Carbonate precipitation in acidic environments, a potential biosignature for searching for life on Mars

Sánchez-Román, M.; Fernandez-Remolar, D.; Sanchez-Navas, N.; Romanek, N.; Schmid, T.; Nieto, F.; Oggerin, M.; Rodriguez, N.; Preston, L.; Izawa, M. R.; Southam, G.; Banerjee, N.; Osinski, G.; Dyar, D.; Gomez-Ortiz, D.; Prieto-Ballesteros, O. and Amils, R. (2012). Carbonate precipitation in acidic environments, a potential biosignature for searching for life on Mars. In: Astrobiology Science Conference 2012: Exploring Life: Past, Present, Near and Far, 16-20 Apr 2012, Atlanta, Georgia, USA.


Recent observations of carbonate minerals in ancient Martian rocks and in the extreme environment of Rio Tinto (15-0.1 gL-1 Fe3+; pH 1.5-3) have been interpreted as evidence for the presence of optimal neutral solutions. In order to understand carbonate formation in such extreme environments, we have conducted a geo-biochemical study of Rio Tinto sediment, where carbonates are occurring, and laboratory culture experiments using a bacterial strain isolated from Rio Tinto.We show that carbonates can precipitate in environmental settings and in laboratory culture experiments with acidic to neutral solutions. A multi-technique approach including optical microscopy, SEM-EDS, AFM, TEM, XRD, carbon isotope analyses and Raman and Mössbauer spectroscopy demonstrate that Rio Tinto carbonates are related to microbial activity.
Apparently, under acidic conditions, carbonate precipitation in Rio Tinto is microbially mediated by the neutralization of the solution because of the bacterial degradation of the organic matter and the reduction of ferric iron (Fe3+), as in the culture experiments occur. Bacteria change the physico-chemical conditions (e.g., Fe2+/Fe3+, pH) of their surrounding environment and overcome kinetic barriers to mineral precipitation (e.g., carbonates, sulfates, phosphates). The neutralization of Rio Tinto water occurs in regions where the sediment is rich in organic matter and ferric iron (Fe3+). Although, Mars surface sediments are devoid of organic matter, if life did emerge there, ancient sediments could have been much richer in organic matter than today, providing the carbon source for carbonate minerals precipitation. In conclusion, the formation of carbonates in the extreme environment of Rio Tinto could explain the occurrence of carbonates on Mars. Finally, this environmental and experimental study provides potential mineralogical biosignatures that may be useful to test life on Mars and other extraterrestrial habitats.

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