Effect of nitrogen limitation on growth, biochemical composition, and cell ultrastructure of the microalga Picocystis salinarum

In recent years biodiesel production has attracted worldwide attention due to the awareness of fossil fuel depletion and microalgae biomass is considered a promising raw material for its formulation. The present study evaluated the effects of different levels of nitrogen limitation (37.5, 18.75, 9.375 mg L⁻¹ NaNO₃) on the growth, cell ultrastructure, and biochemical composition of a halophilic native strain of the green alga Picocystis salinarum as a potential raw material source for biodiesel. During a culture period of 20 days, growth measurements and photosynthetic pigments were estimated. Cell density, dry weight, and chlorophylls a, b content decreased with time as nitrogen limitation increase; however, carotenoid content increased. In addition, nitrogen limitation caused an progressive increase in the lipid and carbohydrate yield and a decrease in protein. The high N limitation (9.375 mg L⁻¹) had a significant effect on the accumulation of total lipid content (33.87% dry weight). Carbohydrate content (30.98% dry weight) and protein content (1.89% dry weight) decrease. The lipid content showed a differential FAME profile with high saturated fatty acid values (996.08 μg g⁻¹ dry weight) mainly palmitic acid, compare with the unsaturated ones that showed low values under high N limitation. The gradual increase of lipid content was also corroborated by transmission electron microscopy images with a single large lipid droplet cell formation. Therefore, evaluation of the algal culture conditions such as N limitation, as a strategy to maximize lipid content and improve the fatty acid profile in unexplored strain of P. salinarum, showed a potential biomass yield as a suitable candidate for biodiesel production. Graphical abstract: [Figure not available: see fulltext.].