Physiological metal uptake by Nostoc punctiforme

Abstract

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 Co(2+), Cu(2+), Mn(2+), Ni(2+), and Zn(2+) and the non-essential divalent cation Cd(2+) in the cyanobacterium Nostoc punctiforme. For each metal, a dose response study based on cell viability showed that the highest non-toxic concentrations were: 0.5μM Cd(2+), 2μM Co(2+), 0.5μM Cu(2+), 500μM Mn(2+), 1μM Ni(2+), and 18μM Zn(2+). Cells exposed to these non-toxic concentrations with combinations of Zn(2+) and Cd(2+), Zn(2+) and Co(2+), Zn(2+) and Cu(2+) or Zn(2+) and Ni(2+), had reduced growth in comparison to controls. Cells exposed to metal combinations with the addition of 500μM Mn(2+) showed similar growth compared to the untreated controls. Metal levels were measured after one and 72h 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 (65)Zn showed that after 72h of exposure Zn(2+) uptake was reduced by most metals particularly 0.5μM Cd(2+), while 2μM Co(2+) increased Zn(2+) uptake. This study demonstrates that N. punctiforme discriminates between different metals and favourably substitutes their uptake to avoid the toxic effects of particular metals.