The purpose of research was to elucidate the peculiarities of the process of spontaneous biofunctionalization of photoactive metal nanoparticles by live cells of cyanobacterium Arthrospira platensis (spirulina). Here we have investigated the effect of enshrouding polymer - coated silver nanoparticles (NPs) with polyethylene glycol (PEG) with 5 nm diameter. One of the tests confirming the biological functionalization of nanoparticles in spirulina biomass involved the recording of spirulina biomass absorption spectra in the presence of nanoparticles. The most specific changes were identified in the recording of the absorption spectrum of ethanolic extract from spirulina biomass, obtained in the presence of AgNPs supplemented to the culture medium in concentration of 10.0 μM on the 5th day of cultivation. The nanoparticles added at the end of spirulina cultivation cycle did not induce large changes in biomass content. In the extract obtained from the control biomass, two wavelength absorption maxima were visible, one peak at 666 nm, specific to chlorophyll a and the second maximum registered at 260 nm, specific to protein components (Figure 1A).figuraFig. 1. Absorption spectrum of 50% ethanol extract from spirulina biomass, control sample (A) and in the presence of AgNPs supplemented to culture medium in concentration of 10,0 μM on the 5th day of cultivation (B). The absorption spectrum recorded for ethanolic extract obtained from biomass grown with AgNPs identified a third peak at 473 nm, which did not appear on spectrum diagram of the control sample. The spectrum recorded for silver nanoparticles displayed an irregular shape with a peak at 411 nm, which was characteristic of nanoparticle conglomerates. Spectral shift may be the evidence of biological functionalization of silver polyethylene glycol - coated (Ag - PEG) nanoparticles in spirulina biomass.