Light-Enhanced Dark CO2 Fixation

Abstract

In 1939, McAlister discovered that wheat leaves can “pick up” CO2 in darkness immediately following a high rate of photosynthesis. Benson and Calvin (1947, 1948) found that dark 14CO2 fixation in green algae (Chlorella and Scenedesmus) was greatly enhanced by prior illumination in the absence of CO2. The prior illumination in the absence of CO2 and the following enhanced dark CO2 fixation are termed “preillumination” and “light-enhanced dark CO2 fixation (LED)”, respectively. Analysis of 14CO2 fixation products revealed that the radioactivity was incorporated into phosphoglyceric acid (PGA), alanine and also sugars during LED. Degradation of 14C-glucose obtained after LED in Chlorella showed that 75%–79% of the total radioactivity was in the 3 and 4 positions. Benson and Calvin therefore assumed that a certain amount of reducing power accumulated in green algae during preillumination and could later be used for CO2 reduction. On the other hand, Gaffron et al. (1951) found that 96% of the total 14C fixed during LED (60 s) was in PGA and pyruvic acid. After 10 s of photosynthetic 14CO2 fixation, only 62% of the total 14C was incorporated into these compounds, although the total radioactivity incorporated was practically equal to that observed after 60 s of LED. Therefore they concluded that after preillumination, the dark reaction consists of only one step, namely, the fixation of the added CO2 into the carboxyl groups of PGA and pyruvic acid. There the reaction stops and reduction beyond the stage of PGA occurs in light only.