Metabolic activities in Cyanophora paradoxa and its cyanelles : II. Photosynthesis and respiration.

Isolated cyanelles of Cyanophora paradoxa perform photosystem I and II dependent Hill reactions. The photosynthetic electron transport of the cyanelles does not show special features uncommon in cyanobacteria or chloroplasts of red algae. A preparation of cyanelles performs photosynthetic O2-evolution with approximately 1/3 of the rate of intact Cyanophora, in only, however, the first three minutes of the experiment. All attempts to stabilize the CO2-fixation activity of isolated cyanelles failed. Isolated cyanelles do not perform KCN-sensitive O2-uptake, indicating that respiratory cytochrome oxidase is lacking in cyanelles. O2-consumption by crude extracts from Cyanophora is inhibited by KCN when N-tetramethyl-p-phenylenediamine/ascorbate or NADH but not NADPH are supplied as the electron donors in contrast to the situation in cyanobacteria. These findings suggest that cyanelles do not respire. It is concluded that cyanelles are not so much related to cyanobacteria as formerly believed, but share many properties with chloroplasts of eukaryotic cells.