Cu2+ efflux and its regulation in Cu-resistant (Cur) cyanobacterium Nostoc calcicola BRÉB

Cu²⁺-tolerant/resistant (Cu^r) strain of the cyanobacterium Nostoc calcicola BRÉB ., developed in the laboratory, showed remarkable growth in ambient medium containing 40 μM Cu²⁺ (5 μM Cu²⁺ being lethal for wild type strain). It showed 22% less Cu²⁺ uptake compared to Cu^s (Cu-sensitive) cells. Cu^r cyanobacterial cells were characterized by a time-dependent Cu²⁺ efflux in contrast to no cation efflux by Cu^s cells. Cu²⁺ efflux pattern in Cu^r cells consisted of two distinct phases: First rapid efflux (10 min) followed by slower second one at least up to 1 h. Cu²⁺ efflux depended to a major extent on light generated ATP¹), as the dark exposed Cu^r cells showed 87% reduction in Cu²⁺ efflux. The lowering in Cu²⁺ efflux in presence of various metabolic inhibitors/uncouplers like carbonyl cyanide-m-chloro phenyl hydrazone (CCCP)¹), N,N'-dicyclohexycarbodiimide (DCCD), azide (NaN₃) and p-chloromercuribenzoate (pCMB) indicated the involvement of -SH group, membrane potential and ATP hydrolysis in the regulation of this process. Cu²⁺ efflux was also initiated in such a medium containing 100-fold lower Cu²⁺ concentration (0.4 μM) compared to 40 μM Cu²⁺ (saturing concentration for metal uptake) and not before. It is suggested that Cu²⁺ efflux in N. calcicola cells may be the predominant mechanism of Cu²⁺ resistance/tolerance in this organism and are regulated at different levels by different modifying factors.