Electron Paramagnetic Resonance Study of a Photosynthetic Microbial Mat and Comparison with Archean Cherts

International audienceOrganic radicals in artificially carbonized biomass dominated by oxygenic and non-oxygenic photosynthetic bacteria, Microcoleus chthonoplastes-like and Chloroflexus-like bacteria respectively, were studied by Electron Paramagnetic Resonance (EPR) spectroscopy. The two bacteria species were sampled in mats from a hypersaline lake. They underwent accelerated ageing by cumulative thermal treatments to induce progressive carbonization of the biological material, mimicking the natural maturation of carbonaceous material of Archean age. For thermal treatments at temperatures higher than 620 °C, a drastic increase in the EPR linewidth is observed in the carbonaceous matter from oxygenic photosynthetic bacteria and not anoxygenic photosynthetic bacteria. This selective EPR linewidth broadening reflects the presence of a catalytic element inducing formation of radical aggregates, without affecting the molecular structure or the microstructure of the organic matter, as shown by Raman spectroscopy and Transmission Electron Microscopy. For comparison, we carried out an EPR study of organic radicals in silicified carbonaceous rocks (cherts) from various localities, of different ages (0.42 to 3.5 Gyr) and having undergone various degrees of metamorphism, i.e. various degrees of natural carbonization. EPR linewidth dispersion for the most primitive samples was quite significant, pointing to a selective dipolar broadening similar to that observed for carbonized bacteria. This surprising result merits further evaluation in the light of its potential use as a marker of past bacterial metabolisms, in particular oxygenic photosynthesis, in Archean cherts

Habitability on Mars from a Microbial Point of View

International audienceExtraterrestrial habitability is a complex notion. We briefly review what is known about the origin of life on Earth, that is, life based on carbon chemistry and water. We then discuss habitable conditions (past and present) for established life and for the survival of microorganisms. Based on these elements, we propose to use the term habitable only for conditions necessary for the origin of life, the proliferation of life, and the survival of life. Not covered by this term would be conditions necessary for prebiotic chemistry and conditions that would allow the recognition of extinct or hibernating life. Finally, we apply this concept to the potential emergence of life on Mars where suitable conditions for life to start, proliferate, and survive have been heterogeneous throughout its history. These considerations have a profound impact on the nature and distribution of eventual traces of martian life, or any precursor, and must therefore inform our search-for-life strategies. Key Words: Mars-- Microbial life--Punctuated habitabilit

Experimental silicification of the extremophilic Archaea Pyrococcus abyssi and Methanocaldococcus jannaschii: applications in the search for evidence of life in early Earth and extraterrestrial rocks

International audienceHydrothermal activity was common on the early Earth and associated micro-organisms would most likely have included thermophilic to hyperthermophilic species. 3.5–3.3 billion-year-old, hydrothermally influenced rocks contain silicified microbial mats and colonies that must have been bathed in warm to hot hydrothermal emanations. Could they represent thermophilic or hyperthermophilic micro-organisms and if so, how were they preserved? We present the results of an experiment to silicify anaerobic, hyperthermophilic micro-organisms from the Archaea Domain Pyrococcus abyssi and Methanocaldococcus jannaschii, that could have lived on the early Earth. The micro-organisms were placed in a silica-saturated medium for periods up to 1 year. Pyrococcus abyssi cells were fossilized but the M. jannaschii cells lysed naturally after the exponential growth phase, apart from a few cells and cell remains, and were not silicified although their extracellular polymeric substances were. In this first simulated fossilization of archaeal strains, our results suggest that differences between species have a strong influence on the potential for different micro-organisms to be preserved by fossilization and that those found in the fossil record represent probably only a part of the original diversity. Our results have important consequences for biosignatures in hydrothermal or hydrothermally influenced deposits on Earth, as well as on early Mars, as environmental conditions were similar on the young terrestrial planets and traces of early Martian life may have been similarly preserved as silicified microfossils

2018 MAX-C/ExoMars Mission: The Orleans Mars-Analogue Rock Collection for Instrument Testing

International audienceIn order to reply to the exobiological goals of the 2018 MAX-C/ExoMars mission, the Orléans-OSUC analogue rock collection and database contains well characterised Mars analogue rocks and minerals for use in instrument testing and in situ missions

Metal cation binding by the hyperthermophilic microorganism, Archaea Methanocaldococcus Jannaschii, and its effects on silicification

