The present work reports on the antibacterial and antioxidant activity of gold and silver nanoparticles, synthesized by an extract of the brown macroalga Sargassum muticum (SM). These nanoparticles were synthesized and fully characterized by means of UV–Vis spectroscopy and electron microscopy. These techniques confirmed that spherical nanoparticles, with mean diameters of 10.4±1.2 nm for Au@ SM and 41.0±5.7 nm for Ag@ SM, were formed. To investigate the possible role of these biomolecules in this green synthetic process, Fourier-transformed infrared spectroscopy analysis was performed before and after the synthesis of nanoparticles. Furthermore, the carbohydrate composition was examined along with other variations observed after the synthesis of nanoparticles by size-exclusion chromatography. The study of the changes observed indicated that the polysaccharide fraction of the extract plays a …