Magnesium Stable Isotope Fractionation on a Cellular Level Explored by Cyanobacteria and Black Fungi with...

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.