Photosynthesis in Chlorella during Intermittent Illumination of Long Periodicity

Abstract

Subsequent to a dark period of several minutes the rate of photosynthesis during the first seconds of illumination is generally less than that in the subsequent steady state. This phenomenon has been described by the general term induction. Recently, Burk and Warburg (1), using a manometric method, determined pressure changes during light and dark for a suspension of Chlorella which was illuminated for periods of three minutes or less alternated with equal dark periods. The pressure changes were interpreted as demonstrating that under these conditions the rate of oxygen production and carbon dioxide consumption during the light periods exceeded that of photosynthesis during the steady state; similarly the oxygen consumption and carbon dioxide production in the dark were greater than the steady state rate of respiration. These experiments are subject to criticism because it appears improbable that during periods as short as one minute the manometer can respond sufficiently rapidly so that the observed pressure changes represent the metabolic rates without the complication of physical lag. Subsequently Damaschke, T?dt, Burk and Warburg (2) determined the consumption and production of oxygen using an electrochemical method. Few results have yet been published; these show that after a period of illumination from 5 to 30 seconds the oxygen consumption immediately following the cessation of illumination was greater than in the preceding or subsequent steady state. The effect was considerably smaller than that observed in the earlier manometric studies. The production of oxygen during the period of illumination corresponded to a quantum efficiency of 0.69 mole/mole quantum whereas measurements in the steady state gave a value of approximately 0.3. On the basis of these observations Warburg, Geleick and Briese (3) formulated a mechanism for photosynthesis which postulated that during illumination the photochemical process was accompanied by an accelerated combustion process. During the steady state the net effect of the combined processes is observed; the two can only be observed independently as transitory phenomena during periods of alternate light and dark. Brackett, Olson and Crickard (4), who also used a polarographic method to determine the time course of oxygen consumption and production by Chlorella, were unable to observe transient phenomena of the type described by Burk, Warburg and collaborators. The present paper reports observations on the time course of oxygen production and consumption by Chlorella in which oxygen was determined spectrophotometrically after combination with hemoglobin. The method has been used previously (5) but in the present investigation the change in optical density of the suspension consequent upon the formation of oxyhemoglobin was detected by a phototube and automatically recorded. Both the sensitivity and precision of the method were thereby improved.