THE EFFECT OF POTASSIUM IODIDE ON PHOTOPHOBIC RESPONSES IN EUGLENA: EVIDENCE FOR TWO PHOTORECEPTOR PIGMENTS *

Abstract— Potassium iodide, a quencher of flavin fluorescence, inhibits the shock reaction which Euglena experiences upon a sudden decrease in light intensity (inverse photophobic response) completely at a concentration of 150 mM. The rate of swimming of the cells at the same concentration of KI is reduced to 30% of the control. The direct photophobic response, a shock reaction which appears identical but occurs upon an increase in light intensity, is unaffected by KI as is negative phototaxis of Euglena. It is concluded that a non‐flavin pigment system mediates photoreception for the direct photophobic response and negative phototaxis.