THE MODE OF UPTAKE OF DISSOLVED INORGANIC CARBON IN THE EXTREMELY ACID-RESISTANT GREEN-ALGA DUNALIELLA-ACIDOPHILA - THEORETICAL CONSIDERATIONS AND EXPERIMENTAL-OBSERVATIONS

V-max and K-m values for CO2 of photosynthesis of the extremely acid r esistant green alga Dunaliella acidophila (growth at pH 1.0) and of th e salt tolerant Dunaliella pawn (growing at pH 7.6) acclimated to air or air +3% CO2 have been experimentally determined. In order to get in formation concerning the mode of uptake of dissolved inorganic carbon (DIC) into the cells these data are mathematically analysed in two dif ferent ways. In the first approach simple Michaelis-Menten kinetics ar e applied assuming that CO2 uptake is due to a pure lipid solubility d ependent diffusion across the plasma membrane following the chemical g radient of CO2. In the second approach Michaelis-Menten kinetics of ph otosynthesis using the experimentally determined V-max and the apparen t K-m value for external CO2 are compared with Michaelis-Menten kineti cs using the experimentally determined V-max values (in vivo data) on one hand and the K-m values of Rubisco for CO2 (in vitro data) on the other hand. The analysis of both mathematical approaches suggest that CO2 uptake in D. acidophila adapted to air is not compatible with the view of a pure lipid solubility dependent CO2 uptake: CO2 uptake must be a catalysed reaction. Only in D. acidophila cells adapted to air +3 % CO2, a noncatalysed diffusion of CO2 would be sufficient, but the ex istence of a catalysed diffusion of CO2 cannot be excluded. For D. par va adapted to air data imply an active uptake of DIC into the cells. C oncerning D. parva cells grown with air +3% CO2 an active uptake of DI C is required, at least at lower CO2 concentrations in the medium. At high external CO2 concentrations a passive diffusion is sufficient to supply photosynthesis with CO2.