Carboxylating enzymes and pathway of photosynthetic carbon assmilation in different marine algae—Evidence for the C4-pathway?

Experiments on short-term photosynthesis in H¹⁴CO₃^- (2–5 s) using various species of different algal classes resulted in predominant ¹⁴C-labelling (>90% of total ¹⁴C-incorporation) of phosphorylated compounds. The percentage of malate and aspartate usually accounts for distinctly less than 10% of the total ¹⁴C-labelling. These findings are consistent with data from enzymatic analyses, since 97–100% of the carboxylation capacity is due to ribulose-1.5-biphosphate carboxylase (EC 4.1.1.39) in Rhodophyceae and Chlorophyceae. Phaeophyceae are generally characterized by considerable activity of phosphoenolpyruvate carboxykinase (EC 4.1.1.32): at least 10% of carboxylation is confined to this enzyme. Similar ratios are obtained when rates of photosynthesis and of light-independent CO₂-fixation are compared. Activity of phosphoenolpyruvate carboxylase (EC 4.1.1.31) could not be detected in the species investigated. The results are discussed with emphasis on the pathway of photosynthetic carbon assimilation in marine algae.Abbreviations PEP-CK phosphoenolpyruvate carboxykinase (EC 4.1.1.32) - PEP-C phosphoenolpyruvate carboxylase (EC 4.1.1.31) - RubP-C ribulose-1.5-biphosphate carboxylase (EC 4.1.1.39)Dedicated to Professor H. Fischer, Bonn, on his 65th birthday