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 carboxylase/oxygenase (Rubisco) measured for the green alga Scenedesmus ecornis are affected by the experimental procedure and they are not sufficiently high to account for the rates of C-14 fixation by photosynthesis. The very low beta-carboxylase activities detected (less than 3% of the Rubisco total activity) cannot explain the difference in CO2 fixation. Attempts to obtain possible optimal conditions (pH, duration of activation with Mg2+ and HCO3, absence of proteases, linearity of C-14 fixation with time) did not lead to increased activity yields. The substrate ribulose-1,5-bisphosphate was found to decrease the initial activity at concentrations higher than 25 muM for algae harvested by centrifugation and having thus experienced several minutes of darkness. Deactivation seems to be primarily responsible for this loss of activity. Furthermore, initial and total activities decrease when the delay before freezing increases, suggesting accumulation of an inhibitor from the light-dark transition metabolism during the first minutes of harvesting.