Optimization of nitrogen sources for heterotrophic production of eicosapentaenoic acid by the diatom Nitzschia laevis

The nitrogen (N) sources including nitrate, ammonium, urea, tryptone and yeast extract were optimized for eicosapentaenoic acid (EPA) production by the diatom Nitzschia laevis in heterotrophic cultures. First, nitrate and urea were found to be the preferred N sources for both cell growth and EPA content. The optimal concentrations of nitrate and urea for cell growth and EPA yield were 620 mg/liter and 600 mg/liter, respectively. Secondly, tryptone and yeast extract were respectively added to the medium and both of them were found to enhance EPA production compared with the control. To investigate the co-effects of tryptone and yeast extract, in the subsequent experiments, a central composite experimental design was used to optimize the two complex nitrogen sources. The results showed that the EPA production was influenced not only by tryptone or yeast extract, but also by their interactions. The cell dry weight (DW), EPA content and EPA yield could be described by the second-order polynomial equations with high confidence levels (>99%). Based on the surface plots of the responses, the optimal concentrations of tryptone and yeast extract were determined to be 1.6 g/liter and 0.8 g/liter, respectively. To verify the predicted models, the alga was grown in the medium with tryptone and yeast extract at optimal concentrations. The resulting DW, EPA content and EPA yield reached 6.48 g/liter, 2.74% (w/w), and 175 mg/liter, respectively, which agreed with the predicted values, and were much higher than those obtained in the previous studies.