[On the effect of potassium iodide on light-enhanced endogenous respiration of algae]

The endogenous respiration of an achlorophyllous mutant of Chlorella vulgaris is enhanced by small amounts of blue light. The action spectrum for this effect shows two peaks at λ 460 and 375 mμ, which points to a flavin or a carotenoid in cis configuration as the likely photoreceptor responsible. WEBER'S (1950) observation that in vitro potassium iodide (KI) quenches the fluorescence of riboflavin was employed to distinguish between these two pigments. - KI in concentrations from 0.033 to 0.50 M lowers the oxygen uptake in blue light increasingly (Fig. 1, Table 1), but not specifically: KI inhibits the respiration of exogenous glucose even more (Fig. 2). Furthermore neither the inhibition of endogenous nor that of exogenous respiration is iodide-specific; a decrease in both of them takes place with KNO3 of corresponding concentrations, too (Table 2). The somewhat smaller inhibition with KNO3 compared to that with KI fits the known observation that iodide has a greater inhibiting effect on metabolic reactions of plant cells than nitrate (HEWITT and NICHOLAS, 1963). - Finally it was observed that irradiation with blue light (λ>366 to <550 mμ) of the iodide solutions used liberates some iodine (Fig. 3). Since the promoting effect of iodine on the isomerisation of carotenoids is well documented (ZECHMEISTER, 1962), it might be impossible to determine whether a flavin or a cis-carotenoid participates in a blue light specific reaction by using an iodide solution, as has been done several times recently.