Chapter 9 Electrical events in photomovement of green flagellated algae

Abstract

Phototaxis and photophobic responses in green flagellated algae are mediated by a rhodopsin-type photoreceptor. Its photoexcitation triggers a rapid cascade of electrical phenomena in the cell membrane. The photoinduced electrical responses in green flagellated algae can be recorded extracellularly from an individual cell by a suction pipette technique, or from a cell suspension. Photoexcitation leads to the onset of a photoreceptor current (PC) across the patch of the cell membrane overlaying the eyespot. The PC consists of at least two components activated by different mechanisms. The first mechanism most likely involves translocation of ions across the membrane either by the rhodopsin itself, or through an ion channel directly coupled to it. The second mechanism of PC generation appears to operate via a cascade of biochemical amplification. Membrane depolarization induced by PC leads to the unbalanced motor response of the flagella, which is the basis for phototaxis. If depolarization exceeds a critical level, a voltage-gated flagellar current (FC) is triggered across the flagellar membrane, which gives rise to the photophobic response of the cell. FC, and, upon prolonged light stimulation, PC are associated with depolarization-activated K + currents across the cell membrane. PC generation is the earliest so far detectable event in the signal transduction pathway. Therefore, investigation of PC allows in vivo probing the photoreceptor function and the role of the phototaxis directional antenna. The processes of energy metabolism provide the negative feedback loop for the light control of behavior in green flagellated algae by regulation of the phototaxis sign.