Ryoji Funadani

We found that vegetative cells of Colpoda cucullus Nag-1 accumulated in shaded areas of a container when grown in the laboratory and then formed resting cysts. The photodispersal (negative photoaccumulation) of C. cucullus was mediated, at least in part, by a difference in forward swimming velocity between the illuminated region and the shaded area of the Petri dish (motion slowed or stopped in the shaded area). When C. cucullus was stimulated by continuous light irradiation, the forward swimming velocity increased and reached a steady state within 10 s. When the light intensity decreased, the forward swimming velocity gradually decreased, and eventually returned to its original level for approximately 1 min. The action spectrum of the photokinetic response (steady-state swimming acceleration driven by continuous light stimulation) implies the involvement of blue light receptors.

Resting cysts of the terrestrial ciliate Colpoda cucullus (Nag-1 strain) are highly resistant to UV light. It has been speculated that auto-fluorescent (blue fluorescent) particles surrounding the nuclei and yellowish fluorescent layers of the cyst wall are the candidate structures for the protection of the cellular components from UV light. The UV resistance of encysting cells was quickly acquired up to 5 h after the onset of encystment induction, and then gradually increased for several days. The less fluorescent ectocyst layer, yellowish fluorescent first-synthesized endocyst layer (en-1) and the NSPs were formed within 5 h after the onset of encystment induction, and thereafter endocyst layers became gradually thicker for several days. The cyst wall sample (ectocyst and endocyst layers) markedly absorbed a broad range of UV light. This result indicates that the cyst wall evidently has UV-cut function. These results support that the cyst wall and NSPs of C. cucullus play a role in the shielding of the cell components from UV light.