Behavioural Responses to Light in Paramecium Bursaria in Relation to Its Symbiotic Green Alga Chlorella

The behavioural responses to light in the ciliate Paramecium bursaria Focke, which normally contains hundreds of the symbiotic green alga Chlorella in its cytoplasm, were analysed quantitatively to clarify the mechanisms governing photoreception in the cell. P. bursana was found to possess three kinds of photoreceptor systems for (1) the step-up photophobic response (system I), (2) the step-down photophobic response (system II), and (3) the photokinetic response (system III). Under the influence of light, the symbiotic algae inhibited systems I and III, but activated system II. Thus the cells showed the avoiding reaction when they encountered shade (the step-down photophobic response), and consequently gathered in the light region (photoaccumulation). Inhibition of system I and activation of system II were assumed to be mediated by products of the blue-light effect of the algae, while inhibition of system III was due to photosynthetic products of the algae. The cells whose algae were experimentally removed gathered in the shade (photodispersal) due to the avoiding reaction exhibited by them when they encountered a lighted region (the step-up photophobic response mediated by system I). Lowered swimming velocity and increased frequency of spontaneous changes in the swimming direction in the shade (photokinetic responses mediated by system III) also caused photodispersal.