Abstract

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.