The uptake of Hg2+ and its regulation in the cyanobacterium Nostoc calcicola Breb. was studied. Hg2+ uptake pattern consisted of two distinct phases: (a) rapid binding of the cation to the negatively charged cell surface (first 10 min) and (b) its subsequent metabolism-dependent intracellular import, at least up to 40 min (saturating concentration 1.5 muM Hg2+, K(m) = 1.0 muM Hg2+ and V(max) 0.21 nmol Hg2+ mg-1 protein min-1). Hg2+ influx, to a major extent, depended on photosynthetically generated energy, and the supply of exogenous ATP (10 muM) or DCMU (5 muM) suggested the vital role of PS II-mediated energy to support the process. The significant lowering in Hg2+ uptake rate as well as total cellular Hg2+ in the presence of p-chloromercuribenzoate (pCMB), azide (NaN3), N,N'-dicyclohexycarbodiimide (DCCD), and thiol (mercaptoethanol) indicated the role of membrane potential, -SH groups, and ATP hydrolysis in regulating Hg2+ transport. While Cu2+ antagonized Hg2+ intake, Ni2+ showed synergism.