Diaphototaxis and gravitaxis in a freshwater Cryptomonas.

Phototaxis and gravitaxis are characterized in a freshwater species of the flagellate Cryptomonas. The phototactic orientation in this limnetic species is unusual and differs from all other Cryptomonas species studied so far: At both low (< or = 1O W m-2) and higher fluence rates it orients perpendicular to the light beam (diaphototaxis) while another freshwater Cryptomonas species (strain CR-1) is restricted to positive phototaxis and the marine species, C. maculata, shows both a positive and a more pronounced negative phototaxis. The mechanism of light direction detection seems to depend on a periodic shading or irradiation mechanism as confirmed by the disturbance of phototaxis in the presence of high viscosity media. In addition, this freshwater species possesses a rather pronounced negative gravitaxis which is only partially modified by phototaxis. The ecological consequences of this behavior are discussed.