In a controlled growth experiment we found that the cyanobacterium Nostoc punctiforme has a bulk cell 26Mg/24Mg ratio (expressed as δ26Mg) that is -0.27‰ lower than the growth solution at a pH of ca. 5.9. This contrasts with a recently published δ26Mg value that was 0.65‰ higher than growth solution for the black fungus Knufia petricola at similar laboratory conditions, interpreted to reflect loss of 24Mg during cell growth. By a mass balance model constrained by δ26Mg in chlorophyll extract we inferred the δ26 Mg value of the main Mg compartments in a cyanobacteria cell: free cytosolic Mg (-2.64‰), chlorophyll (1.85‰), and the nonchlorophyll-bonded Mg compartments like ATP and ribosomes (-0.64‰). The lower δ26Mg found in Nostoc punctiforme would thus result from the absence of significant Mg efflux during cell growth in combination with either (a) discrimination against 26Mg during uptake by desolvation of Mg or transport across protein channels or (b) discrimination against 24Mg in the membrane transporter during efflux. The model predicts the preferential incorporation of 26Mg in cells and plant organs low in Mg and the absence of isotope fractionation in those high in Mg, corroborated by a compilation of Mg isotope ratios from fungi, bacteria, and higher plants.

Department 4 Materials and Environment BAM Federal Institute for Materials Research and Testing 12205 Berlin Germany Department of Biology Chemistry and Pharmacy Freie Universität Berlin 14195 Berlin Germany

GFZ German Research Centre for Geosciences Section 3 3 Earth Surface Geochemistry Telegrafenberg 14473 Potsdam Germany

GFZ German Research Centre for Geosciences Section 3 3 Earth Surface Geochemistry Telegrafenberg 14473 Potsdam Germany Institute of Geological Sciences Freie Universität Berlin 12249 Berlin Germany

Institute of Geological Sciences Freie Universität Berlin 12249 Berlin Germany Department 4 Materials and Environment BAM Federal Institute for Materials Research and Testing 12205 Berlin Germany Department of Biology Chemistry and Pharmacy Freie Universität Berlin 14195 Berlin Germany

Institute of Microbiology Centre Algatech 379 81 Třebon Czech Republic

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