Microalgae‐Mediated Synthesis and Characterization of Metal Oxide Nanoparticles for Potential Enhancement of Tomato Plant Growth and Their Effects on Methylene Blue Dye Degradation

ABSTRACT Agricultural productivity and environmental quality are increasingly threatened by the limitations and hazards associated with conventional chemical inputs. To address this, the present study explores the biosynthesis of metal oxide nanoparticles using microalgal extracts of Tetradesmus nygardii and evaluates their potential as biofertilizers, antifungal agents, and photocatalytic materials. Synthesized nanoparticles were structurally and optically characterized by UV–Visible spectroscopy, Fourier transform infrared spectroscopy (FTIR), and X‐ray diffraction (XRD), which confirmed phase‐pure crystalline formation at nanoscale dimensions. AgONPs exhibited ultra‐small crystallites (1.23–2.70 nm), while CuONPs and NiONPs ranged from 11.2–13.9 nm and 11.8–18.9 nm, respectively. When applied to tomato ( Solanum lycopersicum ) plants, AgONPs produced a 166% increase in chlorophyll content (37.8 μmol m −2 ) and a shoot fresh weight of 4.5 g, while reducing disease prevalence from 38.46% to 3.03%. A combined nanoparticle treatment further reduced disease incidence to 6.25% and extended shoot length to 55 cm, indicating synergistic growth‐promoting effects. In antifungal assays against Rhizopus spp., complete inhibition was recorded at 200 μg/mL across all tested nanoparticles, with AgONPs and ZnONPs showing superior dose‐dependent activity. For photocatalytic performance, AgONPs achieved a methylene blue degradation efficiency of 98.27% under sunlight at a rate of 30.84%/h. These results highlight microalgae‐derived AgONPs as a multifunctional and scalable solution for enhancing crop health, controlling phytopathogens and degrading textile dye pollutants, offering a sustainable alternative to synthetic agrochemicals and chemical‐based remediation approaches.