ABSTRACT
Abstract Isochrysis galbana is a brown microalgae widely used as a feed for marine organism in aquaculture. The aim of present study is to investigate the growth, biochemical composition, fatty acid profile, photosynthetic parameters and antioxidant activity (radical scavenging activity) of Isochrysis galbana cells cultivated under different levels (sub-optimum; 50 ± 1.5,optimum; 125 ± 2.5 and supra-optimum; 325 ± 3.5 µmol photons m-2 s -1) of photosynthetic active radiation (PAR), and subsequently treated with different concentration of nitrate deprivation (8mM, 2mM and 0.5mM). The experiment was carried out under controlled conditions utilizing a factorial design 3x3 (light intensity and nitrate concentration). The result depicts that PAR positively influences the growth of Isochrysis galbana which is maximum under supra-optimum PAR. Nitrate deprivation (2mM & 0.5mM) induced decline in growth in terms of dry weight is observed as 60.1% & 61.9% in suboptimum and 26.5% and 34.9% in supra-optimum respectively over the values recorded in their respective controls. Supra-optimum PAR decreased primary photosynthetic pigment Chl a and Chl c by 15.7% and 8.5%, whereas carotenoid content increased by 45.9% in supra-optimum PAR which displays potential interest as antioxidant agent in addition to total phenolic content and radical scavenging activity. The results suggest that combined stress of high light and nitrogen deprivation shifts the metabolic physiology from protein synthesis to energy reserve (carbohydrate and lipid) and accumulation of saturated fatty acid on expense of unsaturated fatty acid except docosahexaenoic acid. These valuable compounds exhibit potential applications in mariculture, nutraceutical and biofuel industry.