Effects of light on gravitaxis and velocity in Chlamydomonas reinhardtii

The effects of light on gravitaxis and velocity in the bi-flagellated green alga Chlamydomonas reinhardtii were investigated using a real time automat ic tracking system. Three distinct light effects on gravitaxis and velocity with parallel kinetics were found. Photosynthetically active continuous re d light reversibly enhances the swimming velocity and increases or decrease s the precision of gravitaxis, depending on its initial level. Blue light f lashes induce fast transient increases in velocity immediately after the ph otophobic response, and transiently decrease or even reverse negative gravi taxis. The calcium dependence oi this response, its fluence-response curve and its spectral characteristics strongly suggest the participation of chla my-rhodopsin in this effect. The third response, a prolonged activation of velocity and gravitaxis, is also induced by blue light ilashes, which can b e observed even in calcium-free medium.