Habitability, and Evolution of Microorganisms under Extreme Conditions

The choice of a solvent determines the possible biochemistry of life. Life on Earth is based on carbon biochemistry and has evolved in an environment with water as a solvent. As a polar solvent abundant on Earth, water has unique physical properties, including a large range of liquidity and low viscosity, making it a very good solvent for terrestrial life. Liquids other than water are abundant in the universe, and the chemical nature of these liquids might lead to different chemistries of life. In the first chapter, we review the main characteristics of a good solvent, and then we use this knowledge to examine the similarities and differences between water and cryogenic liquid hydrocarbon, methane, and ethane, as potential solvents for life. We argue that at cryogenic temperatures, mobility, and the reaction rate slow down. We discuss that Titan might not be habitable for terrestrial life but having a rich atmosphere and surface lakes of methane and ethane, it might be a habitat for exotic living systems. We then review multiple investigations on two proposed alternative chemistries for life on Titan, Azotosomes, and Silanes, as terrestrial cell membranes cannot form in cryogenic organic solvents. We conclude that there is a need for directing future investigations to planetary bodies that support solvents other than water. As we discuss in the first chapter, life on Earth has evolved around liquid water. Therefore, the presence of liquid water on a planetary body might make it a potential habitat for life. Jupiter’s moon, Europa, is one of the best candidates in the solar system due to the presence of a global saline ocean beneath its icy surface. In the second chapter, we argue that although the extreme conditions of Europa’s ocean (high pressure, low temperature, and high salinity) are not optimal for terrestrial life, microorganisms such as bacteria have shown extraordinary abilities to survive and occupy extreme habitats. Cells constantly adapt themselves to changes in the internal and external environments. Studying the adaptive evolution of bacteria and investigating the signatures of the adaptation under specified simulated conditions in the laboratory can provide a better understanding of the habitability of extreme terrestrial and extraterrestrial environments. We then review a research study done by Yazdani et al. (2019) at the University of Arkansas with an objective to investigate the growth, gene expression, and general strategies used by a mesophilic bacterium, Escherichia coli, to survive and adapt to high concentrations of magnesium sulfate, the proposed dominant salt in Europa’s ocean. We argue that although adaptation to a new environment might take a long time, the adaptive evolution experiments were feasible in laboratory time scales. We also discuss that the bacteria from the laboratory adaptive evolution experiments (called the “adapted sample”) were capable of growing in high concentrations (20% (w/v)) of magnesium sulfate, in which the control population of cells could not grow. We then discuss that a strategy used by bacteria to overcome the osmotic stress was to balance the intake of sulfate and magnesium and prevent water loss based on the study of the regulation of gene expression of adapted and control samples