Ultrastructural observations on the resting cyst of the soil ciliate, Territricha stramenticola and the freshwater ciliate, Urostyla grandis

The ultrastructure of non-dividing cells and resting cysts of soil ciliate Territricha stramenticola and freshwater ciliate Urostyla grandis were observed using scanning and transmission electron microscopy. The results show that the resting cell of T. stramenticola shared many similarities with that of U. grandis. Inside the cytoplasm of both resting cysts, there existed organelles such as mitochondria and autophagic vacuoles. The resting cyst of the freshwater ciliate U. grandis is spherical or ellipsoid in shape, but its cyst wall is composed of three layers: the ectocyst, the endocyst and the granular layer. However, the cyst wall of T. stramenticola was quite different from U. grandis. The cyst wall of T. stramenticola was composed of the ectocyst and the endocyst; the former was made up of small regular rectangular units tightly packed together like that of beehive, while the latter was an irregular layer of varied thickness between the ectocyst and the pellicle. Such cyst wall, quite unlike those of other freshwater ciliates, might help the cyst keep the moisture inside, thus playing an important role in tiding the resting cells over the unfavorably dryer conditions and in maintaining its existence under extreme circumstances. Those suggested that T. stramenticola would take form of another kind of kinetosome-resorbing cyst. The results of this study would help reveal how soil ciliate T. stramenticola adapt to their environment and structural differences under certain conditions.