AbstractChlamydomonascells immobilized on micropipettes were filmed at high‐speed (500 f/s) while photostimulated either by one continuous light stimulus or by pulsed light in the frequency range of cell rotation (1 or 2 Hz). Two kinds of photophobic step‐up and two kinds of photophobic step‐down beat frequency changes without a reversal of flagellar beat were observed after frame‐by‐frame evaluation of 141 records. So far, a single step‐up response, the “shock” response, has been considered the only photophobic response. However, the present results show that the cells always responded to step‐up as well as to step‐down light stimuli. Either a decrease of beat frequency by step‐up was combined with an increase by step‐down (type I), or an increase by step‐up was combined with a decrease by step‐down (type II). Whether type I or type II was observed depended on the preirradiation of the cells. All four responses are blue‐light responses with a lag‐time of ∼40 ms. Nothing can be said about the photoreceptor site. Regardless, these responses cannot be the basic mechanism for phototaxis, as assumed till now, because the flagella remain synchronized during the flagella beat frequency changes. Even if they are uncoupled before and after stimulation, both flagella respond in the same sense, i.e., either both increase or both decrease their beat frequency. The behavioral relevance of these responses
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