Chlamydomonas: Adaptation Phenomena in Phototaxis

Abstract

THE oriented movement of unicellular algae, such as Chlamydomonas, is usually regarded as being either positive or negative, when evoked by light of a fixed intensity. Until recently it has been difficult to measure phototactic responses of algae quantitatively and rapidly, but methods have now been described for following the phototactic response of a population of algae^1,2. These methods are based on a photoelectric monitoring of the distribution of the algae in a suspension, during exposure to light which evokes oriented response. We have adopted the method of Feinleib and Curry² essentially unmodified to follow the initial response of cultures of Chlamydomonas reinhardii (WT + 2). The experiments were undertaken to study the variability of the phototactic response of cultures of Chlamydomonas which we previously observed^3,4. We will show that the same culture of Chlamydomonas can be negative or positive in its response to a given fixed intensity of orienting light. The organism seems to adapt rapidly to conditions prevailing before measurement. Cultures were grown as described before^3,4 in continuous light in an inorganic medium², with constant stirring by a magnetic stirrer. The phototactic response of the cultures immediately after the orienting light was switched on was observed. Cultures frequently did not respond in a clear cut manner; a slight positive response was often followed by a negative one. If cultures were exposed to light in the apparatus for increasing periods of the time and their phototactic responses were measured periodically, the initial positive response increased and the subsequent negative one decreased (Fig. 1). These results indicated that the cells were adapting to the light so that their phototactic response changed. The following experiment was therefore undertaken. A culture was selected the initial response of which was negative. This culture was now illuminated for 2 h with 200 ft.-candles light from an incandescent lamp and its response again measured. The culture which had shown a strong and immediate negative response now responded positively to the same light intensity. The culture was now returned to darkness for 75 min and its response was measured again. As Fig. 2 shows, it now responded negatively.