Toxicity evaluation of iron oxide nanoparticles to freshwater cyanobacteria Nostoc ellipsosporum

The extensive usage of iron oxide nanoparticles (FeO NPs) in commercial and biomedical applications raises the risk of releasing their remains into the aquatic ecosystems and this could possibly cause cytotoxic effects on aquatic organisms. Thus, the toxicity assessment of FeO NPs on cyanobacteria, which are primary producers at the bottom of food chain in aquatic ecosystems, is essential to gain information about the potential ecotoxicological threat on aquatic biota. The present study investigated the cytotoxic effects of FeO NPs on Nostoc ellipsosporum using different concentrations (0, 10, 25, 50 and 100 mg L-1) to track the time-dependent and dose-dependent effects and compared with its bulk equivalent. In addition, the impacts of FeO NPs and bulk counterpart on cyanobacterial cells were assessed under nitrogen as well as nitrogen-deficient conditions, because of ecological role of cyanobacteria in nitrogen fixation. The study revealed that the highest protein content was observed in the control in both types of BG-11 media compared to treatments of nano and bulk particles of Fe2O3. A 23% reduction in protein in nanoparticle treatment and a 14% reduction in bulk treatment at 100 mg L-1 was observed in BG-11 medium. At same concentration, in BG-11(0) media, this decline was even more intense with 54% reduction in nanoparticle and a 26% reduction in bulk. Catalytic activity of catalase and superoxide dismutase was found to be linearly correlated with the dose concentration for nano and bulk form in BG-11 as well as BG-11(0) media. The increased levels of lactate dehydrogenase act as biomarker of the cytotoxicity brought on by nanoparticles. Optical, scanning electron, and transmission electron microscopy all demonstrated the cell entrapment, nanoparticle deposition on the cell surface, cell wall collapse and membrane degradation. A cause for concern is that nanoform was found to be more hazardous than bulk form.