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Skeletonema marinoi is one of the most widespread marine planktonic diatoms in temperate coastal regions and sometimes can form massive blooms. Yet, the molecular mechanisms of triacylglycerol (TAG) synthesis in nutrient-deficient conditions for this species are still unknown. This study aimed to investigate how the TAG biosynthetic pathway of S. marinoi reacts to the culture age and nitrogen (N) or phosphorus (P) deficiency at molecular levels. Meanwhile, we also described the physiological and biochemical changes of S. marinoi in response to N or P starvation over time. To obtain reliable qRT-PCR data, six putative reference genes were identified for assessing expression stability using geNorm and BestKeeper software, and Actin exhibited the most stable expression across 45 tested S. marinoi samples. We found that the expression of TAG biosynthesis-related genes and ACCase enzyme activity varied in response to the different nutrient conditions and culture age. Taken together, we speculated that the capacity of TAG biosynthesis in S. marinoi is induced by N or P stress, and increases with culture age. Furthermore, TAG biosynthesis appears to respond more strongly to P deficiency than to N deficiency. Our study provides important insights into how diatoms regulate the TAG biosynthetic pathway when stressed by nutrient limitation. Besides, the data obtained from this study also provide useful clues for further exploring genes that can be used for metabolic engineering to enhance lipid production.

Diatoms are unicellular algae with a set of extraordinary genes, metabolic pathways, and physiological functions acquired by secondary endosymbiosis, especially for their efficient photosynthetic carbon fixation mechanisms, which can be a reason for their successful environmental adaptation and great contribution to primary production. Based on the available genomic information, the expression patterns of carbon fixation genes were analyzed using transcriptomic sequencing and reverse transcription-quantitative polymerase chain reaction (RT-qPCR) in Skeletonema marinoi. Meanwhile, suitable reference genes applying to specific experimental treatments were selected. In our results, carbon fixation genes were standardized by actin and TATA box-binding protein-coding genes in growth phase samples and stress conditions, respectively. It was found that a series of carbon fixation genes, such as the pyruvate orthophosphate dikinase (PPDK)-coding gene, had significantly up-regulated expression in nitrogen-starvation, phosphate-starvation, and low-temperature conditions, but consistently down-regulated in silicate-starvation treatment. These carbon fixation genes exhibited variable expression levels in different conditions and will be useful for investigating gene expression mechanisms in S. marinoi and improve our understanding of diatom carbon fixation pathways.