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 (CO2) in water and the dehydration kinetics of bicarbonate to CO2 and by ribulose-1,5-bisphosphate carboxylase/oxygenase's (RubisCO) low affinity for its inorganic carbon substrate, CO2. Nearly all marine phytoplankton have adapted to these limitations and evolved inorganic carbon (or CO2) concentrating mechanisms (CCMs) to support photosynthetic carbon fixation at the concentrations of CO2 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, HCO3⁻ transport, and perhaps a C4 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 CO2 affinity of their form II RubisCO. The energetic and nutrient costs of CCMs may modulate how variable CO2 affects primary production, element composition, and species composition of phytoplankton in the ocean.