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 P _CO2 we show that growth of large diatoms in the California upwelling is not limited by CO₂ availability.

1. Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey 08544,
   Email: USAptortell@phoenix.princeton.edu
2. Department of Environmental Sciences, Rutgers University, New Brunswick, New Jersey 08903, USA
3. Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA