Assimilation and metabolism of glucose by Dunaliella tertiolecta, 1

Dunaliella tertiolecta, a green euryhaline flagellate, is unable to use glucose as a substitute for photosynthetically fixed CO2 to maintain growth. Glucose, acetate, pyruvate, succinate, sucrose, glycerol, alanine and α-ketoglutarate do not stimulate endogenous respiration in this alga. By incubating whole cells with these compounds labelled with 14C, it was shown that only acetate, pyruvate and glycerol penetrated the cell at rates which might affect growth. These rates were still only of the order of 10 m/(moles/hr/mg protein. Only acetate and pyruvate were metabolized to CO2 at appreciable rates, 20 and 80% of the total assimilated, respectively. Cell free preparations of D. tertiolecta metabolized glucose rapidly, up to 2 μmoles/hr/mg protein, with over 75% of the 14C-label being recovered as triose phosphate. Both the hexose monophosphate shunt and the Embden-Meyerhof pathway were active. When specifically labelled glucose was supplied, CO2 from the C-6 carbon was released more rapidly than from the C-6 position, in both whole cells and in the cell free extract. It is concluded that the failure of D. tertiolecta to use glucose is due to membrane impermeability, not lack of hexokinase. A possible basis for this impermeability is discussed in the light of the metabolic sequence which seems to be active in this alga.