A fundamental problem related to the astrobiological study of terrestrial (e.g. early Earth rocks and minerals) and planetary materials (e.g. asteroidal and Mars meteorites, Martian rocks) is not only recognizing and quantifying carbon-related compounds that may be present, but also differentiating those molecules formed abiotically from those generated by extinct or extant life. Whereas biological markers or ‘biomarkers’ are molecular fossils (Simoneit 2004a, b), there is not a clear and official definition of the term ‘geomarker’, and it has been ambiguously used to refer to different topics, not always following a formal concept. Biomarkers are defined as complex organic compounds, which originated from formerly living organisms and which are composed of carbon, hydrogen, and other elements. Abiotic organic compounds are not biomarkers per se because they do not originate from biosynthesis

(Simoneit 2004a, b). Biomarkers occur in many different types of materials and geological settings (sediments, rocks, crude oils, etc.) and normally display negligible or no structural changes from their parent organic molecules in living organisms (see Eglinton and Calvin 1967, Summons and Powell 1986, Summons and Walter 1990, Becker et al. 1997, Simoneit et al. 1998, Brocks et al. 1999, Brocks 2001, Simoneit 2002, 2004a, b, Brocks and Summons 2004, Brocks and Pearson 2005, Zhang et al. 2005). In accordance with Peters et al. (2006) three principal characteristics permit biomarkers to be distinguished from many other organic compounds: “a) biomarkers have structures composed of repeating subunits, indicating that their precursors were components in living organisms; b) each parent biomarker is common in certain organisms, and c) these organisms can be abundant and widespread”. Taking into account these concepts, and considering that hydrothermal systems have been proposed as logical candidates for the location of the emergence of life (an assumption that is supported by genetic evidence that modern hyperthermophilic organisms are closer to a common ancestor than any other forms of life) (Shock 1997, Russel et al. 2005), the present contribution aims to offer a synthetic overview of a) the main geological (mineralizing hydrothermal) scenarios regarding geo and biomarkers; b) the main microbiological features regarding hyperthermophilic bacteria and thermophilic viruses. This information is extremely useful for the understanding of hydrothermal ecosystems, both past and present (Barns et al. 1997), and for the possible extrapolation of such knowledge to the astrobiological exploration of Mars and other planetary bodies.