The relationship between calcite production and dissolved inorganic carbon (DIC) utilization was investigated using high- and low-calcifying strains of Emiliania huxleyi aerated with either air (0.03 % v/v CO2) or CO2-free air. With cultures grown on air, calcite production was eight-fold greater in the high-calcifying culture, but growth rates for high- and low-calcifying cultures were similar. Growth and calcite production were accompanied by a concomitant decrease in DIC and free CO2 in the high-calcifying culture, showing that HCO3− provides inorganic carbon for calcite synthesis. In low-calcifying cultures DIC and free CO2 were relatively constant, confirming that cells acquire inorganic carbon mainly by the diffusive entry of free CO2.
When cultures were aerated with CO2-free air the free CO2 concentration was below the K0.5[CO2] for high- and low-calcifying cells and the low-calcifying cells were unable to grow. Growth of high-calcifying cells was observed, and the calcite yield was little changed from cultures gassed with air (0.03 % v/v CO2). At these minimal CO2concentrations HCO3− may provide inorganic carbon for calcite synthesis and CO2 for photosynthesis, allowing growth to occur. Calcite synthesis by E. huxleyi decreases DIC and carbonate alkalinity in cultures not in equilibrium with the gas phase.