Induction Phenomena of Photosynthetic Algae at Low Partial Pressures of Oxygen

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The effect of a dark period at very low partial pressures of oxygen (10 −4 –10 −1 mm. Hg) upon the subsequent photosynthetic production of oxygen by Chlorella pyrenoidota with saturating intensities of illumination has been investigated. With the lowest partial pressures of oxygen used the time course of oxygen production separates into two phases; an initial burst followed by a subsequent rise to the final steady state. The separation of the two phases in time is greater the lower the concentration of carbon dioxide and the longer the preceding dark period. The initial burst of oxygen as distinct from the steady state production is not inhibited by 10 −3 M. iodoacetamide. In contrast with the ‘Hill’ reaction, as measured after addition of quinone, the initial burst of oxygen was inhibited by p -chloromercuribenzoate 10 −4 M.). Concentrations of sodium fluoride and of 2:4-DNP which had no effect on the steady state of photosynthesis shortened the half-time for attainment of the steady state so removing or obscuring the initial burst. The effect with fluoride was observed only if added at the beginning of the preceding dark period. It is suggested that the initial oxygen burst results from reductive amination and carboxylation of keto acids present at the beginning of illumination. Iodoacetamide is assumed to inhibit steady state photosynthesis by preventing reduction of phosphoglyceric acid (PGA) to triose and hence its regeneration. It is assumed not to inhibit reductive amination or carboxylation. With long dark periods the initial burst is separated in time from the final acceleration to a steady state; this phase appearing as a decreased rate of production of oxygen may be also due to inhibition by fermentation products or to their conversion to amino acid without change in oxidation-reduction. The separation in time is minimized if fluoride or 2:4-dinitro-phenol is added at the beginning of the dark period; both probably prevent accumulation of pyruvic acid during fermentation limiting the initial reactions to the reduction of PGA.