Rate-limiting diffusion phytoplankton

It is not clear that the acquisition of carbon is the rate-limiting step for photosynthesis even in the larger phytoplankton cells. Phytoplankton are not limited to the passive diffusion of CO2 to the cell surface for carbon acquisition. HCO may be taken up directly by phytoplankton and used as a source of CO2 for photosynthesis (Korb et al, 1997 Nimer et al, 1997 Raven, 1997 Tortell et al, 1997). The considerably greater abundance of HCO than dissolved CO2 in seawater would imply that carbon-fixation rates of marine phytoplankton are not CO2 limited, and increasing concentrations of CO2 will not have an impact on rates of primary production. 

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