Photosensitivity in the Ciliated Protozoon Loxodes: Pigment Granules, Absorption and Action Spectra, Blue Light Perception, and Ecological Significance1

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Abstract
The ciliate Loxodes has a faint yellow‐brown coloration due to large numbers of electron‐dense granules located in the pellicle. Absorption and action spectra both indicate the presence of a blue‐light receptor, which may be a flavin. Absorbance is minimal at wavelengths greater than 500 nm; there is a major peak at 360 nm and a pronounced shoulder at 435 nm. An action spectrum based on light‐induced escape from oxic water shows a peak at 435 nm and a peak or shoulder at 360 nm. The pigment will generate superoxide when illuminated in the presence of oxygen. Loxodes living in an oxygen gradient in a spectrophotometer cell swims into and remains in anoxic water at light levels ≤10 Wm‐2. Loxodes can be exposed to light levels of 2–20 Wm‐2 in stratified lakes so its photobehavior can explain its periodic absence from oxic water. Photosensitivity in Loxodes may function as part of a predator‐avoidance strategy.