VARIATIONS IN CARBOXYLASE ACTIVITY IN MARINE-PHYTOPLANKTON CULTURES - BETA-CARBOXYLATION IN CARBON FLUX STUDIES

Inorganic carbon may be assimilated through the Calvin-Benson cycle via the enzyme ribulose-1,5-bisphosphate carboxylase (Rubisco) and/or by beta-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 (mu-g chl a+b+c)-1 h-1] while the second is the ratio of beta-carboxylase activity to Rubisco activity, expressed as % (beta-C/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) during 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, beta-carboxylation remained low (beta-C/R < 40). The beta-C/R ratio increased significantly when R/Chl began to decrease at the end of the growth phase of a culture of Skeletonema costatum. The heterotrophic dinoflagellate Crypthecodinium cohnii, grown on an organic medium, incorporated inorganic carbon in the dark through PEPCK activity. The wide range in beta-C/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; beta-carboxylation also fixes inorganic carbon but as energy source uses metabolites synthesized by other pathways in the cell or from the external medium.