A study was conducted to determine the effect of silver nanoparticles (AgNPs) on the growth and productivity of cyanobacterium Arthrospira platensis (spirulina) in dependence on the age of culture. Polyethylene glycol (PEG)-coated AgNPs of around 5 nm were applied to spirulina nutrient medium in the concentration range of 1.0 - 20.0 μM on the first day of cultivation, on day 3 corresponding to the beginning of the exponential growth phase, and on day 5 corresponding to the stationary phase. The amount of spirulina biomass produced for each experimental variant has been established (Figure 1).figuraExperimental variants, which were grouped by culture age, highlighted the specific response of spirulina to the concentrations of supplemented nanoparticles. Concentrations in the range of 2.5 to 5.0 μM, applied on the first day of cultivation, enhanced biomass production of more than 20 % (Figure 1A). Addition of AgNPs in concentrations of 15 - 20 μM reduced biomass production by 20 - 24%, which revealed the inhibitory or toxic effect of used concentrations. Concentration range of 2.5 - 5.0 μM AgNPs, applied when culture was at the end of the latent phase/the beginning of the exponential growth phase (Figure 1B), also stimulated biomass production by 18 - 24%. Applying doses of 15 - 20 μM AgNPs, no decrease in the amount of biomass was observed. In this case, the age of the culture was a determining factor in the response of spirulina to the presence of AgNPs. Hence, the introduction of AgNPs on the 3rd day of cultivation in concentration range of 1.0 - 20.0 μM did not change biomass productivity, the general trend being towards reduction. Supplementing the cultivation medium with AgNPs on the 5th day of cultivation (the end of the exponential phase), spirulina growth was minimal and the biosynthetic activity of culture lost its intensity (Figure 1C). Concentrations of 1.0 - 20.0 μM did not change the biomass content, the general trend being towards reduction.Experimental variants, which were grouped by culture age, highlighted the specific response of spirulina to the concentrations of supplemented nanoparticles. Concentrations in the range of 2.5 to 5.0 μM, applied on the first day of cultivation, enhanced biomass production of more than 20 % (Figure 1A). Addition of AgNPs in concentrations of 15 - 20 μM reduced biomass production by 20 - 24%, which revealed the inhibitory or toxic effect of used concentrations. Concentration range of 2.5 - 5.0 μM AgNPs, applied when culture was at the end of the latent phase/the beginning of the exponential growth phase (Figure 1B), also stimulated biomass production by 18 - 24%. Applying doses of 15 - 20 μM AgNPs, no decrease in the amount of biomass was observed. In this case, the age of the culture was a determining factor in the response of spirulina to the presence of AgNPs. Hence, the introduction of AgNPs on the 3rd day of cultivation in concentration range of 1.0 - 20.0 μM did not change biomass productivity, the general trend being towards reduction. Supplementing the cultivation medium with AgNPs on the 5th day of cultivation (the end of the exponential phase), spirulina growth was minimal and the biosynthetic activity of culture lost its intensity (Figure 1C). Concentrations of 1.0 - 20.0 μM did not change the biomass content, the general trend being towards reduction.