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Inorganic carbon may be assimilated through the Calvin-Benson cycle via the enzyme ribulose-1.5-bisphosphate carboxylase (Rubisco) andlor by 0-carboxylation [via the enzymes phosphoenolpyruvate carboxylase (PEPC), phosphoenolpyruvate carboxykinase (PEPCK) or pyruvate carboxylase]. Here, carboxylase activity measurements for marine phytoplankton are described. Two indices measuring carboxylase activity in marine phytoplankton were used. The first measures Rubisco activity per unit chlorophyll [R/Chl; nmol CO2 (pg chl a+b+c)-' h-'] while the second is the ratio of P-carboxylase activity to Rubisco activity, expressed as % (PC/R), which reflects the proportion of inorganic carbon fixed by these 2 groups of carboxylases. These ratios were studied in (1) different algal species in culture, (2) dunng the different growth phases of a culture, and (3) after a light-dark transition to measure the time response of carboxylase activities. These indices were different from one species to another at the same stage of growth. In autotrophic cells, P-carboxylation remained low (PC/R<40). The PCIR ratio increased significantly when R/Chl began to decrease at the end of the growth phase of a culture of Skeletonerna costaturn. The heterotrophic dinoflagellate Crypthecodinjurn cohnii, grown on an organic medium, incorporated inorganic carbon in the dark through PEPCK activity. The wlde range in PC/R ratio observed among the species confirm that in phytoplankton there may exist a continuum between autotrophy and heterotrophy. From a carbon budget point of view the 2 mechanisms are not equivalent. Rubisco fixation uses light as an energy source and results in gross production; 0-carboxylation also fixes inorganic carbon but as energy source uses metabolites synthesized by other pathways in the cell or from the external medium.

Variations in carboxylase activity in marine phytoplankton cultures, ß-carbolxylation in carbon flux studies