The microbial world and the case for Mars

Microbial prokaryotes have flourished on Earth for more than 3.5 Ga. They dominated the Earthʹs biosphere during the first 2 Ga of its history before the first unicellular mitotic eukaryotes appeared. Therefore, in a search for extant life beyond the Earth, microorganisms are the most likely candidates for a putative biota of an extraterrestrial habitat. On Earth, life has developed strategies to cope with the so-called extreme conditions, such as hot vents, permafrost, permanent ice, subsurface regions, high atmosphere, rocks or salt crystals. By analogy with terrestrial extremophile communities, potential protected niches have been postulated for Mars, such as sulfur-rich sub-surface areas for chemoautotrophic communities, rocks for endolithic communities, permafrost regions, hydrothermal vents, soil, or evaporite crystals. Several methods exist to trace and identify microbial communities on Earth. Some of these biogenic signatures and biomarkers may also be applicable in attempts to search for life on Mars. However, the search for signatures indicative for putative extant life on Mars can only be the final steps in a research strategy in the quest for extraterrestrial life. In particular, prior to any “search for extant life” experiment, more data are required on the geology (paleolakes, volcanism, hydrothermal vents, carbonates), climate (hydrosphere, duration of phases which allow liquid water) and radiation environment, present state and past evolution as well as organic molecules in sediments. The search for possible biological oases will be connected with the detection of areas where liquid water still exists under the current conditions on that planet.