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Microorganisms occupy almost every habitable niche on Earth. Some of them may use inorganic or
organic substances rather than light as energy sources. We refer to this group as chemotrophs in order
to distinguish them from those organisms that use light as an energy source (phototrophs). Both
chemotrophs and phototrophs are abundant in environments where one or more physical, or chemical
parameters show values far from the lower or upper limits known for life. These ambient habitats are
referred to as normal environments. When microorganisms succeed in adapting themselves to harsh
niches they are referred to collectively as extremophiles (Seckbach 2004, 2006).
We review arguments that militate in favor of microorganisms that in the past and present may have
occupied other niches in the Solar System. Among the extremophiles we find representatives of the
three domains of life (Bacteria, Archaea, and Eukarya). Some of the possible candidates for life in the
Solar System are the extremophiles including chemotrophs, especially sulfur-reducing bacteria (SRB).
Another example of chemotrophs are the methanogens. Those microbes are capable of producing
methane as a metabolic byproduct of the reduction of carbon dioxide, a process that is called
methanogenesis. Searching for new forms of life (within the extraterrestrial regions) is the object of
planning within the Cosmic Vision Program of ESA, in collaboration with NASA, and other space
agencies. Indeed, sulfur traces on Jupiter's moon Europa detected by the Galileo mission have been
conjectured to be endogenic, most likely of cryovolcanic origin, due to their non-uniform distribution
in patches. The Galileo space probe first detected the sulfur compounds, as well as revealing that this
moon almost certainly has a volcanically heated and potentially habitable ocean hiding beneath Europa
surface layer of icy water. In this paper we restrict our attention to possible biomarkers that could
signal on Europa the presence extremophiles in general and chemotrophs, especially the presence of
sulfur reducers.
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