Diatoms are eukaryotic unicellular microorganisms which produce a variety of bioactive compounds and value-added products as well as lipids which can be used as an alternative feedstock for the production of biodiesel. Most of the marine diatoms grow only photothrophically by harvesting sunlight and assimilating CO2. However, the productivity and biomass concentration in phototrophically grown culture is very low. In this research, growth medium f/2 for the cultivation of diatom Nitzschia sp. was optimized by changing concentrations of three limiting macroelements (silica, nitrogen and phosphorus). The highest biomass yield of 0,7611 g Lˉ¹ and productivity of 0,0544 g Lˉ¹ dayˉ¹ was achieved with modified f/2 growth medium, which had a tenfold higher concentration of silica and fourfold higher concentrations of nitrogen and phosphorus, compare to original f/2 medium. Effect of modified f/2 growth medium on the composition of cell biomass was also determined. Cell biomass contained increased content of proteins and polyunsaturated fatty acids. Phototrophic fed-batch cultivation was conducted using optimized f/2 growth media. The highest biomass productivity of 0,7150 g Lˉ¹ dayˉ¹ was accomplished between 7. and 8. day of cultivation. The most abundant macromolecule in cell biomass were proteins, followed by carbohydrate and lipid.