Effect of Nitrogen Concentration on Growth and Fatty Acid Composition in the Freshwater Diatom Stephanocyclus meneghinianus

Microalgae can alter their growth rate and metabolic pathways in response to nutrient concentration in the medium, particularly nitrogen, one of their most important components. Novel freshwater diatom strains of Stephanocyclus meneghinianus (M418 and M456) were discovered in Moscow, Russia. Analysis was based on morphological characters, 18S rDNA and rbcL phylogeny, and the impact of changes in nitrogen concentration (from 0 to 0.6 g N/L) in WC medium on growth and total fatty acid composition. The results demonstrated that increasing nitrogen content in the nutrient medium stimulates culture growth. Strain M418 exhibited sustained exponential growth across all conditions, while strain M456 showed a clear transition to stationary phase, highlighting fundamental differences in their growth strategies. Principal component analysis underscored that strain M418 possesses a distinct and more strongly nitrogen-responsive fatty acid profile compared to strain M456. The main fatty acids for both strains were palmitic, palmitoleic, myristic, and eicosapentaenoic acids. The findings indicate that high nitrogen concentration is optimal for monounsaturated fatty acid accumulation, whereas nitrogen starvation stimulates that of polyunsaturated fatty acids.