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 ye ast extract were optimized for eicosapentaenoic acid (EPA) production by th e diatom Nitzschia laevis in heterotrophic cultures. First, nitrate and ure a were found to be the preferred N sources for both cell growth and EPA con tent. The optimal concentrations of nitrate and urea for cell growth and EP A yield were 620 mg/liter and 600 mg/liter, respectively. Secondly, trypton e and yeast extract were respectively added to the medium and both of them were found to enhance EPA production compared with the control. To investig ate the co-effects of tryptone and yeast extract, in the subsequent experim ents, a central composite experimental design was used to optimize the two complex nitrogen sources. The results showed that the EPA production was in fluenced not only by tryptone or yeast extract, but also by their interacti ons. 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 r esulting DW, EPA content and EPA yield reached 6.48 g/liter, 2.74% (w/w), a nd 175 mg/liter, respectively, which agreed with the predicted values, and were much higher than those obtained in the previous studies. (C) 2001 Else vier Science Inc. All rights reserved.