Comparative Effects of CuNPs, CuONPs and CuSO4 on Biomass Quality, Culture Liquid Composition and Biostimulant Activity of Nostoc linckia

The chemical form of copper is a key determinant of metal bioavailability and may profoundly influence cyanobacterial metabolism and the biological activity of culture-derived products. In this study, Nostoc linckia was cultivated in the presence of metallic copper nanoparticles (CuNPs), copper oxide nanoparticles (CuONPs), or CuSO4·5H2O (15 mg Cu L−1). Biomass productivity, biochemical composition, and antioxidant capacity, together with the physicochemical and biochemical characteristics of the culture liquid, were determined. The culture liquid was subsequently evaluated through the seed priming of four maize (Zea mays L.) hybrids. The different copper forms induced distinct metabolic responses in Nostoc linckia. CuONPs promoted biomass accumulation while maintaining protein and photosynthetic pigment contents and inducing an adaptive oxidative response, whereas CuNPs, and particularly CuSO4, impaired biomass production and primary metabolism. These metabolic changes were reflected in the culture liquid properties and were associated with genotype-dependent differences in maize germination dynamics, seedling vigor, and biochemical composition. Integrated correlation, hierarchical clustering, and principal component analyses revealed coordinated relationships linking cyanobacterial physiology, culture liquid profiles, and plant responses. Collectively, these findings demonstrate that the chemical form of copper drives metabolic remodeling in Nostoc linckia, thereby shaping the biochemical composition and biostimulant properties of its culture liquid.