The effect of chaotropic magnesium chloride on the growth of microbes

Chaotropic agents denature biological macromolecules whereas kosmotropes stabilise macromolecular structures. Magnesium chloride (MgCl₂) is one of the most widespread chaotropic solutes that is not expected to support growth above 2.3 M, but few studies have focused on the effects of MgCl₂ on microbial growth. This study investigated the effects of MgCl₂ in comparison to kosmotropic sodium chloride (NaCl), on microbial growth and community composition, with the focus on MgCl₂. Solid (1.5% agar) and liquid media were supplemented with 1% yeast extract and different concentrations of MgCl₂ and NaCl, using samples from a salt marsh and agricultural soil (Colchester, UK). Viable counts decreased for both solutes as concentrations increased but MgCl₂ had no viable counts at a concentration of 1.5 M and above. PCR amplification showed that salt marsh fungi dominated in MgCl₂ enrichments and DGGE analysis of enrichments revealed high community diversity for Bacteria and Archaea but low community diversity for fungi. Sequencing of selected DGGE bands showed the presence of an Acremonium-related species in MgCl₂ at 1.5 M and Baeospora myosura at 1.75 M. Several isolated fungal strains tested in MgCl2 concentrations up to 2.2 M proved to be chaotolerant. A strain from salt marsh potentially grew in 2.2 M MgCl₂ but further testing is needed to confirm this as the small sizes of suspected flocks render visual confirmation difficult. Identification and improved knowledge on chaophiles and chaotolerant microbes would have implications for astrobiology and in the search for extra-terrestrial life or the potential of life in chaotropic (or similar) extra-terrestrial environments.