A Study of Oxidation-Reduction Potentials in Relation to the Functions of Light and Carbon Dioxide in Chlorella Photosynthesis

A study has been made of the oxidation-reduction potentials in suspensions of algae. The potential was found to be governed by three systems, viz. 1, the respiratory system, 2, a metabolic process occurring in the dark in the absence of oxygen, and 3, a system connected with the photosynthetic processes, exclusively determining the potential changes in the light below a certain oxygen tension. The activities of these systems may be distinguished on the basis of their different sensitivities to oxygen, pH and enzyme poisons. At the start of illumination the potential shows a pronounced induction phenomenon and a stationary potential is reached after about 15 min. illumination. Oxygen and hydrogen peroxide are not involved in the establishment of this stationary potential. The essential feature of the potential behaviour in the light and in the absence of carbon dioxide irrespective of pH is that a relatively reduced state is reached some minutes after the start of illumination. Afterwards the potential rises again slowly to its stationary value. During the initial phase of low potentials, methylene blue and possibly also oxygen may be reduced by the suspensions. Similar experiments with disintegrated Chlorella cells demonstrated that both oxidising and reducing reactions occur. The presence of carbon dioxide during illumination results in a more positive potential. This can be explained by assuming that the reducing compounds formed by the action of light are removed by reaction with carbon dioxide. The action is exerted by extremely small concentrations of carbon dioxide. Its maximum effect occurs about 1-2 min. after the start of illumination. No persistence of the ability to reduce carbon dioxide in the dark after preillumination in the absence of carbon dioxide was observed. Our observations lead to the hypothesis that in the dark a very low concentration of the photosynthetic carbon dioxide acceptor is present. The establishment of steady state photosynthesis involves the formation of a sufficient amount of this carbon dioxide acceptor under the simultaneous and combined action of light and carbon dioxide. The potentials depend upon the light intensity in an unusual way. There is no relation to the light saturation of photosynthesis. The author is indebted to Prof. Dr E. C. Wassink for the stimulation to start this investigation and for valuable discussions, to Mr M. F. Kerkhof Mogot for experimental assistance and to Mr H. van den Brink for the construction of the manometer vessels.