Algae and Cyanobacteria-Mediated Biosynthesis of Metal and Metal Oxide Nanoparticles: Mechanisms, Characterisation, and Applications in Sustainable Agriculture and Environmental Remediation

Abstract

The biogenic synthesis of metal and metal oxide nanoparticles using algae and cyanobacteria has emerged as a candidate alternative to physical and chemical fabrication routes, which typically depend on hazardous reagents, high energy inputs, and toxic by-products. This critical narrative review synthesises the accessible peer-reviewed literature on algal and cyanobacterial nanoparticle biosynthesis, with particular attention to the biochemical mechanisms underlying metal ion reduction and capping, the analytical techniques used to confirm nanoparticle formation, and the evidence supporting applications in sustainable agriculture and environmental remediation. Evidence indicates that both intracellular and extracellular synthesis routes are biochemically plausible, mediated principally by NAD(P)H-dependent oxidoreductases, sulphated polysaccharides, phycobiliproteins, and phenolic metabolites, although the relative contribution of each pathway remains incompletely resolved and appears to vary by taxon, growth phase, and reaction condition. Characterisation practice is reasonably consistent across studies, relying on ultraviolet-visible spectroscopy, transmission electron microscopy, X-ray diffraction, and Fourier-transform infrared spectroscopy, but methodological heterogeneity and limited replication constrain cross-study comparison. In agriculture, algal and cyanobacterial nanoparticles show consistent short-term benefits for seed germination, seedling vigour, and antimicrobial protection in laboratory and greenhouse trials, but field-scale and life-cycle evidence remains sparse. In environmental remediation, biosynthesised nanoparticles demonstrate measurable efficacy in heavy metal adsorption, dye photodegradation, and pollutant sequestration, though reported removal efficiencies vary widely with experimental design and are rarely benchmarked against conventional adsorbents under comparable conditions. Across both application domains, confidence in translational readiness is limited by inconsistent reporting of nanoparticle stability, scale-up feasibility, and ecotoxicological risk. The review identifies methodological standardisation, mechanistic elucidation through omics approaches, and rigorous field- and life-cycle-level evaluation as priority research needs. It concludes that while algal and cyanobacterial biosynthesis constitutes a scientifically credible and environmentally motivated alternative to conventional nanoparticle production, the current evidence base supports cautious optimism rather than confirmed practical readiness for large-scale agricultural or remediation deployment.