Multiple plasma membrane SLC4s contribute to external HCO3– acquisition during CO2 starvation in the marine diatom Phaeodactylum tricornutum.

This article focuses on the characterization of three solute carrier 4 (SLC4) bicarbonate (HCO3–) transporters—PtSLC4-1, PtSLC4-2, and PtSLC4-4—in the marine diatom *Phaeodactylum tricornutum* and their roles in aquatic photosynthesis under varying CO2 conditions. PtSLC4-1 and PtSLC4-4 were localized to the plasma membrane and shown to facilitate HCO3– uptake, with PtSLC4-1 exhibiting a strong dependence on sodium ions (Na+) similar to PtSLC4-2, while PtSLC4-4 displayed broader cation selectivity including potassium (K+) and lithium (Li+). Transcript analyses revealed that PtSLC4-1 is the predominant transporter under low CO2, PtSLC4-2 plays a minor but stable role, and PtSLC4-4 is transiently induced under very low CO2 conditions, suggesting a coordinated function of these paralogs in the CO2-concentrating mechanism (CCM) of *P. tricornutum* to optimize dissolved inorganic carbon uptake in response to environmental CO2 fluctuations.