Studies on the resting cyst of ciliated protozoan Colpoda cucullus: resistance to temperature and additional inducing factors for en-or excystment

Abstract

The resting cysts of Colpoda cucullus were resistant to not only drying, but high and low temperatures. The critical temperatures for the survival of wet cysts ranged from 40 ℃ (3-hr exposure) to 45℃ (10-min exposure), and those of the dried cysts ranged from 80 ℃ (3-hr exposure) to 100 ℃ (30-min exposure). Frozen (-30 ℃, 3 hr) and remelted wet cysts also survived. Ca²⁺-induced resting cysts excysted when the external Ca²⁺ was removed. However, the excystment-inducing effect by Ca²⁺ removal was canceled by the addition of cations such as Na⁺, K⁺, and Mg²⁺ in the surrounding medium. When the mature cysts transformed in a diluted buffer without any other salts were temporally exposed to Ca²⁺ and subsequently resuspended in the buffer without Ca²⁺, excystment was induced in a majority of cysts. The addition of chlorophyll-derived molecules in the surrounding medium induced excystment, but suppressed encystment. This suggests that the excystment-inducing and encystment-suppressing activities of cereal infusion are attributed to water-soluble porphyrins derived from chlorophyll. The encystment was induced when the vegetative cells were suspended at a high density, and the substitution of non-living particles such as polystyrene latex particles (PLP) for the living cells also showed an encystment-inducing effect. The result suggests that mechanical cell-to-cell contact induces encystment.