Effects of the metalloid oxyanion tellurite (TeO32-) on growth characteristics of the phototrophic bacterium Rhodobacter capsulatus

Abstract

This work examines the effects of potassium tellurite (K2TeO3) on the cell viability of the facultative phototroph Rhodobacter capsulatus. There was a growth mode-dependent response in which cultures anaerobically grown in the light tolerate the presence of up to 250 to 300 microg of tellurite (TeO3(2-)) per ml, while dark-grown aerobic cells were inhibited at tellurite levels as low as 2 microg/ml. The tellurite sensitivity of aerobic cultures was evident only for growth on minimal salt medium, whereas it was not seen during growth on complex medium. Notably, through the use of flow cytometry, we show that the cell membrane integrity was strongly affected by tellurite during the early growth phase (< or =50% viable cells); however, at the end of the growth period and in parallel with massive tellurite intracellular accumulation as elemental Te0 crystallites, recovery of cytoplasmic membrane integrity was apparent (> or =90% viable cells), which was supported by the development of a significant membrane potential (Deltapsi = 120 mV). These data are taken as evidence that in anaerobic aquatic habitats, the facultative phototroph R. capsulatus might act as a natural scavenger of the highly soluble and toxic oxyanion tellurite.

FIG. 1.

Electron micrographs of R. capsulatus B100 cells grown in the absence (A) or in the presence (B, C, D) of 50 μg of K₂TeO₃ per ml. (A) Photosynthetic-anaerobic growth. (B) Aerobic growth. (C) Photosynthetic-anaerobic growth on minimal medium. (D) Photosynthetic-anaerobic growth on rich medium.

FIG. 2.

(A) Growth curves of cells grown in the presence of 25 μg of K₂TeO₃ per ml. Solid lines, K₂TeO₃-grown cultures; dashed lines, control cultures. (B) Variation of tellurite concentration in the growth medium. The same symbols represent the same growth conditions in the two panels. Symbols: ▪, aerobic conditions on rich medium; •, photosynthetic conditions on minimal medium; ▴, photosynthetic conditions on rich medium.

FIG. 3.

Three-dimensional fluorescence cytograms of R. capsulatus cells grown on minimal medium and stained with SYBR I and propidium iodide. (A) Cells grown in the absence of potassium tellurite; total count, 2.8 × 10⁸ cells/ml. (B) Cells grown in the presence of 50 μg of potassium tellurite per ml; total count, 3.3 × 10⁸ cells/ml.

FIG. 4.

Light-induced and oxygen-dependent uptake of TPP⁺ ions by R. capsulatus cells grown in the absence (continuous trace) or in the presence (interrupted trace) of potassium tellurite. Following the calibration addition of TPP⁺ (1 μM final concentration), cells (2.8 and 3.3 mg of protein for control and tellurite-grown cells, respectively) were added to 2 ml of air-saturated medium in 50 mM TES buffer, pH 7.5-10 mM KCl at 28°C. TPP⁺, tetraphenylphosphonium cation; FCCP, carbonyl cyanide p-trifluoromethoxyphenylhydrazone; ON, light on; OFF, light off.