Regulation of carbon assimilation in Skeletonema costatum under diel variations

Here, we explored the molecular responses of photosynthetic carbon fixation in a globally distributed marine diatom, Skeletonema costatum, based on incubation experiments over a 24-h period under natural light. A model of carbon metabolism was proposed based on the transcriptome of S. costatum. We examined the diurnal expression patterns and relative importance of the C₄ pathway in S. costatum photosynthesis by analyzing the transcript abundance and key enzyme activities during the day/night cycle. This study suggested that the photosynthetic pathways of S. costatum were diverse and could involve combined CO₂-concentrating mechanisms. The analysis at the transcriptional level revealed that the transcripts of the C₄ pathway-related genes phosphoenolpyruvate carboxylase (PEPC) and pyruvate-phosphate dikinase (PPDK) were upregulated under the photoperiod and had obvious diurnal rhythms, indicating that light obviously affected photosynthetic carbon fixation. Moreover, we studied the pathway of photosynthetic carbon assimilation in S. costatum with carbon isotope δ¹³C techniques. The data provided evidence that carbon fixation in S. costatum involves the synergy of C₃ and C₄ photosynthesis. Under low pCO₂ levels in the low light photoperiod, the affinity of inorganic carbon increased, supported by the C₄ pathway, to acclimate to variations in the environment. This study promotes a better understanding of the photosynthetic carbon fixation mechanism in S. costatum during the diel cycle.