Carbon concentrating mechanisms in eukaryotic marine phytoplankton

The accumulation of inorganic carbon from seawater by eukaryotic marine phytoplankton is limited by the diffusion of carbon dioxide (CO ₂) in water and the dehydration kinetics of bicarbonate to CO ₂ and by ribulose-1,5-bisphosphate carboxylase/oxygenase's (RubisCO) low affinity for its inorganic carbon substrate, CO ₂. Nearly all marine phytoplankton have adapted to these limitations and evolved inorganic carbon (or CO ₂) concentrating mechanisms (CCMs) to support photosynthetic carbon fixation at the concentrations of CO ₂ present in ocean surface waters (<10-30 μM). The biophysics and biochemistry of CCMs vary within and among the three dominant groups of eukaryotic marine phytoplankton and may involve the activity of external or intracellular carbonic anhydrase, HCO ₃ ^- transport, and perhaps a C ₄ carbon pump. In general, coccolithophores have low-efficiency CCMs, and diatoms and the haptophyte genus Phaeocystis have high-efficiency CCMs. Dinoflagellates appear to possess moderately efficient CCMs, which may be necessitated by the very low CO ₂ affinity of their form II RubisCO. The energetic and nutrient costs of CCMs may modulate how variable CO ₂ affects primary production, element composition, and species composition of phytoplankton in the ocean.