Microbial Life under Extreme Environments of Permafrost: Tolerance Limits of Methanogenic Archaea as Keystone Organisms for the Investigation of Extraterrestrial Life.

Extraterrestrial Permafrost is a common phenomenon within the solar system and the main centre of extraterrestrial research in Astrobiology, with focus on searching for extraterrestrial life. Mars is considered as one of the most similar planets to Earth of our solar system, even if it is characterized by extreme coldness and dryness today. When living conditions on Mars were similar to that on early Earth, the evolution of microorganisms had already started on Earth. Prokaryotic microfossils, found in early Archaean rocks, implies that the earliest life forms between 3.5 3.8 Ga ago. Therefore it is legitimate to assume that life also emerged on early Mars as on early Earth.The newest ESA Mission to Mars (Mars Express) determined water on Mars, which is fundamental requirement for life initiation. Further investigations for the first time demonstrated the presence of methane in Mars atmosphere, which could be only of active volcanic or biological origin. Data obtained by the Mars Express probe show that water vapour and methane gas are concentrated in the same regions of the Martian atmosphere. This finding may have important implications for the possibility that microbial life could exist on Mars.Martian life, if present, must have adapted to drastically changing environmental conditions. One possibility of survival might be subsurface lithoautotrophic ecosystems, comparable to permafrost on Earth, in which microorganisms have survived for million years.Methanogenic Archaea, which colonised terrestrial permafrost, are highly specialized organisms from the view point of metabolism. The capability these organisms to lithoautotrophic growth, whereby methane is gained by the oxidation of hydrogen and carbon dioxide is the only carbon source under strictly anaerobic conditions, tolerance to low temperatures and survival under extreme conditions of permafrost for several millions of years make methanogens to one of the most suitable keystone organism for the investigation of possible Martian life.The goal of this work was to examine the tolerances of these organisms under unfavourable life conditions of terrestrial or extraterrestrial permafrost. The borders of growth influenced by desiccation, temperature extremes and high salt concentration were analyzed for the organisms in pure cultures as well in their natural environment of Siberian permafrost.The investigation area is situated on Samojlov Island in the Lena Delta, Siberia. The influence of high salt concentrations in combination with low incubation temperatures was determined. Methanogenic Archaea revealed methane production under in situ conditions (at temperatures between 0 and 2°C) and salt concentration up to 6 mol with a rate about 0.02-0.09 nmol CH4 h-1 g-1. Further laboratory tests with isolated cultures confirm those results. Incubation of pure cultures with different salt concentration (from 0,1 to 6 mol) showed a significant methane production rate even at salt concentration 6 mol (0.013-0.022 nmol CH4 h-1 ) The ascendancy of temperature on the experiments was also analyzed. Methanogenic Archaea showed better adaptation to high salt concentration at low temperatures, 4°C, with methane production rate 0.019-0.026 nmol CH4 h-1 as at incubation temperature 28°C (0.012-0.014 nmol CH4 h-1). Consideration of methanogenic Archaea natural environments makes those results conclusive.