Transcriptional responses to nitrogen and phosphorus costress conditions reveal nitrogen-dominant control in a marine diatom, <i>Skeletonema tropicum</i>

Abstract To elucidate the responses of phytoplankton to combined nitrogen (N) and phosphorus (P) stress, we conducted a full factorial experiment to investigate the transcriptional regulation of nutrient-responsive genes in the diatom Skeletonema tropicum. This study aimed to characterize how nutrient availability and physiological status modulate gene expression, with a particular emphasis on the interplay between N and P in shaping transcriptional responses. A matrix of N/P combinations was employed to simulate diverse nutrient regimes consistent with the theoretical framework of nutrient colimitation. The expression of the nitrate transporter gene (Nrt2) was strongly repressed in the presence of ammonium but highly induced following ammonium depletion, demonstrating an inverse relationship with ammonium concentrations in the medium and highlighting direct regulation by nitrogen availability. In contrast, the expression of phosphorus-responsive genes, including phosphate transporters (Pho and Npt2bl) and alkaline phosphatase (Ap1), was upregulated under P deficiency, yet their expression was also modulated by ammonium concentrations and the N:P ratio of the medium. Under combined N and P stress, S. tropicum prioritizes nitrogen acquisition, suggesting a regulatory hierarchy between N and P uptake. The results highlight the complex transcriptional strategies that enable diatoms to adapt to fluctuating nutrient regimes in marine environments.