Mg Isotope Fractionation during Uptake by a Rock-Inhabiting, Model Microcolonial Fungus Knufia petricola at Acidic and Neutral pH

Abstract

The model rock-inhabiting microcolonial fungus Knufia petricola fractionates stable Mg isotopes in a time- and pH-dependent manner. During growth, the increase of Mg/Mg in the fungal cells relative to the growth media amounted to 0.65 ± 0.14‰ at pH 6 and 1.11 ± 0.35‰ at pH 3. We suggest a constant equilibrium fractionation factor during incorporation of Mg into ribosomes and ATP as a cause of enrichment of Mg in the cells. We suggest too that the proton gradient across the cell wall and cytoplasmic membrane controls Mg transport into the fungal cell. As the strength of this gradient is a function of extracellular solution pH, the pH-dependence on Mg isotope fractionation is thus due to differences in fungal cell mass fluxes. Through a mass balance model we show that Mg uptake into the fungal cell is not associated with a unique Mg isotope fractionation factor. This Mg isotope fractionation dependence on pH might also be observed in any organism with cells that follow similar Mg uptake and metabolic pathways and serves to reveal Mg cycling in ecosystems.

Mg Isotope Interlaboratory Comparison of Reference Materials from Earth‐Surface Low‐Temperature Environments

“…The intermediate precision of this measurement procedure (including dissolution, Mg column separation, dilution and MC‐ICP‐MS analyses) was evaluated by repeat analyses of reference materials (silicate rocks, IAPSO Atlantic seawater) over the course of about 3 years (e.g., Pokharel et al . , Uhlig et al . ).…”

“…Magnesium recovery (quantitative column yield), purity of the Mg solutions (> 90% Mg was found to produce no bias using our measurement instrumental set‐up; see Pokharel et al . ) and the Mg content in total procedural blanks were checked by ICP‐OES (Varian 720ES) and quadrupole ICP‐MS (Thermo Scientific iCAP‐Qc), respectively.…”

“…Total procedural blanks of the column chemistry and the H 2 O 2 / HNO 3 treatment contained < 12 ng Mg, which is < 0.5% Mg compared with the amount of Mg processed through column chemistry and is therefore considered insignificant (potential bias in d 26 Mg is estimated < 0.03‰). Magnesium recovery (quantitative column yield), purity of the Mg solutions (> 90% Mg was found to produce no bias using our measurement instrumental set-up; see Pokharel et al 2017) and the Mg content in total procedural blanks were checked by ICP-OES (Varian 720ES) and quadrupole ICP-MS (Thermo Scientific iCAP-Qc), respectively.…”

“…The intermediate precision of this measurement procedure (including dissolution, Mg column separation, dilution and MC-ICP-MS analyses) was evaluated by repeat analyses of reference materials (silicate rocks, IAPSO Atlantic seawater) over the course of about 3 years (e.g., Pokharel et al 2017, Uhlig et al 2017. For example, for BHVO-2 basalt we obtained a mean d 26 Mg of -0.24 ± 0.10‰ and a mean d 25 Mg of -0.13 ± 0.06‰ (2s, n = 24 measurements on N = 6 full dissolution and column chemistry replicates), which is identical within stated precisions to results produced independently in other laboratories (d 26 Mg = -0.24 ± 0.08‰, d 25 Mg = -0.12 ± 0.05‰, Teng 2017 and references therein).…”

Magnesium isotope fractionation during microbially enhanced forsterite dissolution

“…Plants have been found to be generally enriched in heavy Mg isotopes compared with their substrates; however, the extent of the fractionation is dependent upon plant species and environmental conditions (Black, Epstein, Rains, Yin, & Casey, 2008;Bolou-Bi, Poszwa, Leyval, & Vigier, 2010;Bolou-Bi, Vigier, Poszwa, Boudot, & Dambrine, 2012;Uhlig, Schuessler, Bouchez, Dixon, & Blanckenburg, 2017). Variable isotope fractionation effects have been attributed to Mg uptake into bacterial cells (Oelkers et al, 2015;Pokharel et al, 2018) and fungi (Fahad, Bolou-Bi, Kohler, Finlay, & Mahmood, 2016;Oelkers et al, 2015;Pokharel et al, 2017Pokharel et al, , 2018, and the extent and mechanisms of fractionation appear to be largely species dependent. However, Mg isotopes have not previously been applied to systematically elucidate the specific effects of biological surface chemistry on chemical weathering.…”