Heterologous expression in E. coli reveals that bicarbonate transporter BicA2 mediates carbon uptake in marine Prochlorococcus spp.

The widespread oceanic cyanobacterial Prochlorococcus genus is a major contributor to global carbon fixation, yet mechanisms enabling this lineage to elevate intracellular inorganic carbon as a substrate for photosynthesis remain unresolved. Cyanobacterial CO2-concentrating mechanisms typically rely on membrane-bound bicarbonate (HCO3-) transporters SbtA1, SbtA2, BicA and BCT1, and CO2-to-HCO3- conversion uptake systems (CO2 pumps; NDH-I3 and NDH-I4), to elevate a cellular HCO3- pool for use by Rubisco-containing carboxysomes. Evidence suggests Prochlorococcus harbours carboxysomes with a low-CO2-specificity Rubisco, implying a functional CCM dependent on active HCO3- uptake. However, canonical CO2 pumps are absent, leaving distant HCO3- transporter homologues, BicA2 and SbtA2, as prime candidates for HCO3- transport in this group. Yet these have not been functionally characterized. Here we demonstrate that BicA2 from P. marinus CCMP1375 mediates Na+-dependent HCO3- uptake in Escherichia coli, whereas BicA2 from P. marinus CCMP1986 is inactive in its native form but acquired transport function through a single amino acid substitution during adaptive laboratory evolution. These findings confirm BicA2 as a low-affinity, Na+-dependent bicarbonate transporter with variable flux, revealing a previously uncharacterized CCM component in Prochlorococcus. This mechanistic insight reshapes our understanding of carbon acquisition strategies in the most abundant photosynthetic organism on Earth and highlights evolutionary plasticity in transporter function with implications for global biogeochemical cycles.