Some Factors Affecting Excystment in the Ciliate Tillina magna

Abstract

Resting cysts were produced when the food supply (Pseudomonas fluorescens) became exhausted in lettuce-infusion cultures. These cysts, which never excysted spontaneously, were then transferred to various fluids, the excystment-inducing capacity of which was under investigation (always at 20[degree]C). The % of excyst-ment and the mean excystment time were computed for each fluid. Distilled water was found to be a highly effective excysting agent. In a typical expt. with 500 cysts, 498 excysted with a mean excystment time of 2.86 hrs. The dilution of the original encystment medium with distilled water was likewise effective, the % of excystment increasing with the degree of dilution. The results indicate that excystment under these conditions is induced by the hypotonicity of the medium and that the primary excystment-inducing factor for T. magna is of an osmotic nature. The following concns. of sterile aqueous plant infusions and organic compounds induced practically 100% excystment more rapidly than distilled water, as the mean excystment times show 0.05% lettuce, 2.48 hours; 0.05% timothy, 2.43; 0.05% elodea, 2.35; 0.05% alfalfa, 223; 0.01% Na acetate, 2.58; 0.005% K citrate, 2.45; 0.025% dextrose, 2.38; 0.025% glycine, 2.36. In general, concns. weaker than these had the effect of distilled water; concns. of increasing strength lengthened the mean excystment time and decreased the % of excystment, which finally fell to zero. The results indicate that these substances at certain levels of concn. possess specific excystment-inducing properties and that the presence of these substances in the medium constitutes a secondary excystment-inducing factor. Yeast extract, ethyl alcohol, urea, inorganic salt medium (a mixture of salts of Na, K, Ca and Mg), Na bicarbonate and Na sulfate in low concns. likewise induced excystment, but never more rapidly than distilled water. Higher concns. inhibited excystment. Hence, specific excysting properties can not be attributed to these substances, their effect in low concns. being the result of the osmotic properties of these concns.