Abstract
We perform quantitative studies of the growth, death, and gene expression of Escherichia coli in a wide range of magnesium sulfate (MgSO) concentrations (0–2.5 M). Elevated concentration of MgSO causes the inhibition of cell growth, leading to an increase in the population doubling time. We find that cells exhibit three distinct morphological phenotypes—(i) normal, (ii) filamentous, and (iii) small cells at M MgSO. Filamentous cells arise due to the lack of cell division, while the small cells arise due to the partial plasmolysis of the cells. We further find that cell death starts for salt concentrations >1.25 M and increases with an increasing concentration of MgSO. For salt concentrations ≥1.66 M, the growth of cells stops and all the cells become smaller than the control cells, suggesting the plasmolysis of the population. Cells grown at salt concentration up to M are reversible in both the growth rate and morphology upon the removal of the salt stress. The time scale of reversibility increases with increasing salt concentration. Finally, we investigate the expression of an osmotically inducible gene (osmC), genes involved in magnesium transport (corA), sulfate transport (cysP), and osmotically driven transport of water (aqpZ). We find that a high concentration of magnesium sulfate leads to the upregulation of cysP and osmC.
Highlights
Europa, one of the Galilean moons of Jupiter, is a significant candidate in the search for life beyond Earth, due to the presence of a global subsurface liquid water ocean in contact with a dense rocky core [1,2,3,4,5]
In order to explore the cellular response to a high concentration of magnesium sulfate, we study the cell growth and death, morphology, and gene expression of a number of genes involved in osmolarity regulation and the transport of magnesium and sulfate of a halotolerant bacterium, E. coli, at different concentrations of MgSO4
We studied the effect of high concentrations of magnesium sulfate, presumably the most abundant salt on Europa and Mars, on cell growth, death, cell division, and gene expression of a halotolerant bacterium E. coli
Summary
One of the Galilean moons of Jupiter, is a significant candidate in the search for life beyond Earth, due to the presence of a global subsurface liquid water ocean in contact with a dense rocky core [1,2,3,4,5]. This study suggests that if the endogenic origin of sodium sulfate and magnesium sulfate is confirmed, it would imply an ocean with a low pH and rich in magnesium and sulfate and poor in sodium [10] These geochemical models further predict that the concentrations of Mg2+and SO24− can be as high as 2.9 M and 3.6 M, respectively, depending on the temperature [8,9]. Recent studies have demonstrated the growth of salinotolerant bacteria isolated from the Great Salt Plains (OK), Hot Lake (Oroville, WA, USA), and Basque Lakes under high concentrations of magnesium sulfate [27,28,29,30] While these studies reveal the microbial community structures in hypersaline environments and their potential growth in those environments, they do not provide information regarding the adaptation mechanisms of these microbes to these environments. An interesting question arises whether these cells can tolerate high concentration of magnesium sulfate, presumably the most abundant salt on Europa and Mars
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