This work examines the effects of potassium tellurite (K ₂ TeO ₃ ) 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 μg of tellurite (TeO ₃ ²⁻ ) per ml, while dark-grown aerobic cells were inhibited at tellurite levels as low as 2 μg/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 (≤50% viable cells); however, at the end of the growth period and in parallel with massive tellurite intracellular accumulation as elemental Te ⁰ crystallites, recovery of cytoplasmic membrane integrity was apparent (≥90% viable cells), which was supported by the development of a significant membrane potential (Δψ = 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.

The highly toxic oxyanion tellurite has to enter the cytoplasm of microbial cells in order to fully express its toxicity. Here we show that in the phototroph Rhodobacter capsulatus , tellurite exploits acetate permease (ActP) to get into the cytoplasm and that the levels of resistance and uptake are linked.