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 ([Tl+]i/[Tl+]o) could be used to calculate the apparent membrane potential (Δ­i,o) of the cells (ETI+i,o = Δ­i,o). The permeability of the plasmalemma to TI+ (PTI+≈ 1 to 5 nms−1) is higher than that of K+. Valinomycin does not increase the permeability of TI+. Transient changes in the Δ­i,o of cells, because of electrogenic transport of ions, could be detected from its effects upon the uptake rate of TI+. HCO−3 hyperpolarized Synechococcus cells, whereas NH+4, CH3NH+, and K+ led to depolarization. The use of TI+ as a reporter of Δ­i,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 TI+ takes about 2 h, which limits its value as a membrane potential probe. Large amounts of TI+ bind to the surface of the cells making the method impracticable for measuring accumulation ratios of less than about 10 (Δ­i,o) values smaller than about −60 mV). Cultures continuously exposed to Tl+ (10 mmol m−3) eventually become TI+ resistant by actively extruding TI+ (ΔμTI+i,o= −3±0.2 kJ mol−1) and so thallium cannot be used as a Δ­i,o probe in such cells.