Physiological metal uptake by Nostoc punctiforme
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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.
KeywordsCyanobacteria Divalent cations Metal uptake Adsorption Absorption Bioremediation
We are grateful to Professor John C. Meeks for technical assistance and helpful discussions.
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