Invisible pollutants, visible impact: ZnO nanoparticles (nZnO) induce nanoecotoxicity, oxidative stress and functional decline in the freshwater alga Nostoc spongiaeforme Agardh

Abstract

Zinc oxide nanoparticles (ZnO NPs), widely used in pharmaceuticals, cosmetics, and various industrial products, are increasingly detected in freshwater ecosystems, posing potential ecological risks. This study investigated the nanoecotoxicological effects of ZnO NPs (10-1000 ppm) on the freshwater nitrogen-fixing cyanobacterium Nostoc spongiaeforme Agardh. Exposure to ZnO NPs led to a concentration-dependent decline in growth rate by up to 65 percent at 1000 ppm, and chlorophyll a content decreased by 58.3 percent compared to control. Photosynthetic oxygen evolution was reduced by 54.7 percent, and respiration rates dropped by 46.2 percent. Thin layer chromatography (TLC) and high-performance liquid chromatography (HPLC) analyses revealed a progressive loss of pigments, with neoxanthin and beta-carotene declining by 51.1 percent and 49.8 percent, respectively. Sodium dodecyl-sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed altered protein expression, including the emergence of low molecular weight stress-associated bands at higher doses. These results confirm oxidative stress and membrane destabilization as key mechanisms of ZnO NP toxicity. The study highlights the vulnerability of filamentous cyanobacteria to nanoparticle exposure and emphasizes the need for robust environmental risk assessments targeting freshwater primary producers.