Disclosed is a method for obtaining a unialgal biomass of cells of small multicellular macroalgae, including: preparing a unialgal sample of cells of multicellular macroalgae from a sample of microalgae taken from the natural environment; culturing the unialgal sample of cells of multicellular macroalgae obtained in step in sea water to which at least one nitrogen source is added, in order to obtain an aqueous suspension of the unialgal biomass of cells of small multicellular macroalgae; a step of harvesting the unialgal biomass of cells of small multicellular macroalgae from the aqueous suspension obtained at the end of step; and a step of producing a powder of the unialgal biomass of cells of small multicellular macroalgae obtained in step. Also disclosed is glycolic extract of the aforementioned biomass, to a method for producing the extract, to the use thereof in cosmetics and pharmaceuticals, and to compositions containing same.