Cyanobacteria and microalgae are known as possible facilities for the manufacture of bionanoparticles. Copper nanoparticles (CuNPs) have found numerous applications in nanotechnology, and biological properties of Cu have determined the application of CuNPs as antibacterial, imaging, immunomodulatory agents. The biological effect of nanoparticles, especially CuNPs on microalgal and cyanobacterial metabolism is currently poorly elucidated. The effect of PEGylated Cu (5nm) nanoparticles (CuPEGNPs) on Spirulina platensis Nordst (Geitl) CNMN CB-02 strain was studied. CuPEGNPs were added into nutrient medium of spirulina in concentrations from 1.0 up to 10.0 µM/L. Higher concentrations than 3.75 µM/L were toxic to spirulina strain. On the background of an obvious decrease of produced biomass, CuPEGNPs (2.25-3.75 μM/L) caused the protein content in spirulina biomass to decrease by 2440%. Under the action of high concentrations of CuPEGNPs, carbohydrates recorded variations in their content in biomass from an increase of about 90% to a reduction of 13%. Modification of secondary photosynthetic pigments (phycobilins) towards reducing their content as a result of the effect exerted by high concentrations of CuPEGNPs was an indication of their toxicity. Thus, concentrations of 1.0 to 3.75 μM/L CuPEGNPs reduced phycobilin content in spirulina biomass by 63-87%, their toxicity being evident. The content of chlorophyll a and β-carotene also fluctuated in relation to the applied concentrations of tested nanoparticles. The chlorophyll content in spirulina biomass, grown by supplementing the medium with CuPEGNPs in concentrations of 2.5 and 3.75 μM/L, was reduced by 59-44%.In the case of β-carotene, the same concentrations of CuPEGNPs caused the reduction of β-carotene content by 50-70%, similar to chlorophyll content. The oscillations of chlorophyll and carotene contents in spirulina biomass confirm the involvement of nanoparticles in the biosynthetic activity of the strain. The tested concentrations of CuPEGNPs resulted in a reduction of lipid content with 13-30%. The content of lipid oxidation end products were increased by 84-147%, which demonstrated the toxic effect of high concentrations of these nanoparticles. Thus, CuPEGNPs in high concentrations (from 1.0 to 10.0 µM/L) altered the biosynthetic activity of cyanobacterial strain Spirulina platensis CNMN-CB-02, mainly in the direction of diminishing the levels of this activity. These concentrations determined the toxic effect of CuPEGNPs on the main qualitative parameters (proteins, phycobilins, chlorophyll a and β-carotene) of spirulina biomass, characterized by their quantitative decrease, in some cases drastically. Except for carbohydrates from spirulina biomass, for which the increase of their content on the background of protein decreasing represented a reorientation of biosynthetic processes towards the formation of carbon reserves in order to stabilize the membrane structural components. The toxic effect of high concentrations of CuPEGNPs was confirmed by the results of MDA assay, which showed the increased levels of lipid oxidation end products in spirulina biomass subjected to the action of these nanoparticles.