Magnesium (Mg) is an important macronutrient for all living cells and is abundant in the environment. At the Earth's surface its stable isotopes are fractionated by both abiotic (mineral dissolution, primary and secondary mineral formation from solution) and biotic processes (uptake into and translocation through organisms). The objectives of this doctoral thesis were to investigate, in controlled laboratory experiments, the fractionation of Mg stable isotopes during (1) uptake and

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... of Mg into microbes focusing on microbial species, cell physiology, and the dependence on pH; and (2) mineral dissolution under abiotic conditions and in the presence of a fungus. Given that large Mg fluxes pass through biological pools, the fingerprints I identified is useful for Mg cycling studies, and for tracing Mg pathways through ecosystems. To reduce the complexity of natural systems I conducted laboratory batch experiments using unicellular model organisms. Growth experiments showed a pH-dependence of Mg stable isotope fractionation during uptake by the rock-inhabiting microcolonial fungus Knufia petricola. The fungal cell was enriched in heavy Mg isotopes relative to the growth solution, where at pH 6 the 26Mg/24Mg ratio (expressed as δ26Mg) was 0.65 ± 0.10‰ higher than that in the growth solution, and at pH 3 δ26Mg was 1.11 ± 0.34‰ higher. In contrast, the cyanobacterium Nostoc punctiforme incorporated lighter Mg isotopes from the growth solution and the cells' δ26Mg was -0.27 ± 0.14‰ lower than that of the growth solution. Chlorophyll extracted from the cyanobacterium, however, preferentially incorporated the heavier Mg isotopes, with a δ26Mg value that was 1.85 ± 0.14‰ higher than the growth solution. To explore how Mg metabolic pathways in these organisms set these isotope composition, a mass-balance model was designed. The results can be explained by three fractionation processes and the mass balance associated with them: (1) during uptake, where Mg with a slightly lower 26Mg/24Mg isotope ratio is preferred [...]