J. Plant Physio!. 157.247-254 (2000) © Urban & Fischer Verlag http: //www. urbanfischer.de/journals/jpp • JOURNAL OF • PLANT PHYSIOLOGY Effects of light on gravitaxis and velocity in Chlamydomonas reinhardtii Oleg Sineshchekov\ Michael Lebert 2 , Donat-P. Hader 2 * 1 Moscow State University, BioI. Dept., 119899 Moscow, Russia 2 Friedrich-Alexander-Universitat, Erlangen-NOrnberg, Institut fOr Botanik und Pharmazeutische Biologie, Staudtstr. 5, D-91058 Erlangen, Germany Received January 4, 2000 . Accepted May 15, 2000 Summary The effects of light on gravitaxis and velocity in the bi-flagellated green alga Chlamydomonas rein- hardtii were investigated using a real time automatic tracking system. Three distinct light effects on gravitaxis and velocity with parallel kinetics were found. Photosynthetically active continuous red light reversibly enhances the swimming velocity and increases or decreases the precision of gravi- taxis, depending on its initial level. Blue light flashes induce fast transient increases in velocity imme- diately after the photophobic response, and transiently decrease or even reverse negative gravitaxis. The calcium dependence of this response, its fluence-response curve and its spectral characteris- tics strongly suggest the participation of chlamy-rhodopsin in this effect. The third response, a pro- longed activation of velocity and gravitaxis, is also induced by blue light flashes, which can be observed even in calcium-free medium. Key words: Chlamydomonas reinhardtii - Flagellates - Gravitaxis - Image analysis - Light effects - Swimming velocity Introduction Many motile microorganisms use external cues to move to and stay at a depth in the water column possessing optimal conditions for growth and reproduction. In addition to light, gravity is a key factor for orientation in the water column (Hader 1987, Bean 1984). The unicellular bi-flagellated Chla- mydomonas reinhardtii responds to light with positive and negative phototaxis, depending on the irradiance of the actin- ic radiation (Feinleib and Curry 1971). The photoreceptor for photoorientation has been identified as a novel rhodopsin (Foster et al. 1984, Lawson et al. 1991 , Hegemann 1997) • E-mail correspondingauthor: dphaeder@biologie.uni-erlangen.de which is capable of mediating responses to extremely low light intensities (Ruth and Uhl 1997) . The event of photoper- ception is believed to trigger an inward calcium current across the cytoplasmic membrane overlaying the stigma region of the cell (Litvin et al. 1978, Harz and Hegemann 1991 , Sineshchekov 1991 a, b). The intracellular sensory trans- duction chain operates on the basis of light-induced elec- trical potential changes, which result in a modulation of the flagellar beat pattern (Sineshchekov 1991 a, b, Holland et al. 1997, Sineshchekov and Govorunova 1999). In the absence of light, movement is controlled by gravitaxis in many motile microorganisms (Roberts 1970, Eggersdorfer and Hader 1991, Rhiel et al. 1988). Chlamydomonas shows a pronounced negative gravitaxis which allows the organism to move to the surface (Kessler 1989a, Bean 1977) . 0176-1617/00/157/03-247 $15.00/0