The diversity of inorganic carbon acquisition mechanisms in eukaryotic microalgae

Eukaryotic microalgae have developed CO2concentrating mechanisms to maximise the concentration of CO2 at the active site of Rubisco in response to the low CO2 concentrations in the external aquatic medium. In these organisms, the modes of inorganic carbon (Ci) uptake are diverse, ranging from diffusive CO2 uptake to the active transport of HCO3 ^-and CO2 and many have an external carbonic anhydrase to facilitate HCO3^- use. There is unequivocal evidence for the mechanisms of Ci uptake in only about 25 species of microalgae of the chlorophyte, haptophyte, rhodophyte, diatom, and eustigmatophyte groups. Most of these species take up both CO2 and HCO3^-, but the rates of uptake of each of these substrates varies with the algal species. A few species take up only one of the two forms of Ci, an adaptation that is not necessarily correlated with their ecological distribution. Evidence is presented for the active uptake of HCO3^- and CO2 in two marine haptophytes,Isochrysis galbana Parke and Dicrateria inornata Parke, and for active transport of CO2 but lack of HCO3^- uptake in two marine dinoflagellates, Amphidinium carteraeHulburt and Heterocapsa oceanica Stein.