Conditions of survival and extinction of Paramecium multimicronucleatum cultured with P. tetraurelia in a closed environment

Abstract

Summary With two species of Paramecium, P. multimicronucleatum (Pm) and P. tetraurelia (Pt), we explored in more detail the classical experiments of Gause from which the competitive exclusion principle was formulated. We found that when they were cultured together, Pm cells, but not Pt cells, became extinct after reaching their maximal density. The extinction of Pm in the mixed culture occurred not only in a semi-open system in which 1/10 of the culture was replaced daily with fresh culture medium, but also in a closed system to which no food was added. Pm cells survived, however, in the mixed culture when different stocks of Pm were used, when cultured in higher cell density of Pm, and when cultured in medium supplemented with stigmasterol or in medium prepared from Wheat Grass Powder of a new lot number. These results indicate that survival and extinction of Pm cultured with Pt are a physiological phenomenon rather than an ecological problem. We propose that the fate of Pm is affected by survival substances in the surrounding medium, which are composed largely of the remaining food and partly of the secreted growth factor, and by a threshold level for survival substances for Pm.