INADEQUACY OF RUBISCO INITIAL AND TOTAL ACTIVITIES TO ACCOUNT FOR OBSERVED RATES OF PHOTOSYNTHETIC CARBON-DIOXIDE ASSIMILATION BY SCENEDESMUS-ECORNIS

Both initial and total activity of ribulose-1,5, bisphosphate carboxyl ase/oxygenase (Rubisco) measured for the green alga Scenedesmus ecorni s are affected by the experimental procedure and they are not sufficie ntly high to account for the rates of C-14 fixation by photosynthesis. The very low beta-carboxylase activities detected (less than 3% of th e Rubisco total activity) cannot explain the difference in CO2 fixatio n. Attempts to obtain possible optimal conditions (pH, duration of act ivation with Mg2+ and HCO3, absence of proteases, linearity of C-14 fi xation with time) did not lead to increased activity yields. The subst rate ribulose-1,5-bisphosphate was found to decrease the initial activ ity at concentrations higher than 25 muM for algae harvested by centri fugation and having thus experienced several minutes of darkness. Deac tivation seems to be primarily responsible for this loss of activity. Furthermore, initial and total activities decrease when the delay befo re freezing increases, suggesting accumulation of an inhibitor from th e light-dark transition metabolism during the first minutes of harvest ing.