Impact of Horizontal Gene Transfer on Adaptations to Extreme Environments.

Horizontal (or lateral) gene transfer - an acquisition of genetic material not associated with the organismal reproduction - is known to alter genomes of most, if not all, living organisms. There is mounting evidence for the importance of gene exchange in organismal adaptations to new or changing environmental conditions. In comparison to accumulation of de novo mutations, acquisition of a gene already beneficial in the environment is fast and less costly, and thus an advantageous, way to adjust to survival and growth in new conditions. Adaptation to extreme environments at the boundaries of habitat conditions beyond which cellular integrity, metabolism and growth are not possible, is not an exception. Here we review the impact of horizontal gene transfer on organismal adaptations to natural and human-made extreme environments. This includes thermophiles living at high temperatures, psychrophiles found at low temperatures, acidophiles inhabiting high acidity environments, alkaliphiles thriving at high pH, halophiles found in high salt environments, xerophiles that can tolerate extremely low water availability, oligotrophes thriving at low nutrient availability, piezophiles inhabiting high pressure environments, and organisms that can withstand high levels of ionizing radiation. We also discuss the challenges and future directions for deciphering genetic determinants and horizontal gene transfer events of extremophiles' adaptations.