The cell composition of Nannochloropsis sp. changes under different irradiances in semicontinuous culture

Nannochloropsis sp. was grown semicontinuously with a rate of daily renewal of the culture media of 40% of the volume of the culture under different irradiances (40, 60, 80, 220 and 480 μmol quanta m−2 s−1). Under the conditions tested, light saturation was achieved at 220 μmol quanta m−2 s−1 with no significant increase in steady-state cell density or of dry weight productivity with higher irradiance, reaching values of 115 × 106 cells ml−1 and 375 mg l−1 day−1 respectively. C/N ratios clearly indicated the point of light saturation, decreasing with increasing irradiance for light-limited conditions and increasing for light-saturated conditions. Under light-limited conditions, an increase in the irradiance produced an increase in the protein percentage of the organic fraction to the detriment of lipids and carbohydrates, while small changes were recorded under light-saturated conditions. The degree of unsaturation of fatty acids was lower with increasing irradiance, with a three-fold decrease of the percentage of total n−3 fatty acids, from 29 to 8% of total fatty acids, caused mainly by a decrease of eicosapentaenoic acid (EPA) (20:5n−3). The microalga reached its maximal value of dry weight productivity (375 mg l−1 day−1), EPA productivity (3.2 mg l−1 day−1) and maximal protein content (36% of the organic content) at the point at which light saturation was achieved. Results demonstrate the efficiency of the use of the irradiance for the modification of the biochemical composition of Nannochloropsis sp.