Soil extract induces excystment and inhibits encystment in the terrestrial ciliate Colpoda cucullus

Abstract

The terrestrial ciliated protozoan Colpoda cucullus inhabits soil and survives water-limited condition by forming a resting cyst which is known to be highly resistant to harsh environments. Although encystment and excystment of this ciliate can be induced experimentally under laboratory conditions, little is known about the actual environmental factors that control such dynamic changes in cell morphology in nature. Therefore, in this study, we investigated the effects of soil extracts on C. cucullus and attempted to identify the active substances that may be present in the soil extract. The soil extract induced excystment of the resting cyst, and it inhibited encystment of the vegetative cell. These effects of the soil extract might be brought by fulvic acid, one of humic substances contained in soil, together with some soil inorganic elements.