J. Photochem. Photobiol. B: Biol., 13 (1992) 119-134 119 Photoelectric responses in phototactic flagellated algae measured in cell suspension 0. A. Sineshchekov”~b~c~t, E. G. Govorunovaa*‘, A. DerC, L. Keszthelyi’ and W. Nultschb aBiology Department, it4oscow State University, 119899 iMoscow (Russia) bFachbereich Biologic, Philipps-Universitiit Marburg W-3550 Marburg (Germany) “Biological Research Centre, Hungarian Academy of Sciences, H-6701 Szeged (Hungary) (Received September 6, 1991; accepted November 19, 1991) zyxwvutsrqponmlkjihgfedcbaZYXWVUTS Abstract A new method for the investigation of electric responses involved in the light reception in microorganisms has been developed. It is based on the detection of photoelectric signals in suspensions of cells (instead of a single cell) by two different techniques: (a) by unilateral excitation of non-oriented cells and (b) after preorientation of the cells (e.g. by gravitaxis or weak, phototactically active light). The method was applied to the flagellated green algae Haematococcus and Chlamydomonas (and several of its mutants). Three main com- ponents of the electric signal, which differs in their origin and the mechanisms underlying that registration, were identified. Fast (microsecond) responses reflect charge separation in reaction centres of photosynthesis and are due to the classical light gradient effect on unilateral flash excitation. The later components of the electric signal are involved in photoreception and represent the photoreceptor potential of phototaxis and the calcium- dependent regenerative response. They are measured because of the directional sensitivity of the photoreceptor antenna and the asymmetry of localization of the electric currents involved in the sensory transduction chain. A general similarity between the electric responses in both organisms shows that the sensory transduction chain of photomovements in Chlamydomonas is similar to that described previously for Haematococcus. The advantages of the proposed method are discussed. Keywords: Photoelectric response, receptor potential, regenerative response, sensory transduction, phototaxis, photophobic response, light gradient, charge separation, ion current, flagellated algae, mutants, Chlamydomonas, Haematococcus. 1. Introduction Unicellular flagellated algae possess highly specialized light-sensing mechanisms which enable them to orient their movement in relation to light direction and to accumulate in areas of light conditions favourable for photosynthesis and of minimal photodamage [l, 21. It has been shown previously, using the suction pipette-electrode technique [3, 41, that photoelectric phenomena play a key role in phototactic light reception and sensory transduction in Haematococcus pluvialis. After the onset of +Author to whom correspondence should be addressed. loll-1344/92/$5.00 0 1992 - Elsevier Sequoia. All rights reserved