Transcriptomic Responses of the Marine Diatom <i>Phaeodactylum tricornutum</i> to High Carbon and Low Nitrogen Stress

Diatoms play a pivotal role in global biogeochemical cycling and marine primary productivity, making them ideal model organisms for understanding how phytoplankton respond to environmental fluctuations associated with global climate change. In natural marine systems, diatoms frequently encounter simultaneous variations in carbon and nitrogen availability, yet most previous studies have examined the effects of these factors in isolation. To elucidate the integrated transcriptional mechanisms underlying diatom acclimation to coupled carbon-nitrogen (C-N) imbalance, we employed RNA sequencing (RNA-Seq) to characterize the global transcriptional response of the model diatom <i>Phaeodactylum tricornutum</i> to high CO₂ (~2000 μatm) and low nitrogen (10% of nitrogen concentration in f/2 medium) under parallel culture conditions. The results revealed both shared and distinct transcriptional responses between the two treatments. Key genes involved in carbon metabolism, such as phosphoglycerate mutase (PGAM_7) and dihydrolipoamide succinyltransferase (PHATRDRAFT_40430), were significantly upregulated, indicating enhanced glycolytic and TCA cycle activity. In contrast, the Calvin-cycle enzyme fructose-1,6-bisphosphatase (FBPC4) was downregulated. Genes associated with nitrogen assimilation-including nitrate reductase (PHATRDRAFT_54983), nitrite reductases (PHATRDRAFT_13154, PHATRDRAFT_8155), and ferredoxin-nitrite reductase (PHATRDRAFT_27757)-were strongly induced under both conditions. Pathway enrichment analysis further indicated the activation of lactic acid fermentation and nitrogen salvage pathways, suggesting a metabolic shift toward energy conservation and nutrient recycling. Collectively, these findings provide an overview of the transcriptional adjustments that enable <i>P. tricornutum</i> to maintain C-N homeostasis under high CO₂ and low nitrogen stress, offering new insights into diatom metabolic plasticity under changing ocean conditions.