The impact of silver nanoparticles (AgNPs) on the biosynthetic activity of cyanobacterium Arthropsira platensis (spirulina) was studied in dependence of culture age. The presence of 5 nm polyethylene glycol-coated silver nanoparticles in cultivation medium induced changes in biochemical composition of spirulina biomass. Carbohydrate content in spirulina biomass exhibited the most significant changes (Figure 1).figureFig. 1. Carbohydrate content in spirulina biomass upon introduction of PEG-AgNPs into culture medium in concentration range of 1.0 - 20.0 μM on the first day of cultivation (A) and on the third day of cultivation (B). The concentration of 1.25 μM AgNPs, applied on the first day of cultivation cycle, enhanced carbohydrate content by 20% (Figure 1A). In concentration range from 2.5 to 20 μM, the carbohydrate content in spirulina biomass increased from 14% to 17%, which was with 29 - 59% more above the value of the control samples. Moreover, higher concentrations of nanoparticles in the medium promoted carbohydrate synthesis. Applying AgNPs on the 3rd day of cultivation, in concentration range from 1.0 to 5.0 μM, no change in carbohydrate content was noticed (Figure 1B). The beginning of the exponential phase was marked by an increase in biosynthetic activity involving significant use of carbon. In the case of AgNPs concentrations from 10 to 20 μM, the carbohydrate content increased by 34 - 44% compared to control samples. Carbohydrate values were within the limits of control samples when using nanoparticle concentrations of 1.0 - 5.0 μM. In the presence of low concentrations of nanoparticles, no significant changes in carbohydrate content was observed. The culture age was a determinant factor for carbohydrate synthesis in the presence of AgNPs in cultivation medium, the most sensitive being the latent phase. Spirulina at the beginning of the latent phase reacted intensely to silver nanoparticles through the synthesis of carbohydrates. At the beginning of the exponential growth phase, low concentrations of 1.25 - 5.0 μM did not alter the carbohydrate content in biomass. The significant increase in the carbohydrate content in spirulina biomass indicated a reorientation of biosynthetic processes towards the formation of carbon reserves, in order to stabilize the structural components of the cell membrane.