Pyruvate orthophosphate dikinase is required for the acclimation to high bicarbonate concentrations in Phaeodactylum tricornutum

The model marine diatom P. tricornutum possesses efficient CO 2 -concentrating mechanisms (CCMs) involving the bicarbonate transport and the rapid inter-conversion of HCO 3 − and CO 2, and it has a complete set of enzymes essential for biochemical CCM C 4 pathway. However, the role of potential C 4 pathway in the diatom is still controversial. Based on the analysis of previously published proteome and transcriptome data in P. tricornutum, the expression of pyruvate orthophosphate dikinase (PPDK), a key enzyme generating the primary acceptor for bicarbonate fixation in C 4 pathway, was upregulated in long-term high CO 2 -selected populations. In our study, P. tricornutum PPDK ( PtPPDK ) transcript abundance was higher with short-term treatment of high NaHCO 3 concentrations and overexpression of PtPPDK resulted in a cell density increase of 5–16 % from day 2 to 8. Knockdown (KD) or knockout of PtPPDK inhibited the growth and the maximal photosystem II electron transport rate, and the inhibited growth became more and more evident with the increasing NaHCO 3 concentration from 1 to 8 mM in PtPPDK KD lines. Furthermore, elevating NaHCO 3 from 1 to 16 mM dramatically decreased the cell density both in wild-type and PtPPDK mutants, suggesting that 1 mM NaHCO 3 is enough for the photosynthesis carbon assimilation. PtPPDK was localized in the plastid stroma by eGFP fusion protein analysis at 2 mM NaHCO 3, while PtPPDK-eGFP fluorescence was observed both in the stroma and periplasmic space at higher NaHCO 3. It is suggested that PPDK is necessary for consuming excessive inorganic carbon (C i ) both from the plastid and extracellular environment to maintain pH homeostasis, indicating a potential function of adapting to high C i concentrations.