Physiological metal uptake by Nostoc punctiforme

Trace metals are required for many cellular processes. The acquisition of trace elements from the environment includes a rapid adsorption of metals to the cell surface, followed by a slower internalization. We investigated the uptake of the trace elements Co²⁺, Cu²⁺, Mn²⁺, Ni²⁺, and Zn²⁺ and the non-essential divalent cation Cd²⁺ in the cyanobacterium Nostoc punctiforme. For each metal, a dose response study based on cell viability showed that the highest non-toxic concentrations were: 0.5 μM Cd²⁺, 2 μM Co²⁺, 0.5 μM Cu²⁺, 500 μM Mn²⁺, 1 μM Ni²⁺, and 18 μM Zn²⁺. Cells exposed to these non-toxic concentrations with combinations of Zn²⁺ and Cd²⁺, Zn²⁺ and Co²⁺, Zn²⁺ and Cu²⁺ or Zn²⁺ and Ni²⁺, had reduced growth in comparison to controls. Cells exposed to metal combinations with the addition of 500 μM Mn²⁺ showed similar growth compared to the untreated controls. Metal levels were measured after one and 72 h for whole cells and absorbed (EDTA-resistant) fractions and used to calculate differential uptake rates for each metal. The differences in binding and internalisation between different metals indicate different uptake processes exist for each metal. For each metal, competitive uptake experiments using ⁶⁵Zn showed that after 72 h of exposure Zn²⁺ uptake was reduced by most metals particularly 0.5 μM Cd²⁺, while 2 μM Co²⁺ increased Zn²⁺ uptake. This study demonstrates that N. punctiforme discriminates between different metals and favourably substitutes their uptake to avoid the toxic effects of particular metals.