Low-CO2 inducible bestrophins in diatom thylakoid membranes sustain high photosynthetic efficacy at distant locations from the pyrenoid

Anion transporters are important to sustain a variety of physiological states in cells. Bestrophins are a family of Cl− and/or HCO3− transporters conserved in bacteria, animals, algae, and plants. Recently, bestrophin paralogs were found in the green alga Chlamydomonas reinhardtii as up- regulated components in low CO2 conditions that play an essential role in the CO2- concentrating mechanism (CCM). Bestrophin orthologs are also conserved in diatoms, a group of secondary endosymbiotic algae harboring red-type plastids, but their physiological functions are not known yet. Here, we characterized the subcellular localization and expression profile of bestrophins in the marine diatoms Phaeodactylum tricornutum (PtBST1−4) and Thalassiosira pseudonana (TpBST1 and 2). PtBST1 and PtBST2 were localized at the stromal thylakoid membrane outside of the pyrenoid, and PtBST3 was localized in the pyrenoid. Contrarily, TpBST1 and TpBST2 were both localized in the pyrenoid. These bestrophin proteins were accumulated in cells grown in atmospheric CO2 but not in 1% CO2-grown cells. To assess the physiological functions, we generated knock-out mutants for PtBST1 by genome editing. The lack of PtBST1 decreased affinity of photosynthesis for dissolved inorganic carbon closer to that of the cells grown in 1% CO2. Additionally, non-photochemical quenching was 1.5–2.0 times higher in the mutants than that of the wild type cells. These data suggests that HCO3− transport at the stroma thylakoid membranes by PtBST1 is a critical part of the CO2 evolving system of the pyrenoid in the fully induced CCM, and simultaneously that PtBST1 modulates photoprotection in response to CO2 availability in P. tricornutum . Significant statement Marine diatoms are responsible for nearly half of oceanic primary production, owing to the high-affinity photosynthesis for dissolved inorganic carbon which is supported by CO2- concentrating mechanism (CCM). This study uncovered that a bestrophin family protein at the stoma thylakoid membrane operates to import HCO3− to the thylakoid lumen and mobilizes it towards the CO2 evolving system at the pyrenoid-penetrating thylakoid in the diatom Phaeodactylum tricornutum . This HCO3− collecting system not only enhances the CCM but also down regulates the photoprotection capacity of photosystem II, presumably by affecting the thylakoid lumen acidification. This study experimentally demonstrates the molecular mechanism how diatoms optimize the use of CO2 and light energy, giving an insight into the reason of ecological successfulness of marine diatoms. ### Competing Interest Statement The authors have declared no competing interest.