Active uptake of bicarbonate by diatoms

Marine diatoms play a predominant role in the biological carbon pump1 transferring carbon dioxide from surface to deep waters. Laboratory studies show that a number of species take up HCO3− and concentrate inorganic carbon intracellularly allowing rapid growth despite low CO2 availability2,3. In contrast, many oceanographers, particularly when interpreting carbon isotope data4,5, have made the assumption that diatoms do not utilize the abundant HCO3− in seawater but rather take up CO2 by diffusion6. This has led to the hypothesis that large diatoms may be CO2-limited in the oceans7. We now demonstrate active uptake of HCO3− in the field and a carbon-concentrating mechanism in coastal Atlantic diatoms. By manipulating pCO2 we show that growth of large diatoms in the California upwelling is not limited by CO2 availability.