Dr. SYNERGISTIC EFFECTS OF NITROGEN DEPRIVATION AND HIGH IRRADIANCE TO ENHANCE BIOMASS AND LIPID PRODUCTION IN NANNOCHLOROPSIS

Abstract

Nannochloropsis, due to its high lipid content, small size and growth rate, can be exploited for the production of biofuel and other value-added products. This study analyzed synergistic responses of high irradiance and nitrogen deprivation on Nannochloropsis cells to enhance biomass and lipid production. The growth of Nannochloropsis was first optimized under different irradiance 50, 100, 150, 200 and 250 μmol photons m^-2 s^-1. The maximum specific growth rate (0.25 day^-1) and dry weight (0.67 g L^-1) were obtained (p<0.05) at photosynthetically active radiation (PAR) of 200 µmol photons m^-2 s^-1, further increase in light intensity decreased the growth rate and dry weight up to 0.2 day^-1 and 0.41g L^-1, respectively. The stress of high light for a short period increased the lipid content by 51.1% and decreased the protein content by 35.2% over the control values. In the second phase of the study, the combined stress of high light intensity of 200 µmol photons m^-2 s^-1 PAR and nitrate depletion (50%, 75%, 100% N^-) were studied and observed significant (p<0.05) increase in lipid content from 21.3±1.4% to 43.2±1.56% DCW and decrease in protein from 30.7±1.3% to 16.3±1.2% DCW, respectively. The highest lipid productivity (140 mg L^-1 d^-1) had been observed in a culture grown at 75% N^- starved cells (p<0.05). However, the further increase of nitrate deprivation to 100% decreased the lipid productivity to 107 mg L^-1 d^-1. Maximum accumulations of TG (19.6% DCW) were observed on 6 days (early stationary phase) in a 100% nitrogen deprived medium, although significant accumulation was observed on 4 days and 6 days of nitrate reduction.