Physicochemical Salt Solution Parameters Limit the Survival of Planococcus halocryophilus in Martian Cryobrines

Abstract

Microorganisms living in sub-zero environments can benefit from the presence of dissolved salts, as they significantly increase the temperature range of liquid water by lowering the freezing point. However, high concentrations of salts can reduce microbial growth and survival, and can evoke a physiological stress response. It remains poorly understood how the physicochemical parameters of brines (e.g. water activity, ionic strength, solubility and hydration shell strength between the ions and the surrounding water molecules) influence the survival of microorganisms. We used the cryo− and halotolerant bacterial strain Planococcus halocryophilus as a model organism to evaluate the degree of stress different salts assert. Cells were incubated in liquid media at −15 • C containing single salts at eutectic concentrations (CaCl 2 , LiCl, LiI, MgBr 2 , MgCl 2 , NaBr, NaCl, NaClO 4 and NaI). Four of these salts (LiCl, LiI, MgBr 2 and NaClO 4) were also investigated at concentrations with a low water activity (0.635) and, separately, with a high ionic strength (8 mol/L). Water activity of all solutions was measured at −15 • C. This is the first time that water activity has been measured for such a large number of liquid salt solutions at constant sub-zero temperatures (−15 • C). Colony-Forming Unit (CFU) counts show that the survival of P. halocryophilus has a negative correlation with the salt concentration, molecular weight of the anion and anion radius; and a positive correlation with the water activity and anions' hydration shell strength. The survival of P. halocryophilus did not show a significant correlation with the ionic strength, the molecular weight of the cation, the hydrated and unhydrated cation and hydrated anion radius, and the cations' hydration bond length. Thus, the water activity, salt concentration and anion parameters play the largest role in the survival of P. halocryophilus in concentrated brines. These findings improve our understanding of the limitations of microbial life in saline environments, which provides a basis for better evaluation of the habitability of extraterrestrial environments such as Martian cryobrines.

Physicochemical Parameters Limiting Growth of Debaryomyces hansenii in Solutions of Hygroscopic Compounds and Their Effects on the Habitability of Martian Brines

“…Furthermore, chaotropicity, a value describing the entropic disordering of biomacromolecules like proteins or lipid bilayers, has been described as a major biological stressor in many environments [13][14][15]. In addition, the size and strength of hydration shells around the solutes may influence membrane permeability and, consequently, affects the toxicity to organisms [16,17]. Other solute-specific parameters may influence the toxicity as well [17], such as interaction with metabolic processes, changes of pH or an enhanced redox potential of the solutes.…”

“…In addition, the size and strength of hydration shells around the solutes may influence membrane permeability and, consequently, affects the toxicity to organisms [16,17]. Other solute-specific parameters may influence the toxicity as well [17], such as interaction with metabolic processes, changes of pH or an enhanced redox potential of the solutes.…”

Anion-type modulates the effect of salt stress on saline lake bacteria

“…Both studies concluded that physicochemical properties such as ionic strength and water activity alone did not determine the limits of growth, instead the combination of anions and cations also had an effect on bacteria. Waajen et al ( 2020 ) also indicated that anion properties were among the most important parameters that affected the survival of a Planococcus halocryophilus strain at extremely low temperature values when comparing brine solutions of nine different salts, while Banciu et al ( 2004 ) observed higher biomass yield of a Thialkalivibrio halophilus strain in NaCl than in Na 2 CO 3 , and hypothesized that the adaptation to high salinities could, at least partly, be explained with differences in the osmotic pressure caused by these salts at equal sodium ion concentration (4 M). Contrary to these experiments performed on individual bacterial strains, to the best of our knowledge, systematic study on the effect of different dissolved ions on a large set of bacteria isolated from different saline lakes like ours (i.e., 172 bacterial strains isolated from 6 different lakes in 3 different regions and tested on 3 different salts) has not been carried out before.…”

Perchlorate‐specific proteomic stress responses of Debaryomyces hansenii could enable microbial survival in Martian brines

“…The copyright holder for this preprint this version posted May 2, 2022. ; https://doi.org/10.1101/2022.05.02.490276 doi: bioRxiv preprint 3 permeability through variations in ionic hydration shells (9). Some anions like perchlorate additionally evoke chaotropic stress (10), i.e., they destabilize biomacromolecules like proteins, presumably through nonlocalized attractive dispersion forces (11).…”