International audienceA series of experiments was conducted to determine the capacity of an archaeal strain, Methanocaldococcus jannaschii, to bind metals and to study the effects of metal binding on the subsequent silicification of the microorganisms. The results showed that M. jannaschii can rapidly bind several metal cations (Fe3+, Ca2+, Pb2+, Zn2+, Cu2+). Considering the lack of silicification of this strain without metal binding, these experiments demonstrate that Fe(III) ion binding to the cell wall components was of fundamental importance for successful silicification and, especially, for the excellent preservation of the cell wall. This study brings new elements to the understanding of fossilization processes, showing that the positive effect of Fe(III) on silicification, already known for Bacteria, can also apply to Archaea and that this preliminary binding can be decisive for the subsequent fossilization of these organisms. Knowledge of these mechanisms can be helpful for the search and the identification of microfossils in both terrestrial and extraterrestrials rocks, and in particular on Mars

The International Space Analogue Rock Store (ISAR): A key tool for future planetary exploration.

International audienceIn order to prepare the next in situ space missions we have created a " lithothèque " of analogue rocks for calibrating and testing future (and existing) space flight instruments. This rock collection is called the International Space Analogue Rockstore (ISAR) and is hosted in the CNRS and the Observatoire des Sciences de l'Univers en Region Centre (OSUC) in Orléans. For maximum science return, all instruments on a single mission should ideally be tested with the same suite of relevant analogue materials. The ISAR lithothéque aims to fulfill this role by providing suitable materials to instrument teams [1]. The lithothèque is accompanied by an online database of all relevant structural, textural, and geochemical data (www.isar.cnrs-orleans.fr).The data base will also be available during missions to aid interpretation of data obtained in situ. Mars is the immediate goal for MSL-2011 and the new international Mars 2018 mission. The lithothèque thus presently contains relevant Mars-analogue rock and mineral samples, a preliminary range of which is now available to the scientific community for instrument testing [2]. The preliminary group of samples covers a range of lithologies to be found on Mars, especially those in Noachain/Hesperian terrains where MSL will land (Gale Crater) and where the 2018 landing site will most likely be located. It includes a variety of basalts (tephrite, primitive basalt, silicified basalt; plus cumulates), komatiites, artificially synthesized martian basalts [3], volcanic sands, a banded iron formation, carbonates associated with volcanic lithologies and hydrothermalism, the clay Nontronite, and hydrothermal cherts. Some of the silicified volcanic sands contain traces of early life that are good analogues for potential martian life [4]. [1] Westall F. et al., LPI contribution 1608, 1346, 42nd LPSC, 2011; [2] Bost N. et al., in review (Icarus); [3] Bost N. et al., in review (Meteoritics); [4] Westall et al., 2011, Planetary and Space Science 59. ISAR Team: N. Bost, F. Westall, C; Ramboz, F. Foucher, D. Pullan, T. Zegers, B. Hoffman, F. Rull, J. Bridges, A; Steele, H. Amundsen, R. Barbieri, A. Hubert, B. Cavalazzi, J. Bridges, M. Viso, J. Vago, S. Petit, A. Meunier, I. Fleischer, G. Klingelhöfer, N. Arndt..

Biominéralisation et préservation des traces de vie dans les roches précambriennes

La minéralisation des micro-organismes au cours du Précambrien était différente de celle qui se produit actuellement. A l'Archéen inférieur (> 3,3 Ga), c'est à dire avant l'apparition de micro-organismes utilisant la photosynthèse oxygénique, les cellules étaient généralement silicifiés. Cette silicification massive disparaîtra ensuite totalement. Ainsi, à partir du Protérozoïque, et jusqu'à aujourd'hui, les microfossiles sont majoritairement calcifiés. D'autres types de minéraux, comme les oxydes de fer, peuvent également être impliqués dans la fossilisation, en particulier dans certains environnements tels que les sources hydrothermales. Cette particularité de l'archéen inférieur est-elle uniquement due à des conditions environnementales différentes (les océans de l'époque étaient effectivement saturés en silice, ce qui n'est plus le cas ensuite), ou la composition des parois des micro-organismes préservés a-t-elle pu jouer un rôle ? Et quelle a pu être l'influence des minéraux activement bioprécipités par certains micro-organismes, par rapport à une précipitation purement physico-chimique ? Pour mieux comprendre la préservation des micro-organismes précambriens, différentes approches sont actuellement mises en oeuvre : (1) une approche expérimentale où différentes souches de microorganismes sont fossilisées dans des condition physico-chimiques variables, et ce avec différents minéraux, (2) l'étude in situ de micro-organismes dans les milieux fossilisants actuels, avec pour objectif la compréhension de l'environnement physico-chimique au moment de la fossilisation, et (3) des études in-situ intégrés de microfossiles précambriens grâce à une instrumentation de pointe. Les nanotechnologies ont montré que des microfossiles bien préservés selon des standards optiques ont perdu pratiquement toute leur matière organique. Chez ces derniers, la fossilisation en deux temps (calcification suivie d'une silicification) pourrait expliquer la dégradation importante des parois, qui contraste fortement avec la préservation des parois observées chez des espèces ayant subi une silicification directe. Cependant, les analyses expérimentales ont également montré que certains micro-organismes ne sont absolument pas préservés par des dépôts directs de silice

