Alginate oligosaccharides (AOS) are functional oligosaccharides derived from the enzymatic or chemical degradation of alginate, a natural polysaccharide abundantly found in brown seaweeds. Compared to alginate, AOS possess a lower molecular weight, better water solubility, and enhanced bioavailability, enabling them to overcome the limitations of alginate’s high viscosity and poor absorption.
Recent research, including a study by He Haofeng and Li Shaochong from the University of Jinan, has highlighted the broad-spectrum bioactivities of AOS and their application in enhancing fertilizer efficiency. With properties such as root growth stimulation, stress tolerance enhancement, crop quality improvement, antiviral activity, nutrient enrichment, antioxidation, postharvest preservation, and soil microbiota regulation, AOS are emerging as promising biostimulants for sustainable agriculture.
Broad Applications of Alginate Oligosaccharides
1. Food Industry
- AOS serve as natural food additives, acting as thickeners, stabilizers, and gelling agents.
- Due to their non-toxicity and biodegradability, they meet clean-label and green food processing trends.
- Their prebiotic effects promote beneficial gut bacteria when added to functional foods.
2. Medical and Pharmaceutical Fields
- AOS exhibit anti-tumor, antioxidant, and anti-inflammatory properties.
- They are studied as drug carriers for targeted delivery systems due to their biocompatibility.
- Promote immune regulation and support therapies for metabolic and cardiovascular diseases.
3. Functional Animal Feed
- AOS act as prebiotics, improving gut health and nutrient absorption in livestock.
- Enhance growth performance, immune responses, and metabolic efficiency in poultry and swine.
- Can reduce the use of antibiotics in feed, aligning with antimicrobial resistance control policies.
4. Agricultural and Plant Protection
- AOS are widely used as plant biostimulants and biofungicides.
- Help plants withstand environmental stresses and promote healthy, high-yielding crops.
- Compatible with integrated pest management (IPM) systems.
Mechanisms of AOS as Plant Biostimulants
Biostimulants enhance plant growth and productivity by activating natural processes rather than acting as nutrients or pesticides. AOS have been extensively studied for the following effects:
1. Root Growth Promotion
- AOS upregulate nitrate reductase gene expression and increase enzyme activity in crops like wheat.
- Stimulate nitric oxide (NO) production, facilitating cell division and root elongation.
- In rice, AOS increase auxin biosynthesis and reduce IAA oxidase activity, supporting root system development and nutrient uptake.
2. Stress Resistance Enhancement
- AOS modulate hormonal signaling pathways, especially abscisic acid (ABA), enhancing plant responses to:
- Drought
- Salinity
- Temperature extremes
- Oxidative stress
- Pathogen invasion
- Studies show combinations of AOS with chitosan and γ-aminobutyric acid (GABA) further improve cold tolerance and drought resistance.
3. Quality Improvement and Yield Enhancement
- Foliar spraying of AOS increases yield by up to 13% in rice and over 60% in cauliflower during early and mid-growth stages.
- Enhance fruit coloration and nutritional quality by regulating flavonoid and anthocyanin biosynthesis (e.g., in strawberries).
- Promote early flowering and fruit maturation, resulting in extended harvest windows and higher economic returns.
4. Antiviral and Antifungal Effects
- AOS act both directly and indirectly to inhibit pathogens:
- Reduce postharvest diseases (e.g., Botrytis cinerea in kiwifruit and peach rot).
- Enhance production of defense enzymes (e.g., phenolics, peroxidases).
- Induce systemic acquired resistance (SAR) in crops like rice against blast disease.
5. Nutritional Enhancement
- Improve photosynthetic efficiency by increasing chlorophyll synthesis and stability.
- Enhance CO₂ assimilation and stomatal conductance.
- Upregulate genes involved in primary and secondary metabolite biosynthesis, leading to increased accumulation of sugars, proteins, and antioxidants.
6. Antioxidant Activity and Postharvest Shelf-Life Extension
- AOS-treated fruits exhibit:
- Higher firmness
- Lower respiration rates
- Reduced ABA accumulation
- Combined with natural extracts (e.g., ginkgo biloba), AOS extend shelf life by reducing microbial spoilage and deterring fruit flies.
7. Soil Microbiome Improvement
- AOS application significantly increases the diversity and abundance of beneficial soil bacteria such as Azospira and Simplicispira.
- Enhances soil enzyme activity and organic matter turnover.
- Improves soil structure and fertility over time, contributing to sustainable land management.
AOS in Fertilizer Efficiency Enhancement
Challenges in Current Fertilizer Use
- Excessive fertilizer use leads to:
- Low nutrient use efficiency (N: 30–40%, P: 10–20%)
- Soil acidification and compaction
- Environmental contamination (e.g., nitrate leaching into groundwater)
- Decreased microbial diversity
- Improving nutrient use efficiency is vital for reducing input costs and mitigating ecological damage.
Mechanisms of AOS in Fertilizer Efficiency
- AOS possess negatively charged carboxyl groups that can chelate with positively charged nitrogen forms (e.g., NH₄⁺, NH₃).
- Form stable complexes, enabling controlled nutrient release.
- Suppress microbial conversion to nitrate/nitrite, reducing volatilization and leaching.
Applications in Fertilizer Formulation
- Slow-release Fertilizers:
- Double-coated urea granules with sulfur and AOS improved nitrogen use efficiency by 22.9% (MENG et al.).
- Biodegradable Coatings:
- AOS–lignin composites (75:25 ratio) reduce fertilizer hydrolysis and prolong nutrient release (BOUCHTAOUI et al.).
- AOS combined with mate tea powder produced controlled-release materials with 80% nutrient release by day 5 (TEIXEIRA et al.).
Production Methods of AOS
| Method | Description | Pros | Cons |
|---|---|---|---|
| Enzymatic Hydrolysis | Uses alginate lyases to cleave alginate | High specificity, mild conditions, preserves activity | Enzyme cost |
| Microbial Fermentation | Employs strains like Bacillus to produce AOS | Low-cost, eco-friendly | Requires strain optimization |
| Physical Methods | Includes ultrasound, heat, radiation | Quick, simple | May damage activity |
| Chemical Degradation | Acid/base/oxidation treatments | Scalable | Random degradation, pollutive, less suitable for agri-use |
Conclusion
Alginate oligosaccharides offer significant advantages as multifunctional biostimulants in modern agriculture. They promote sustainable crop production by enhancing nutrient uptake, improving plant resilience, and reducing dependency on chemical fertilizers. Their compatibility with slow-release formulations, ability to activate plant defense mechanisms, and positive impact on soil health make them highly suitable for eco-friendly farming practices.
Future Research Directions:
- Elucidation of molecular signaling pathways affected by AOS.
- Optimization of low-cost, high-yield AOS production technologies.
- Development of AOS-based multifunctional fertilizer additives and bioformulations.
With continued innovation and integration into crop management systems, AOS are set to become key players in the global movement toward sustainable and resilient agriculture.
