Episodic outbursts and jets from the nuclei of several comets indicate that localized regimes of liquid water and water vapor can periodically exist beneath the comet crust. Impact craters and pinnacles observed on comet Wild 2 suggest a thick crust. Spacecraft observations of the chemical compositions and characteristics of the nuclei of several comets (Halley, Borrelly, Wild 2, and Tempel 1) have shown that comets contain complex organic chemicals that water is the predominant volatile and that extremely high temperatures (~350-400 K) can be reached on the surface of the very black (albedo~0.03) nuclei of comets when they approach the Sun. Complex organic compounds have been observed in comets and on the water-rich asteroid 1998 KY26 and near IR observations have indicated the presence of crystalline water ice and ammonia hydrate on the large Kuiper Belt object (50000) Quaoar that has resurfacing suggesting cryovolcanic outgassing. Comets are still largely regarded as frigid, pristine bodies of protosolar nebula material that are devoid of liquid water and therefore unsuitable for life. hydrogenovorans are tolerant to high concentrations of sodium salts and extremely tolerant to high pH values, which makes them unique objects for biochemical studies and biotechnological applications.ĭuring the past few decades, the delivery of water, organics, and prebiotic chemicals to the Biosphere of Earth during the Hadean (4.5-3.8 Ga) period of heavy bombardment by comets and asteroids has become more widely accepted. The optimum pH for the cytoplasmic and periplasmic hydrogenases of D. thiodismutans remain active at pH values between 6 and 12. lacustre, and Desulfonatrovibrio hydrogenovorans exhibit different sensitivity to Na+ ions and remain active at NaCl concentrations between 0 and 4.3 M and NaHCO3 concentrations between 0 and 1.2 M. The hydrogenases of the newly isolated sulfate-reducing bacteria Desulfonatronum thiodismutans, D. In haloalkaliphilic sulfate reducers, hydrogenase, particularly if it is periplasmic, functions at high concentrations of Na+ ions and low concentrations of H+ ions. Hydrogenase is the main catabolic enzyme of hydrogen-utilizing sulfate-reducing bacteria. We also discuss the finding of extremophiles from unusual places such as soils, and briefly review recent studies of microfossils in meteorites in the context of the significance of microbial extremophiles to Astrobiology. In this review we show the current status of studies in all fields of extremophiles and summarize the limits of life for different species of microbial extremophiles. This artificial systematics cannot reflect the multiple actions of different environmental factors since one organism could unite characteristics of several extreme-groups. As a result, modern prokaryotic diversity formed in a framework of physico-chemical factors, and it is composed of: thermophilic, psychrophilic, acidophilic, alkaliphilic, halophilic, barophilic, and radioresistant species. Flexibility of the genome probably allowed life to adapt to a wide spectrum of extreme environments.
Prokaryotic extremophiles were the first representatives of life on Earth and they are responsible for the genesis of geological structures during the evolution and creation of all currently known ecosystems.
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