The leaves of the epiphytic fern Polypodium polypodioides, which lives on the branches of trees, are very similar to those of most higher plants except for the presence of scales on the dorsal side of the leaves. The structure of the cells of the chloroplasts and the mitochondria is the same as that of other higher plants. The only obvious difference found was that the contents of the central vacuole solidified when dehydrated. P. polypodioides was not damaged by loss of 97% of its normal water content and photosynthetic activity was found to be proportional to water content between 20 and 100% water content. When a dried leaf was immersed in liquid water, almost all of the original photosynthetic activity reappeared in the first 30 min of rehydration, provided incisions had been made into the leaf before drying.The rate of water uptake by intact (uncut) leaves was strongly inhibited by anaerobic conditions. This inhibition could be relieved by cutting the leaves, by supplying oxygen, or by removing the scales.Since in P. polypodioides the photosynthetic apparatus is not damaged by severe dehydration its quick revival does not depend on a special repair mechanism. Therefore, P. polypodioides should be a suitable object for a number of studies on the mechanism of photosynthesis.
C02 + 4H20 + P > (CH20) + 02 + 3H20 (1) A mass spectrometer inlet and an oxygen electrode in the same vessel allowed the continuous recording of the gases exchanged (H2, C02, 02) by hydrogenase-containing anaerobically adapted Scenedesmius obliquus strain D, (Gaffron) and Chlorella fusca Shihira et Krauss (= pyrenoidosa) 211-15. A light intensity which produces more photosynthetic oxygen than the cells can re-reduce to water leads to de-adaptation and the substitution of normal photosynthesis for photoreduction. The sequence of these metabolic events was recorded in a matter of a few minutes. Upon exposure of these adapted algae to light, an evolution of hydrogen lasting up to 60 seconds preceded any other light-dependent gas exchange. In the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, this initial hydrogen production was inhibited approximately 50%, pointing to a contribution of electrons by photosystem II. At very low hydrogen tensions (0
The effect of glucose and the uncoupler Cl-CCP upon hydrogen production was studied in adapted cells of Scenedesmus obliquus D3. Cl-CCP at 10(-5)M concentration completely inhibited the evolution of H2 in the dark and increased the apparent rate of H2 evolution in the light. At 10(-5)M Cl-CCP, photosynthesis and photoreduction by anaerobically adapted algae were only temporarily inhibited; O2 evolution reappeared after approximately 1 hr of illumination if CO2 was present. Increasing the Cl-CCP concentration to 5 x 10(-5)M led to a maximum rate of photohydrogen production and fully inhibited H2 evolution, photoreduction and dark H2 evolution. H2 evolution was accompanied by a release of varying amounts of CO2 in the light, as well as in the dark. Dark CO2 production was stimulated by Cl-CCP. H2 evolution in the light was stimulated by adding glucose to autotrophically grown cells or by growing the cells heterotrophically with glucose; starvation had an opposite effect. Adapted cells released (14)CO2 from the 3 and/or 4 position of specifically labeled glucose, indicating that degradation occurred via the Embden-Meyerhof pathway. The amount of H2 released by autotrophically grown cells was the same either with continuous illumination or with short periods of light, followed by darkness. Scenedesmus mutant No. 11, which is unable to evolve O2 was not inhibited in its capacity to evolve H2 in the light. These data indicate that the evolution of H2 in the light by adapted Scenedesmus depends upon the degradation of organic material and does not require the production of free O2 by photosystem II.
In order to come to a more firmly based conclusion on the mechanism of hydrogen photoproduction in green algae, we have compared two additional genera of green algae, i.e., Ankistrodesmus and Chlorella, with the previously tested Chlamydomonas and Scenedesmus. None of the algae tested required photosystem II for H2 photoproduction, since this reaction still occurred in the presence of 10(-5)M DCMU. Photophosphorylation was also not required since two potent inhibitors of this process, Cl-CCP and SAL, almost always stimulated H2 photoproduction. However, the effect of the inhibitors was found to vary with the species of alga and also with the age and growth conditions of the culture. The highest concentration of SAL tested (10(-2)M) always stimulated H2 photoproduction by photoheterotrophically grown cells, but often inhibited this reaction in autotrophically grown cells. When present, this inhibition by SAL was associated with gross pigment damage. The variation in the effect of Cl-CCP upon H2 photoproduction due to different growth conditions was particularly striking for Chlorella vulgaris.Cl-CCP gave very little if any stimulation of this reaction in autotrophically grown cells of this alga, but stimulated H2 photoproduction by photoheterotrophically grown cells approximately 450%. Chlamydomonas cells were found to be about ten times as sensitive as the other cells to both poisons. We conclude that all of the algae tested are able to photoproduce H2 via non-cyclic electron flow through photosystem I to hydrogenase.
The contribution of PS II to H2 photoproduction by several unicellular green algae was measured both when O2 evolution and photophosphorylation were unimpaired and also when these processes had been eliminated by Cl-CCP. As judged by the effects of DCMU, a PS II contribution was found under both sets of experimental conditions for several strains of Chlorella, Ankistrodesmus and Scenedesmus. However, H2 photoproduction by Chlamydomonas moewusii was insensitive to DCMU and thus was entirely due to PS I. With cells treated with Cl-CCP, the relative amount of PS II contribution varied from zero in autotrophically grown Chlamydomonas reinhardii, to ≈ 20% in photoheterotrophically grown and ≈ 50% in autotrophically grown cells of Ankistrodesmus braunii, Chlorella fusca, Chlorella vulgaris and Scenedesmus obliquus. The dehydrogenation of reduced H-donors by PS II of Scenedesmus treated with Cl-CCP showed the same biphasic kinetics previously described for H2 photoproduction by PS I of this alga.
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