Cyclostat, light/dark-synchronized cultures of the marine microalga Tetraselmis suecica were carried out with six nutrient concentrations: 0.5, 1, 2, 4, 8, and 16 mmol N/l. A renewal of 50% culture volume, equivalent to a dilution rate of 1 day−1 was applied. Maximal steady-state cell density (2.5 × 106 cells/ml) and dry-weight productivity (0.2 g l−1 day−1) were achieved with 4 mmol N/l. Maximal protein (94.3 pg/cell) and lipid contents (25.4 pg/cell) were achieved with8 mmol N/l. The high variability reported in chlorophyll cellular content invalidates any estimation of biomass or cell number based on chlorophyll concentration or fluorescence, especially when nutrient concentration is being varied. The total fatty acid cellular content increased with increasing nutrient concentration although the amount of eicosapentaenoic acid decreased. Results indicated that a different desaturation pathway may be present in this species. The change of the nutrient concentration in the cyclostat system is a powerful tool to manipulate the biochemical composition of microalgae regarding protein, lipid, carbohydrates and fatty acid content.