The photoreceptor current of the green algaChlamydomonas

Abstract

The unicellular alga Chlamydomonas possesses a visual system which guides it to places that are optimal for photosynthetic growth. The rhodopsin, serving as the photoreceptor, conveys light information into a cellular signal. This signal is transmitted via several electrical steps to the flagella, where it modulates the flagellar beating pattern. The first detectable electrical process is the photoreceptor current, which is confined to the eyespot region. It is carried mainly by calcium under physiological conditions. After a flash the current rises with no detectable delay in less than 2 ms to the maximum and decays within tens of ms. The peak amplitude increases with the stimulus intensity over a range of two orders of magnitude, and the decay kinetic is accelerated in parallel. The light sensitivity of the cell is reduced by a factor of 8 when the eye is oriented away from the light source instead of facing it. This gives a measure for the `contrast' used by the cell during orientation in light. The rhodopsin action spectrum of the photoreceptor current has a maximum at 494 nm, closely matching the absorption maximum of rhodopsin in purified membranes. When the photoreceptor current exceeds a critical level, flagellar currents appear as all-or-non events. Several lines of evidences suggest that these photocurrents are the link between light absorption and alteration of the flagellar beating during a movement response.