Organic and inorganic nano-Fe3O4: Alga Ulva flexuosa-based synthesis, antimicrobial effects and acute toxicity to briny water rotifer Brachionus rotundiformis

Highlights: • The Phyco-synthesis of Fe₃O₄ nanoparticles using seaweed, Ulva flexuosa (wulfen) J.Agardh. • The strong antimicrobial activity of U. flexuosa extract, organic and inorganic magnetite NPs. • Chemo- and Bio-Fe₃O₄-MgNPs showed low toxic effects against rotifer Brachionus rotundiformis. • Induced rotifer population density reduction using biogenic Fe₃O₄-MgNPs is lower than chemogenic nano-Fe₃O₄. Following the recent progress in magnetic nanotechnology, concern over the optimal benefits and potential risks of iron oxide nanoparticles (Fe NPs), has increased. Hence, to minimize the negative impacts of inorganic Fe NPs, we report the phyco-synthesis and characterization of superparamagnetic Fe₃O₄ NPs via reduction of ferric/ferrous chloride solution (2:1 M ratio; 88 °C) with green macroalga, Ulva flexuosa (wulfen) J.Agardh aqueous extract. The biogenic process is clean, eco-friendly, rapid, and facile to handle. These green fabricated magnetite NPs are characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), particle size analysers (PSA), zeta potential (ZP) measurement, and vibrating sample magnetometry (VSM) techniques. The results confirmed that the cubo-spherical, polydisperse of biosynthesized Fe₃O₄ NPs with an average diameter of 12.3 nm was formed. The antagonistic effects of algal extract, chemo-route and U. flexuosa-derived Fe₃O₄ MgNPs on selective human pathogenic microbes (i.e. n = 11) resulted in strong antibacterial and moderate antifungal activity. The comparative toxic and reproductive effects of the chemo- and bio-routes of Fe₃O₄-MgNPs against rotifer B. rotundiformis exhibited low acute toxicity with a lower inducing effect of biogenic nano-magnetite on reduction rotifer reproductive rate than its chemogenic counterpart. In view of the nanoecotoxicity, though the current study covered a wide range of exposure concentrations (10-500 mg/L) of organic and inorganic nano sizes of Fe₃O₄ in brackish water rotifer, a biotoxicity assay at higher dosage or a comprehensive risk assessment in different aqua-organisms is recommended.