Photo-orientation in a freshwater Cryptomonas species

Abstract

Phototactic orientation is characterized in a freshwater species of the flagellate Cryptomonas. In contrast to the marine species, C. maculata, which shows both pronounced positive and negative phototaxis, and an unidentified red tide Cryptomonas species, which is restricted to positive phototaxis, this limnetic species possesses an unusual phototactic response: at all effective fluence rates it orients perpendicular to the direction of the light beam (diaphototaxis). When grown under nitrogen and phosphorus deficiency, the absorption spectrum does not change considerably but the degree of orientation increases. Addition of p-quinone, a quencher of free radical formation, at concentrations of up to 10 −5 M does not alter the phototactic behaviour at either high or low fluence rates, while 1,4-diazobicyclo [2.2.2] octane (DABCO) as well as other scavengers of singlet oxygen, affects photo-orientation at high fluence rates using a concentration of 10 −4 M, but not at low fluencerates. These results indicate that the high fluence rate response may be mediated by a type II ( 1O 2 production) photodynamic effect.