Effect of Oxygen on Photosynthetic ^<14>CO_2 Fixation in Chroomonas sp. (Cryptophyta) II. : Effects of Inhibitors, Uncouplers and an Artificial Electron Mediator on the Inhibition of ^<14>CO_2 Fixation by Anaerobiosis

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In "air-grown" Chroomonas sp. cells, low concentrations of DCMU (less than 0.1 μM) could prevent the inhibition of ^<14>CO_2 fixation by anaerobiosis under light-saturating conditions (more than 40 W・m^<-2>), with phenazine methosulfate showing a similar effect. Antimycin A, carbonyl cyanide m-chlorophenylhydrazone (CCCP), and N,N'-dicyclohexylcarbodiimide strongly inhibited anaerobic photosynthesis at concen-trations which did not significantly inhibit the rate under 2% O_2 at high light intensity (200 W・m<-2>), although 0.2 μM CCCP stimulated the rate under 2% O_2 to some extent. On the other hand, KCN inhibited the rate much more strongly under 2% O_2 than N_2, although it inhibited the rate very strongly at concentrations above 5 /μM both under N_2 and 2% O_2. These results suggest that the inhibition of photosynthetic ^<14>CO_2 fixation by anaerobiosis in this alga result from ATP deficiency caused by over-reduction of electron carriers of the cyclic electron flow and that oxygen can prevent the over-reduction. Cyclic electron flow seems to be necessary to provide additional ATP for CO_2 reduction under anaerobic conditions, although it seems to be less necessary under aerobic conditions.
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