Physiological, biochemical, and morphological responses to nitrogen starvation and biomass-specific photon supply rates of Nannochloropsis oceanica and Microchloropsis gaditana

Abstract

Physiological, biochemical, and morphological responses of Nannochloropsis oceanica and Microchloropsis gaditana were investigated using a two-phase cultivation strategy, starting with nitrogen (N) replete conditions followed by N-starvation at different biomass-specific photon supply rates (BSPSRs). Cultures were grown in photobioreactors under a 16:8 hours light-dark cycle with a maximum light intensity of 1200 μmolphoton m-2 s-1. Subsequently, two BSPSRs, 9 and 18 μmolphoton gdw-1 s-1, were implemented at the onset of the N-starvation phase by adjusting biomass concentrations. Under N-starvation at the lower BSPSR of 9 μmolphoton gdw-1 s-1, N. oceanica exhibited substantially higher biomass and fatty acid productivities (0.62 and 0.35 g L-1 day-1, respectively), compared to M.gaditana's biomass and fatty acid productivities (0.22 and 0.085 g L-1 day-1, respectively), indicating the presence of a robust stress response mechanism in N.oceanica. While both species experienced decreased photosynthetic efficiency under N-starvation, M.gaditana maintained higher quantum yield values with (0.46 and 0.32 at low and high BSPSRs, respectively) compared to N.oceanica (0.42 and 0.15). Similarly, M.gaditana preserved higher EPA content (35.36 and 30.28 mgEPA gdw-1) than N.oceanica (31.34 and 28.98 mgEPA gdw-1) at low and high BSPSRs, suggesting distinct physiological adaptation strategies. Chlorophyll content substantially declined in both species under high BSPSR and N-starvation, with N.oceanica experiencing a more pronounced degradation. The morphological analysis aligned with these data, unveiling significant alterations in chloroplast structure and oil-body formation. Providing visual evidence of cellular adaptations to stress conditions and the differences between the two species, with N.oceanica formed larger oil bodies while experiencing more extensive chloroplast degradation than M.gaditana under N-starvation at the higher BSPSR. These findings demonstrate species-specific trade-offs between growth, stress tolerance, and biochemical composition, offering valuable insights for optimizing microalgal strains for targeted application, food and biofuel production with N.oceanica or EPA-rich biomass with M.gaditana.