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

The availability of CO2 is one of the restrictions on aquatic photosynthesis. Solute carrier (SLC) 4-2, a plasma membrane HCO3- transporter, was previously identified in the marine diatom, Phaeodactylum tricornutum. In this study, we discovered two paralogs, PtSLC4-1 and PtSLC4-4, and localized both at the plasma membrane. Their overexpression stimulated HCO3- uptake, which were inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic (DIDS), an anion channel blocker. Furthermore, PtSLC4-1, similarly to SLC4-2, required specifically Na + of about 100 mM for its maximum HCO3- transport activity. Unlike PtSLC4-1 and PtSLC4-2, the HCO3- transport of PtSLC4-4 depends equally on Na +, K +, or Li +, suggesting its broad selectivity for cations. Transcript analyses indicate that PtSLC4-1 is the most abundant HCO3- transporter under CO2 below atmospheric level, while PtSLC4-4 showed little transcript induction in atmospheric CO2 but temporal induction to the comparable levels to PtSLC4-1 at an initial acclimation stage from high CO2 (1%) to very low CO2 (<0.002%). The data strongly suggest the major HCO3- transport role of PtSLC4-1 with relatively minor role of PtSLC4-2, and that PtSLC4-4 operates under severe CO2 limitation when other SLC4s do not function to support an urgent HCO3- uptake unselectively to cations.