Enhanced docosahexaenoic acid production by Crypthecodinium cohnii under combined stress in two-stage cultivation with date syrup based medium

In this study, date syrup was used for the first time as an alternative carbon source for Crypthecodinium cohnii growth and docosahexaenoic acid (DHA) production by response surface methodology. Four medium variables including date syrup concentration, pH, yeast extract and sea salt concentrations were considered to optimize biomass production. Maximum biomass concentration, 6.6 g/L, was achieved in 17 g/L date syrup concentration, 2 g/L yeast extract, 20 g/L sea salt and pH 7. After this optimization, a two-step feed strategy in fed batch cultivation was done in the optimized date based medium to enhance DHA production. The first step was dedicated to biomass formation and the second step was designed for DHA production. In the second step, single and for the first time, the combined effects of growth temperature shift from 25 to 20 degrees C, and nitrogen starvation on DHA production were investigated. The results revealed that the most appropriate strategy was a fed batch cultivation combined with both nitrogen depletion and temperature growth shift after 6th days from inoculation, leading to an enhancement of DHA production to 5.65 g/L, which was approximately twice the amount obtained considering the fact that the control was 2.9 g/L. Results revealed that date syrup based medium with two-step fed batch cultivation under combined stress can be an effective strategy for the production of valuable microalgal metabolites in heterotrophic conditions.