Abstract
At high light intensity suspensions of Chlorella pyrenoidosa, supplied with non-limiting concentrations of CO 2, produce oxygen at a greater rate when nitrate is simultaneously present. In that case the photosynthetic quotient, CO 2/O 2, is considerably lower than in the absence of nitrate, even though the rate of CO 2 assimilation is not reduced. From these results it is concluded that the photochemical nitrate reduction, discovered by Warburg and Negelein in 1920, cannot be explained on the basis of a dark reaction which yields CO 2 by the oxidation of cell material with the concomitant reduction of nitrate. It can best be interpreted as a process in which nitrate acts directly as an alternate and additional hydrogen acceptor in photosynthesis.
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