The Quantum Requirement of Photophosphorylation and its Stoichiometric Relation to Reduction in Anacystis in vivo

Abstract

Summary 1. Rates of ATP synthesis in the blue-green alga Anacystis nidulans were measured as a function of light intensity up to 20 kerg · cm -2 · s -1 during the transition from dark to light of 632 nm and of 719 nm. Rates of 14 C0 2 fixation were determined under the same light conditions. Up to light saturation of photophosphorylation also the absorption of light by the cells was measured at both wavelengths. 2. ATP synthesis reached light saturation at 3 kerg · cm -2 · s -1 (632 nm) or at 1 kerg · cm -2 · s -1 (719 nm). By contrast C0 2 fixation was not saturated even at intensities of 20 kerg · cm -2 · s -1 (632 nm). At this intensity of 719 nm light, C0 2 fixation was very low and did not exceed 1.8 µmoles · mg chl -1 · h -1 . 3. The quantum requirement of ATP synthesis was determined. A plot of the rates versus light intensity yielded a requirement of two quanta per ATP at zero light intensity for both wavelengths. 4. ATP synthesis was inhibited by about 50% by 5 µM DCMU [3-(3,4-dichlorophenyl)1,1-dimethylurea] or by 10 µM desaspidin suggesting an equal contribution of both cyclic and non-cyclic electron transport to the total photophosphorylation. Evidence is presented that the open-chain photophosphorylation occurs in part by non-cyclic and by pseudocyclic electron transport, the proportion of which is estimated from the results.