Investigation of Antarctica sub-fossils under Mars relevant radiation: implication for biomarkers detection

One of the primary goals of the upcoming space missions is the search for signs of past or present life in rock deposit on Mars. This research mainly focuses on biosignatures that mostly include microbial fossils that could be preserved in the geological record also under high radiation levels. On Earth, there are limited environmental analogs (cold dry desert, H-UV radiation) in which it is possible to investigate traces of life and biosignatures adapted to it. One of the best-known Mars analogues is the ice-free area of the McMurdo Dry Valleys in Antarctica (Southern Victoria Land). Here, the sandstone shows a peculiar colorization due to the stratification of microorganisms into a cryptoendolithic community (Selbmann et al., 2005). The maintenance of the cryptoendolithic communities depends of a slight balance of biological, geological and climatic factors. An unfavorable change in this equilibrium results in the death of the community, and this may be followed by the formation of trace fossils (Friedmann and Weed, 1987). The same processes could have happened on hypothetical forms of life on early Mars or could reflect a microbial decay mechanism in the last stages of hypothetical life-forms in the history of Mars (McKay et al., 1992; Wierzchos and Ascaso, 2001). During microorganisms’ fossilization process, the rocks are characterized by an exfoliation mosaic patterns (Kappen, 1993; Sun and Friedmann, 1999) and by a reddish color on the top surface layer (Friedmann and Weed, 1987). Although these rocks have a characteristic exfoliation and a typical color in the fossilization phase, under particular conditions, they can be re-colonized by new cryptoendolithic communities. In the context of the European Space Agency BioSigN (BioSigNatures and habitable niches: experiment on the International Space Station (ISS) for the search of biosignatures in microfossils) project, we analyzed irradiated (117 kGy gamma-rays) and non-irradiated sandstone samples, containing sub-fossilized microorganisms collected at Timber Peak (Northen Victoria Land), during the XXXI Italian Antarctic Expedition, with the aim to investigate if and how high radiation environment could affect the microbial community and its biosignatures. Here we present results obtained via a combination of analytical technics including (1) molecular analysis (DNA amplification of Internal Transcribed Sequences - ITS) used to investigate changes in the fungal genetic markers associated with these rocks and optical microscopy to reveal microbial morphologies; (2) optical microcopy and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopic (SEM-EDX) used to identify subfossil microbial morphologies (filaments) and define their in situ mineralogy and chemical composition which included bioessential elements; and (3) Raman and infrared spectroscopies used to detect the carbonaceous nature of the fossil microbes. This study allowed the characterization of sub-fossilized traces of past life and the identification of the microbial community inside rocks in fossilization phase.