Uptake of Thallium, a Toxic Heavy-Metal, in the Cyanobacterium Synechococcus R-2 (Anacystis nidulans, S. Leopoliensis) PCC 7942

Abstract

Uptake of the toxic heavy-metal, thallium, was studied in the cyanobacterium Synechococcus R-2 (PCC 7942) using clinically available 201Tl+. Thallium was found to distribute across the plasmalemma passively, and so the accumulation ratio of the ion ([TI+]i/[Tl+]o) could be used to calculate the apparent membrane potential (Δψi,o) of the cells (ETli,o+=Δψi,o). The permeability of the plasmalemma to Tl+ (PTl+≈1 to 5 nm s-1) is higher than that of K+. Valinomycin does not increase the permeability of Tl+. Transient changes in the Δψi,o of cells, because of electrogenic transport of ions, could be detected from its effects upon the uptake rate of Tl+. HCO3- hyperpolarized Synechococcus cells, whereas NH4+, CH3NH3+, and K+ led to depolarization. The use of Tl+ as a reporter of Δψi,o has some inherent limitations. Tl+ is toxic at very low concentrations (inhibitory effects are apparent after about 6 h at concentrations as low as 1 mmol m-3). The rate of equilibration is slow (t1/2≈5 to 20 min). Equilibration of Tl+ takes about 2 h, which limits its value as a membrane potential probe. Large amounts of Tl+ bind to the surface of the cells making the method impracticable for measuring accumulation ratios of less than about 10 (Δψi,o values smaller than about -60 mV). Cultures continuously exposed to Tl+ (10 mmol m-3) eventually become Tl+ resistant by actively extruding Tl+ (ΔμTli,o+=-3±0.2 kJ mol-1) and so thallium cannot be used as a Δψi,o probe in such cells.