Effect of Oxygen on Photosynthetic 14CO2 Fixation in Chroomonas sp. (Cryptophyta) II. Effects of Inhibitors, Uncouplers and an Artificial Electron Mediator on the Inhibition of 14CO2 Fixation by Anaerobiosis

In "air-grown" Chroomonas sp. cells, low concentrations of DCMU(less than 0.1 µM) could prevent the inhibition of ¹⁴CO₂fixation by anaerobiosis under light-saturating conditions (morethan 40 W.m^–2), with phenazine methosulfate showing asimilar effect. Antimycin A, carbonyl cyanide m-chlorophenylhydrazone(CCCP), and N,N'-dicyclohexylcarbodiimide strongly inhibitedanaerobic photosynthesis at concentrations which did not significantlyinhibit the rate under 2% O₂ at high light intensity (200 W.m^–2),although 0.2 µM CCCP stimulated the rate under 2% O₂ tosome extent. On the other hand, KCN inhibited the rate muchmore strongly under 2% O₂ than N₂, although it inhibited therate very strongly at concentrations above 5 µM both underN₂ and 2% O₂. These results suggest that the inhibition of photosynthetic¹⁴CO₂ fixation by anaerobiosis in this alga result from ATPdeficiency caused by over-reduction of electron carriers ofthe cyclic electron flow and that oxygen can prevent the over-reduction.Cyclic electron flow seems to be necessary to provide additionalATP for CO₂ reduction under anaerobic conditions, although itseems to be less necessary under aerobic conditions. (Received July 21, 1983; Accepted January 23, 1984)