faster, resulting in low residue risks—unlike chemical algaecides (e.g., simazine), which accumulate in sediment over the long term.

Impacts on Ecosystem Functions

Primary productivity: Although L-valine inhibits cyanobacterial growth, it has minor effects on beneficial algae (e.g., green algae, diatoms). Thus, water primary productivity does not decline significantly; instead, reduced cyanobacterial competition may promote the growth of other algae, maintaining material cycling and energy flow in the ecosystem.

Food chain transfer: As a natural amino acid, L-valine is easily utilized or degraded in the food chain and does not bioaccumulate in higher trophic level organisms. Its indirect impacts on birds, mammals, etc., are negligible.

III. Application Limitations and Optimization Directions

Limitations

High concentration requirement: Effective cyanobacterial inhibition typically requires L-valine concentrations ≥1.0 mmol/L, which may increase application costs. At high concentrations, it may slightly inhibit the growth of some aquatic plants (e.g., the submerged plant Vallisneria natans).

Interference from environmental factors: In high-turbidity or high-organic-matter water, L-valine may bind to other substances, reducing its bioavailability and inhibiting effectiveness.

Optimization Strategies

Combined application: Combining L-valine with low-concentration plant-derived extracts (e.g., gallic acid) or microbial agents (e.g., algicidal bacteria) can reduce the required effective concentration, enhance algicidal efficacy, and minimize impacts on non-target organisms.

Sustained-release technology: Encapsulating L-valine in sustained-release materials (e.g., starch microspheres, chitosan gels) to control its release rate in water can extend the action period and reduce single-dose requirements.

L-valine inhibits cyanobacterial growth mainly by interfering with metabolism, damaging photosynthetic systems, and inducing oxidative stress, with concentration-dependent and partially selective effects. From an ecological safety perspective, L-valine, due to its degradability, low toxicity to non-target organisms, and lack of bioaccumulation risks, aligns better with green ecological management concepts than chemical algaecides. However, its high concentration requirement and sensitivity to environmental factors limit practical applications. Future optimization through composite formulations and sustained-release technologies will enhance its efficiency, providing a new approach for the ecological control of cyanobacterial blooms.