Abstract
BACKGROUND: Diatoms are well known for high photosynthetic efficiency and rapid growth rate, which are not only important oceanic primary producer, but also ideal feedstock for microalgae industrialization. Their high success is mainly due to the rapid response of photosynthesis to inorganic carbon fluctuations. Thus, an in-depth understanding of the photosynthetic carbon fixation mechanism of diatoms will be of great help to microalgae-based applications. This work directed toward the analysis of whether C4 photosynthetic pathway functions in the model marine diatom Phaeodactylum tricornutum, which possesses biophysical CO(2)-concentrating mechanism (CCM) as well as metabolic enzymes potentially involved in C4 photosynthetic pathway. RESULTS: For P. tricornutum, differential proteome, enzyme activities and transcript abundance of carbon metabolism-related genes especially biophysical and biochemical CCM-related genes in response to different concentrations of CO(2) were tracked in this study. The upregulated protein abundance of a carbonic anhydrases and a bicarbonate transporter suggested biophysical CCM activated under low CO(2) (LC). The upregulation of a number of key C4-related enzymes in enzymatic activity, transcript and protein abundance under LC indicated the induction of a mitochondria-mediated CCM in P. tricornutum. Moreover, protein abundance of a number of glycolysis, tricarboxylic acid cycle, photorespiration and ornithine-urea cycle related proteins upregulated under LC, while numbers of proteins involved in the Calvin cycle and pentose phosphate pathway were downregulated. Under high CO(2) (HC), protein abundance of most central carbon metabolism and photosynthesis-related proteins were upregulated. CONCLUSIONS: The proteomic and biochemical responses to different concentrations of CO(2) suggested multiple carbon metabolism strategies exist in P. tricornutum. Namely, LC might induce a mitochondrial-mediated C4-like CCM and the improvement of glycolysis, tricarboxylic acid cycle, photorespiration and ornithine-urea cycle activity contribute to the energy supply and carbon and nitrogen recapture in P. tricornutum to cope with the CO(2) limitation, while P. tricornutum responds to the HC environment by improving photosynthesis and central carbon metabolism activity. These findings can not only provide evidences for revealing the global picture of biophysical and biochemical CCM in P. tricornutum, but also provide target genes for further microalgal strain modification to improve carbon fixation and biomass yield in algal-based industry.