Factors Regulating Methyl Mercury Uptake in a Cyanobacterium

The simultaneous addition of dithiothreitol (DTT), mercaptoethanol, and glutathione (30 μM each) and CH3Hg+ to Nostoc calcicola cells reduced CH3Hg+ uptake in the order GSH > DTT > mercaptoethanol. However, the preexposure of cyanobacterial cells to similar thiols resulted in different pattern of CH3Hg+ uptake in the sequence: GSH > mercaptoethanol > DTT. Light-grown cyanobacterial cells demonstrated a faster initial uptake of CH3Hg+ (rate 0.619 μmol CH3Hg+ mg−1 protein min−1, 10 min) with a biphasic pattern saturating at 30 min (bioconcentration FACTOR = 2.7 × 103). 3-(3,4-Dichlorophenyl)-1,1′-dimethyl urea (30 μM) reduced the uptake rate by 5% with a corresponding 33% reduction in CH3Hg+ accumulation. Dark exposure (24 hr) of cells reduced the CH3Hg+ uptake rate (22.3%) accompanied by a considerable decline in the bioconcentration factor (1.4 × 103). Of the four permeabilizers used, p-chloromercuribenzoate (1 μM) proved most effective in altering the CH3Hg+ uptake kinetics while dimethyl sulfoxide (5%) and cetyl trimethylammoniun bromide (1%) lowered the bioconcentration factor to 2.2 × 103 and 1.2 × 103, respectively. After toluene exposure, however, the cells revealed no sign of CH3Hg+ uptake. The data have been discussed in light of the role(s) of thiols, photoautotrophy, and membrane integrity in regulating the cellular influx of CH3Hg+.