Reduction of Carbon Dioxide with Molecular Hydrogen in Green Algæ

A VIGOROUSLY growing pure culture of the alga Scenedesmus sp. D.3 is suspended in a hundredth molar (1/100 M.) solution of potassium bicarbonate and kept for six hours in the dark in an atmosphere of hydrogen containing 2 per cent carbon dioxide. Shortly before the last traces of oxygen in the vessel have disappeared by way of respiration, the cells begin to absorb hydrogen. The amount of hydrogen absorbed in this reaction varies with the condition of the algæ. After this absorption has stopped, practically no further gas exchange is observed. If the light is now turned on, no evolution of oxygen can be detected but the algæ begin to absorb hydrogen instead. The process has a measurable induction period, but thereafter it proceeds at a steady rate. This phenomenon is strikingly similar to the reduction of carbon dioxide with molecular hydrogen brought about by purple bacteria. That this absorption of hydrogen by the green algæ is connected with the reduction of carbon dioxide can be shown by the same method which enables one to establish the ratio H2/CO2 in purple bacteria1. The algæ are suspended in a slightly acid phosphate buffer, treated with hydrogen in the dark, and then illuminated until all carbon dioxide stored or produced by fermentation has been used up in photosynthesis. Addition of a known amount of carbonate at this point is followed by a rapid uptake of hydrogen, the quantity of which exceeds the ratio 2H2 for 1CO2 found in purple bacteria. In Scenedesmus this ratio often reaches a value of about 3.