Life on the 'mild' side: adaptation of an extremophile archaeon to a mesophilic lifestyle

Extremophilic organisms have evolved specialised adaptations for coping with harsh environments. Maintaining genomic integrity under such conditions requires high DNA replication fidelity and repair systems to prevent genome degradation. However, these systems may hinder the generation of novel genetic variation required to adapt to mesophilic conditions. The Aim of this project is to identify evolutionary transitions, and constraints on adaptation, that occur during adaptation of an extremophile archaeon to mesophilic conditions (i.e. environmental cooling and alkalinization). We will characterise mesophilic adaptation in Sulfolobus acidocaldarius, a thermoacidophilic archaeon with optimal growth at 75-80 degrees Celsius and pH 2-3. We will first assess whether S. acidocaldarius is mutation limited under mesophilic conditions. We will then use laboratory experimental evolution to gradually adapt S. acidocaldarius to decreased temperature and increased pH, and use genomic sequencing to identify key adaptive mutations. Finally, we will assess whether adapting to mesophilic conditions was preceded by, or associated with, an increase in mutation rate. We hypothesise that evolutionary 'potentiating' mutations will involve loss-of-function or function-reducing variation in one or more of the highly-specialised DNA repair and replication fidelity systems possessed by S. acidocaldarius. This work will determine, for the first time, both the environment-specific adaptations and the evolutionary mechanisms underpinning mesophilic adaptation. This will shed light on transitions between extremophile and mesophile lifestyles, as may have occurred early in the evolutionary history of life on Earth under changing environmental conditions, on both global and microscopic scales.