GAIN SETTING IN CHLAMYDOMONAS-REINHARDTII - MECHANISM OF PHOTOTAXIS AND THE ROLE OF THE PHOTOPHOBIC RESPONSE

The unicellular green alga Chlamydomonas reinhardtii maintains sensiti vity of its phototaxis response (alignment of swimming direction along the axis of a light beam) over several orders of magnitude of light i ntensities. It is widely accepted that the rotation of the swimming ce ll provides temporal comparisons of light intensities via periodic con trast generated by its asymmetrically positioned refractile eyespot or ganelle. The cells also exhibit a second behavioral response to light called the photophobic (or stop) response, which is a brief cessation of swimming caused by a temporal change in light intensity. The cells are desensitized to photophobic stimuli by light exposure. Through com parative measurements of both responses, we explain the behavioral bas is of the large dynamic range of phototaxis in terms of precise desens itization of the photophobic response. The basis of the explanation is that the flagellar beat changes which cause phototactic orientation a re the residual of the photophobic response after desensitization (i.e ., ''mini-photophobic'' reactions which cause brief reorienting motion s without a full stop). This interpretation predicts quantitatively th e dependence of the extent of desensitization on light intensity and t he dependence of onset and maintenance of phototaxis on extent of dese nsitization. These predictions are tested and confirmed in this report . (C) 1994 Wiley-Liss, Inc.