**Background:** The global population is projected to reach 9.6 billion by 2050, increasing food demand, while arable land is expected to grow only from 1592 million ha (2005-07) to 1661 million ha (2050). Intensive use of chemical fertilizers has deteriorated soil health and caused environmental issues. Microalgae and cyanobacteria offer sustainable alternatives as biofertilizers, biostimulants, and biopesticides. This review summarizes their plant growth-promoting activities, mechanisms of action, application methods, and challenges to commercial adoption.
**Methods:** This is a comprehensive narrative review synthesizing published literature on microalgae-based agricultural additives. It covers three functional categories: biofertilizers (N-fixation, P-solubilization, micronutrient mobilization), biostimulants (phytohormones, polysaccharides, phenolics, C-phycocyanin, micronutrients), and biopesticides (antimicrobial and insecticidal properties). The review also examines application methods (seed treatments, foliar spray, soil/root drenching), abiotic stress amelioration, biomass production challenges, life cycle analysis, wastewater integration, biorefinery approaches, and circular economy models.
**Key Results:** Microalgae contain auxins (IAA predominant, 0.18-99.83 nmol g⁻¹ DW), cytokinins (0.29-21.40 nmol g⁻¹ DW), gibberellins (3-3452.9 pg mg⁻¹), and other phytohormones. Polysaccharides from species like Spirulina platensis and Chlorella vulgaris enhance defense enzymes (LOX, PAL, CAT, POD, APX). Total polyphenol content ranges from 0.16 mg GAE g⁻¹ (Neochloris) to 60 mg GAE g⁻¹ (Nostoc). C-phycocyanin from Spirulina platensis enhances germination index and flavonoid content in tomato and lettuce. Micronutrient content includes P (0.73-1.46% w/w), Ca (0.1-2.9% w/w), K (0.7-2.4% w/w), and Fe (up to 1110 mg 100g⁻¹ in Porphyridium aerugineum). Cyanobacterial biofertilizers can reduce chemical N fertilizer requirements by up to 50% in crops like corn and rice. Foliar spray of microalgae extracts enhances PAL activity more rapidly than soil drenching. Wastewater-grown microalgae achieve nutrient removal efficiencies >70-90% for nitrates, phosphates, and ammonia. The global biostimulant market is valued at USD 3.2 billion, forecasted to reach USD 5.6 billion by 2026, with microalgae fertilizers valued at USD 9479 thousand in 2021 (CAGR 7.7%).
**Clinical Implications:** While not a clinical study, this review has significant implications for agricultural and environmental sustainability. Microalgae-based products can reduce dependency on synthetic fertilizers, mitigate environmental pollution (eutrophication, GHG emissions), and improve crop resilience to climate change-induced abiotic stresses (drought, salinity, temperature). Key challenges include high biomass production costs (12000-22000 kWh per metric ton), water requirements (15000 m³ m⁻² month⁻¹), scalability issues, and risks from xenobiotics/heavy metals in wastewater-grown biomass. The review advocates for closed-loop circular economy models integrating wastewater bioremediation, biorefinery approaches, and hydroponic co-cultivation to achieve commercial feasibility.