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

Marine diatoms are responsible for up to 20% of the annual global primary production by performing photosynthesis in seawater where CO2 availability is limited while 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 CO2-concentrating mechanism 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.

Reactions of Rebamipide with Hypobromous Acid.

Rebamipide is a therapeutic agent for gastric ulcers and chronic gastritis. Hypobromous acid (HOBr) is generated not only by eosinophils but also by neutrophils in the presence of bromide ions in the plasma. At inflammation sites, rebamipide may encounter and react with HOBr to generated various products. When rebamipide was incubated with reagent HOBr in potassium phosphate buffer at pH 4.7 and 37°C for 4 h, several products were generated. A major product was identified as 3-bromorebamipide, a novel compound. Rebamipide does not react with hypochlorous acid (HOCl). However, when rebamipide was incubated with HOCl in the presence of NaBr, 3-bromorebamipide was generated in a dose-dependent manner, probably because of formation of HOBr. These results suggest that 3-bromorebamipide may generate from rebamipide at inflammation sites in humans.