Concentration and Application Timing Modulate Growth and Biochemical Composition of Limnospira platensis Exposed to Multipod ZnO Nanostructures

Zinc oxide nanostructures can induce highly variable biological responses depending on their physicochemical properties, concentration, and exposure conditions. This study investigated the effects of multipod ZnO nanostructures (ZnO-MPs) on Limnospira platensis, considering both concentration and application timing. ZnO-MPs (0.01–30 mg/L) were applied at 0, 72, or 120 h after inoculation, and the final biomass and the contents of proteins, carbohydrates, lipids, phycobiliproteins, phycocyanin, chlorophyll, and carotenoids were determined. Final biomass showed relatively limited and non-monotonic variation, whereas biochemical composition was substantially altered. Low concentrations (0.1–1 mg/L), particularly when applied at inoculation, stimulated total phycobiliprotein and phycocyanin accumulation. Higher concentrations strongly reduced protein and phycobiliprotein contents, while promoting carbohydrate and/or lipid accumulation under specific application conditions. Chlorophyll and carotenoids exhibited comparatively moderate and non-monotonic responses. Overall, the results demonstrate that the effects of ZnO-MPs on Limnospira platensis are strongly modulated by concentration and application timing, with biochemical responses being more pronounced than changes in final biomass. These findings emphasize the importance of considering both nanomaterial concentration and application timing when evaluating nanomaterial–cyanobacterium interactions.