Low-CO2-inducible bestrophins outside the pyrenoid sustain high photosynthetic efficacy in diatoms.

Anion transporters sustain a variety of physiological states in cells. Bestrophins belong to a Cl- and/or HCO3- transporter family conserved in bacteria, animals, algae, and plants. Recently, putative bestrophins were found in the green alga Chlamydomonas reinhardtii, where they are up-regulated under low CO2 conditions and play an essential role in the CO2-concentrating mechanism (CCM). The putative bestrophin orthologs are also conserved in diatoms, secondary endosymbiotic algae harboring red-type plastids, but their physiological functions are unknown. Here, we characterized the subcellular localization and expression profile of bestrophins (BSTs) in the marine diatoms Phaeodactylum tricornutum (PtBST1-4) and Thalassiosira pseudonana (TpBST1 and 2). PtBST1, PtBST2, and PtBST4 localized at the stroma thylakoid membrane outside of the pyrenoid, and PtBST3 localized in the pyrenoid. Contrarily, TpBST1 and TpBST2 both localized in the pyrenoid. These bestrophin proteins accumulated in cells grown in atmospheric CO2 (LC) but not in 1% CO2 (HC)-grown cells. To assess the physiological functions, we generated knock-out mutants for the PtBST1gene by genome editing. The lack of PtBST1 decreased photosynthetic affinity for dissolved inorganic carbon to the level comparable to the HC-grown wild type. Furthermore, non-photochemical quenching in LC-grown cells was 1.5-2.0 times higher in the mutants than in the wild type. These data suggest that HCO3- transport at the stroma thylakoid membranes by PtBST1 is a critical part of the CO2-evolving machinery of the pyrenoid in the fully induced CCM and that PtBST1 may modulate photoprotection under CO2-limited environments in P. tricornutum.