Inhibition of Photosynthesis in Chlorella Pyrenoidosa by the Iodo-acetyl Radical

Photosynthesis in Chlorella pyrenoidosa is inhibited by iodo-acetic acid and iodo-acetamide, both of which attack the Blackman reaction. Since acetamide is without effect, the iodo-acetyl radical must be responsible. The study of the action of the acid is complicated by the fact that its ions penetrate slowly, if at all, so that negative results with this agent are without significance unless penetration can be established. The absorption spectrum of the cells is not affected by concentrations of iodo-acetamide which completely inhibit photosynthesis. This establishes that the chromophore groups of chlorophyll are not involved, and renders it unlikely that any other part of the molecule is. Inasmuch as cyanide likewise inhibits by way of the Blackman reaction, it would seem necessary to postulate that this complex can be attacked at two different loci, which may or may not be on the same molecule. The presence of the iodo-acetyl radical also gives rise to three other effects. (1) Concentrations (10(-5)M or less) too small to inhibit photosynthesis may increase the rate by interacting with the photochemical complex. (2) Concentrations (ca. 10(-4)M) which inhibit photosynthesis increase the rate of respiration. (3) Concentrations (10(-3)M or more) higher than those required to inhibit photosynthesis inhibit respiration.