How nitrogen and phosphorus stoichiometry drive the physiology and photosynthesis of Scrippsiella acuminata (Dinophyceae).

Development and growth of microalgae are mainly sustained by two essential nutrients: nitrogen (N) and phosphorus (P). Although single-nutrient limitation has been extensively studied, the balance between N and P availability remains less explored. Scrippsiella acuminata is one of the most abundant dinoflagellates in coastal ecosystems due to its physiological plasticity, making it a key species in the understanding of acclimation to unbalanced nutrient supply. To test the acclimation of S. acuminata, semicontinuous cultures were exposed to six N:P ratios (1.6, 8, 16, 32, 90, 180). Parameters such as photosynthetic response, biovolume, carbon excretion, lipids, reactive oxygen species production, cell cycle, and alkaline phosphatase activity were analyzed. Growth, regular cell cycle progression, balanced carbon allocation carbon resource, and high photosynthesis efficiency occurred at balanced N:P ratios (16, 32). At low ratios (1.6, 8), growth was reduced but cells maintained active photochemistry, whereas high ratios (90, 180) led to an extension of the G1 phase leading to biovolume increase and a limitation of the protective capacity of non-photochemical quenching leading to reactive oxygen species accumulation. Carbon allocation followed a stoichiometric gradient where more limiting N:P ratios favored soluble extracellular polymeric substances and a pool of cellular carbohydrates production as an overflow mechanism to protect cells, whereas moderate limitation led to lipid accumulation as a metabolic reserve. These results not only highlight the plasticity of S. acuminata to acclimate to nutrient stress but also suggest that this species may be more vulnerable in P-limited environments and has a competitive advantage where N is the primary limiting factor.