Microalgae have been identified as an important source of new bioactive compounds with potential application in biomedicine, food and cosmetic industry. Microalgae cells are often exposed to oxidative stress, therefore they accumulate a large amount of effective compounds with antioxidant activity to prevent cells from damage by free radicals, and represent a potential source of a large number of bioactive molecules with antioxidant activity. Microalgae, especially diatoms (so-called diatoms), secrete a large amount of extracellular polymers (EPS) and are the main producer of EPS in the euphotic zone of the ocean. In this project, the composition, structure and antioxidant activity of EPS secreted by the marine diatom Cylindrotheca closterium will be investigated. C. closterium was selected as an ecologically important species, widely distributed in high and low latitudes, and it is attributed an important role in the process of macroaggregate formation in the northern Adriatic (sea bloom). The isolated EPS of C. closterium will be chemically characterized by determining the content of polysaccharides and proteins, and the supramolecular structure of EPS will be determined using atomic force microscopy (AFM). The antioxidant activity of the isolated EPS in vitro will be determined by measuring the free radical scavenging capacity (hydroxyl and superoxide radicals) and by measuring the reducing power. The obtained results will contribute to a better understanding of the correlation between the composition, structure and antioxidant activity of EPS, and to the assessment of the potential application of EPS of C. closterium as an antioxidant of natural origin.