Photosynthesis modulates the sign of phototaxis of wild‐type Chlamydomonas reinhardtii

We have found that the sign of phototaxis, i.e. the direction of phototactic migration either towards or away from the light source, of wild‐type Chlamydomonas reinhardtii depends on its photosynthetic activity. This accounts for the frequently reported transient positive phototaxis that precedes a negative phototaxis of wild‐type cells, as well as the earlier observations that preillumination affects the sign of phototaxis. The bases for our conclusion are as follows. (1) The transient nature of phototaxis was preferentially observable with blue‐green actinic light rather than with green actinic light. (2) Red background light induces negative phototaxis under the actinic‐light conditions in which, without background light, Chlamydomonas cells show exclusively positive phototaxis. (3) Both the effect of red background light and the transient change in the sign of phototaxis were inhibited by 3‐(3',4'‐dichlorophenyl)‐1,1‐dimethylurea, a specific inhibitor of photosynthesis. The conclusion modifies the accepted view that photosynthesis does not link with the phototaxis of this microorganism, thus constituting a necessary part in elucidating mechanisms of algal phototaxis.