Pyrenoid-core CO2-evolving machinery is essential for diatom photosynthesis in elevated CO2.

Marine diatoms are responsible for up to 20% of annual global primary production by performing photosynthesis in seawater where CO2 availability is limited whilst HCO3- is abundant. Our previous studies have demonstrated that solute carrier 4 proteins at the plasma membrane of the diatom Phaeodactylum tricornutum facilitate the use of the abundant seawater HCO3-. There has been an unconcluded debate as to whether such HCO3- use capacity may itself supply enough dissolved inorganic carbon (DIC) to saturate the enzyme Rubisco. Here, we show that the θ-type carbonic anhydrase, Ptθ-CA1, a luminal factor of the pyrenoid-penetrating thylakoid membranes, plays an essential role in saturating photosynthesis of P. tricornutum. We isolated and analyzed genome-edited mutants of P. tricornutum defective in Ptθ-CA1. The mutants showed impaired growth in seawater aerated with a broad range of CO2 levels, from atmospheric to 1%. Independently of growth CO2 conditions, the photosynthetic affinity measured as K0.5 for DIC in mutants reached around 2 mM, which is about 10 times higher than K0.5[DIC] of high-CO2-grown wild-type cells that have repressed CCM levels. The results clearly indicate that diatom photosynthesis is not saturated with either seawater level DIC or even under a highly elevated CO2 environment unless the CO2-evolving machinery is at the core of the pyrenoid.