The investigation reported in this paper rendered mainly two results which are of importance for the analysis of photosynthesis in chlorophyllous plants. Firstly: Unicellular algae (Scenedesmus and similar species), which under anaerobic conditions are capable of reducing carbon dioxide with molecular hydrogen in the light, will liberate hydrogen slowly in the dark if air is replaced by nitrogen in the surrbunding gas phase. This faculty for a hydrogen fermentation in Scenedesraus agrees with our knowledge of the dark metabolism of bacteria which utilize hydrogen. These reactions with hydrogen have often been found to be reversible. Secondly: Illumination of the fermenting algae enhances the liberation of hydrogen, particularly if the substrates of the' photochemical reduction process, carbon dioxide and hydrogen, are both absent. The release of hydrogen by the algae under the influence of light occurs at a rate about ten times that of the hydrogen formation in the dark. The rate, however, is already limited at low intensities by factors other than the light intensity. Experiments with specific inhibitors, like dinitrophenol, allow us to differentiate between the dark and the photochemical liberation of hydrogen.Upon return of the photosynthesizing cell to aerobic conditions both phenomena disappear as in the case of photoreduction.Besides hydrogen and carbon dioxide, the anaerobic metabolism of Scenedesmus and of similar green algae includes many other substances, notably organic acids formed by fermentation of internal or artificially provided carbohydrates. As far as the nature of these fermentation products is concerned, the present investigation was extended only to the analysis of added glucose and to the identification and determination of lactic acid. MethodsA description of the methods used in the present study has been reported elsewhere (Jack Rubin (27)), so that a few summarizing remarks will suffice.Pure cultures of Scenedesmus species D1, S. species D3, and S. obl~tuus were grown at 20°C. in inorganic saline through which a slow stream of 4 per cent CO2 in air was passed. The culture flasks, each containing 200 co. of medium, were inoculated from an agar slant and illuminated for 3 to 4 days with incandescent lamps yielding about 4,000 lux at the bottom of the flasks. The gas exchange of the algae was determined by Warburg's manometric method 219

No abstract

No abstract

No abstract

Summary During the last decade it has been recognized that the process of photosynthesis in green plants is unique, not because it involves a complicated photochemical decomposition of carbon dioxide for which there is no analogy in the organic world, but because it combines in a unique manner a number of processes each of which may be found in other living cells. If we turn from green plants to purple bacteria, for instance, we find that radiant energy is utilized for the reduction of carbon dioxide. These organisms, however, cannot use simply water as a hydrogen donor and hence are not able to liberate free oxygen. For the reduction of carbon dioxide they depend, in addition to light, upon energetically valuable hydrogen donors such as free hydrogen, hydrogen sulphide, or organic acids. The over‐all energy balances of these photoreductions are, therefore, much less favourable than that of photosynthesis in plants. If we turn to organisms not sensitive to light, we find that carbon dioxide can be reduced in complete darkness by several species of bacteria and even by some animal tissues. In this case the mechanism is a coupled oxido‐reduction in which an excess of hydrogen donors, either of inorganic or organic nature, has to be sacrificed to promote the ‘chemosynthesis’. It is clear that such dark reactions lead not to a gain but to an over‐all loss of chemical energy. Recent advances in the field of respiration and fermentation have taught us that despite the infinite variety of metabolic reactions in living cells the principles governing them are few. Accordingly, it is conceivable that the different reactions involving a reduction of carbon dioxide have many important traits in common, and that the study of any one of them may lead to a better understanding of the process of photosynthesis. The present article is a report on the metabolism of certain unicellular chlorophyllous algae (several species of Scenedesmus, Ankistrodesmus, Rhaphidium) that are able to reduce carbon dioxide either in normal photosynthesis with the evolution of oxygen, or in photo‐reduction with the absorption of an equivalent amount of hydrogen, or in chemosynthesis with the oxyhydrogen reaction as the driving force. The two latter reactions do not occur under normal aerobic conditions. They can be observed only after a few hours' incubation in hydrogen gas. The anaerobic treatment brings into play a hydrogenase which enables the algae to absorb or to release molecular hydrogen. This metabolic change we call adaptation. The adaptation consists in an enzymatic activation or rearrangement of some of the catalytic systems. It can be inhibited by traces of specific poisons like cyanide and hydroxylamine. Upon illumination in the adapted state, in presence of hydrogen and carbon dioxide, the algae reduce carbon dioxide with twice the volume of hydrogen, exactly akin to some purple bacteria. This we call photoreduction. The results of experiments with flashing light and with specific poisons show that in photosynthesis and photo‐reduction t...