Enhanced Transport of Inorganic Carbon Into Algal Cells and Its Implications for the Biological Fixation of Carbon¹,²

Kinetics of uptake of inorganic carbon by the freshwater green alga Chlamydomonas reinhardtii Dang. suggest that rates of fixation may be enhanced at low tensions of CO 2 by transport of bicarbonate from the cell surface to the chloroplast. Results are evaluated in the context of models that treat diffusion and reaction of dissolved inorganic carbon across a 3 dimensional finite boundary layer, and they are consistent with the claim that CO 2 alone is the substrate used during carbon fixation. An alternative hypothesis, which presumes that both CO 2 and bicarbonate are used as substrates, yields predictions which are inconsistent with the data. Instead, bicarbonate seems to act only as a vehicle for the transport of inorganic carbon into the cell, thereby adding its flux to that of CO 2 , and enhancing rates of synthesis that would otherwise be restricted by uptake of CO 2 alone.