Thermophilic microorganisms grow optimally above 45°C and inhabit environments with higher temperatures such as hot springs, terrestrial solfatara, deep-sea hydrothermal vents, and composting organic matter. Extreme thermophiles, also known as hyperthermophiles, grow optimally above 80°C; these species are distributed throughout the archaeal and bacterial domains and are positioned near the root of the microbial phylogenetic tree (Fig.1).

Such placement has led to the speculation that most ancient life forms dwelt in hotter environments, and that these ancestors gradually evolved into modern-day microorganisms, which subsequently adapted to cooler environments.It is noteworthy that hyperthermophilc archaea which grow wide temperature range possess a unique cold-stress inducible molecular chaperonin in addition to the heat-inducible ones. These two chaperonins share high sequence identity, except in their carboxy-terminal regions.Furthermore, depletion of cold-inducible or heat-induciblechaperonin gene results in growth defects under cold stress or heat stress, respectively, but not at the optimal temperature.We speculate that cold stress tolerant hyperthermophileshave adapted to lower temperature environments by acquiring an additional cold-inducible chaperoninduring the course of evolution.Likewise, several extremophilic archaea encode paralogous chaperonins that are differentially regulated during stresses such as heat, cold, high salt, pH, pressure, and nutrient deprivation, suggesting that these chaperonins might encounter different substrates depending on the type of stress confronting the cell.

Fig.1 Pylogenetic situation of hyperthermophiles(Bold line indicates positions of hyperthermophiles)

Keywords:  Archaea, evolution, hyperthermophile, thermostable enzyme.

Shinsuke Fujiwara
Department of Bioscience, School of Science of Technology, Japan