Caesium toxicity, accumulation and intracellular localization in yeasts

Toxicity screening of several yeast strains has shown that lithium and caesium (at concentrations up to 80 mm in MYGP agar) were the only toxic alkali metals of Group I over this concentration range. Rhodotorula rubra [minimum inhibitory concentration (MIC) = 36 mm] and Saccharomyces cerevisiae (MIC = 48 mm) were more sensitive than Candida albicans (MIC > 80 mm). Accumulation of Cs⁺ in the yeasts was metabolically-dependent and affected by two main external factors; pH and K⁺ concentration. Over the pH range 3–5·5, Cs⁺ accumulation increased but after pH 5·5, no noticeable difference was obvious up to pH 9. The presence of equimolar K⁺ lowered Cs⁺ accumulation and this effect was more pronounced at low pH values. In order to assess Cs⁺ toxicity towards the yeast strains, it was necessary to define the K⁺:Cs⁺ ratio in the external medium. In batch culture, greater accumulation of Cs⁺ occurred in logarithmic phase cells compared to those from the stationary phase and this was equivalent to 35·5 and 23·5% of the total monovalent cations present respectively (K⁺ and Cs⁺, Na⁺ was negligible). However, using repeated 2 h incubation periods in 200 mm CsCl, it was possible to replace ∼ 80% of cellular K⁺ in S. cerevisiae with Cs⁺. It was found that ∼ 90% of the Cs⁺ accumulated by K⁺ replacement in S. cerevisiae was localized in the vacuole, the remainder being found in the cytosol. Significant amounts of Cs⁺ were not associated with the cell wall. A similar cellular distribution of Cs⁺ was found with Cs⁺-tolerant C. albicans. In contrast, with Cs⁺-sensitive R. rubra, Cs⁺ compartmentation was not so closely related to the initial distribution of K⁺ within the cells being ∼ 60% in the vacuole and ∼ 40% in the cytosol, none being bound to the cell wall. It is concluded that the vacuole may play an important role in yeast intracellular Cs⁺ compartmentation and detoxification.