Physiological, morphological, and molecular adaptations of Pseudo-nitzschia multistriata to phosphorus limitation.

Phosphorus is a key driver of marine primary production, and its increasing scarcity due to imbalanced anthropogenic nutrient inputs is a growing concern. The widespread diatom genus Pseudo-nitzschia includes several toxigenic species in which phosphorus limitation can induce domoic acid production; however, the responses of Pseudo-nitzschia multistriata, a common coastal species with extensive genomic resources, remain poorly characterised. Here, we provide the first integrated physiological, transcriptomic, and chemical characterisation of P. multistriata under phosphorus limitation. Using a multidisciplinary approach, we assessed growth, cell viability, chloroplast and nuclear biovolumes, elemental stoichiometry, transcriptional responses and domoic acid quantification. Under phosphorus deprivation, growth arrest occurs after three days while cell viability remains high, indicating a shift towards a survival strategy. This is accompanied by reduced chloroplast and nuclear size and marked alterations in C:N:P ratios. Transcriptomic analyses revealed strong induction of phosphorus acquisition genes and recycling pathways, including the SPX-PSR1 regulatory module, and suggested accumulation of TAGs. Intracellular domoic acid content increased under phosphorus limitation, consistent with upregulation of the domoic acid biosynthetic (dab) genes. Overall, P. multistriata displays coordinated metabolic, transcriptional, and morphological adjustments that support persistence under phosphorus limitation and may influence nutrient cycling and ecological interactions in coastal ecosystems.