Magnesium isotope fractionation during microbially enhanced forsterite dissolution

Area of Science:

• Geochemistry
• Biogeochemistry
• Isotope Geochemistry

Background:

• Cell surface reactivity influences mineral weathering and elemental cycling.
• Magnesium (Mg) isotopes fractionate during mineral dissolution and biological processes.
• Bacillus subtilis endospores offer a model for studying isolated cell surface effects due to low metabolic activity.

Purpose of the Study:

• To investigate the impact of Bacillus subtilis endospore surface reactivity on Mg isotope fractionation during forsterite dissolution.
• To differentiate the effects of cell surface adsorption from metabolic processes on Mg isotopes.

Main Methods:

• Forsterite dissolution experiments were conducted with and without Bacillus subtilis endospores.
• Mg isotope compositions (δ26Mg) of the solution and solid phases were analyzed using mass spectrometry.
• Abiotic controls were used to establish baseline Mg isotope fractionation during forsterite dissolution.

Main Results:

• Abiotic forsterite dissolution preferentially released lighter 24Mg, resulting in isotopically light aqueous solutions.
• Endospore presence did not affect Mg release fractionation but led to preferential adsorption of lighter 24Mg onto endospore surfaces.
• Aqueous Mg isotope compositions became heavier with increased Mg adsorption onto endospores, demonstrating surface reactivity's influence.

Conclusions:

• Cell surface adsorption, not metabolism, significantly impacts Mg isotope fractionation during microbial-mineral interactions.
• Mg isotopes can serve as tracers for the role of microbial surface reactivity in environmental Mg cycling.
• Further research can utilize Mg isotopes to understand the biosphere's influence on global Mg cycles.