Magnesium Stable Isotope Fractionation on a Cellular Level Explored by Cyanobacteria and Black Fungi with Implications for Higher Plants

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