The Dark Reductions of Photosynthesis

Abstract

The conditions influencing the fixation of CO2 in the dark and the compds. formed were investigated. A sample of growing algae was split into 2 parts (about 1 ml. algae/15 ml. suspension). Part I was kept in the dark, exposed to 4% CO2 in N2 for about 8 hrs. Part II was exposed to a 150-watt lamp (0.7 g. cal./cm.2/min.) for 1 hr., during which time it was kept free of CO2 by flushing with N2. The 2 samples were evacuated, kept in the dark, and exposed to the same gas containing C14O2 in N2 for 5 mins. The algae were then killed by an acetic acid-HCl mixture and the remaining C14O2 pumped off. The total non-volatile radiocarbon content of the two samples was detd. and its chem. distribution detd. The results showed that the reduction of CO2 to sugar and the intermediates in that reduction did not involve the primary photochemical step itself. This was confirmed by the appearance of an appreciable fraction (up to 15%) of the radiocarbon in the methylene groups of the succinic acid isolated from sample I. It is thus confirmed that the photochemical process establishes a small reservoir of reducing power which can then carry out all the reduction steps needed to bring CO2 to sugar.