Preservation and Evolution of Organic Matter During Experimental Fossilisation of the Hyperthermophilic Archaea Methanocaldococcus jannaschii

International audienceIdentification of the earliest traces of life is made difficult by the scarcity of the preserved microbial remains and by the alteration and potential contamination of the organic matter (OM) content of rocks. These factors can confuse interpretations of the biogenicity and syngenicity of fossilised structures and organic molecules found in ancient rocks. In order to improve our knowledge of the fossilisation processes and their effects at the molecular level, we made a preliminary study of the fate of OM during experimental fossilisation. Changes in the composition and quantity of amino acids, monosaccharides and fatty acids were followed with HPLC, GC and GC-MS analyses during 1 year of silicification of the hyperthermophilic Archaea Methanocaldococcus jannaschii. Although the cells themselves did not fossilise and the accompanying extracellular polymeric substances (EPS) did, our analyses showed that the OM initially present in both cells and EPS was uniformly preserved in the precipitated silica, with amino acids and fatty acids being the best preserved compounds. This study thus completes previous data obtained by electron microscopy investigations of simulated microbial fossilisation and can help better identification and interpretation of microbial biosignatures in both ancient rocks and in recent hydrothermal formations and sediments

Biominéralisation et préservation des traces de vie dans les roches précambriennes

La minéralisation des micro-organismes au cours du Précambrien était différente de celle qui se produit actuellement. A l'Archéen inférieur (> 3,3 Ga), c'est à dire avant l'apparition de micro-organismes utilisant la photosynthèse oxygénique, les cellules étaient généralement silicifiés. Cette silicification massive disparaîtra ensuite totalement. Ainsi, à partir du Protérozoïque, et jusqu'à aujourd'hui, les microfossiles sont majoritairement calcifiés. D'autres types de minéraux, comme les oxydes de fer, peuvent également être impliqués dans la fossilisation, en particulier dans certains environnements tels que les sources hydrothermales. Cette particularité de l'archéen inférieur est-elle uniquement due à des conditions environnementales différentes (les océans de l'époque étaient effectivement saturés en silice, ce qui n'est plus le cas ensuite), ou la composition des parois des micro-organismes préservés a-t-elle pu jouer un rôle ? Et quelle a pu être l'influence des minéraux activement bioprécipités par certains micro-organismes, par rapport à une précipitation purement physico-chimique ? Pour mieux comprendre la préservation des micro-organismes précambriens, différentes approches sont actuellement mises en oeuvre : (1) une approche expérimentale où différentes souches de microorganismes sont fossilisées dans des condition physico-chimiques variables, et ce avec différents minéraux, (2) l'étude in situ de micro-organismes dans les milieux fossilisants actuels, avec pour objectif la compréhension de l'environnement physico-chimique au moment de la fossilisation, et (3) des études in-situ intégrés de microfossiles précambriens grâce à une instrumentation de pointe. Les nanotechnologies ont montré que des microfossiles bien préservés selon des standards optiques ont perdu pratiquement toute leur matière organique. Chez ces derniers, la fossilisation en deux temps (calcification suivie d'une silicification) pourrait expliquer la dégradation importante des parois, qui contraste fortement avec la préservation des parois observées chez des espèces ayant subi une silicification directe. Cependant, les analyses expérimentales ont également montré que certains micro-organismes ne sont absolument pas préservés par des dépôts directs de silice

Cathodoluminescence : an imaging technique for the search of extraterrestrial life

International audienceSolids irradiated by a 10-20 keV electron beam emit ligth in the UV-visible range, which is called cathodoluminescence (CL). CL imagery is a powerful tool for visualizing minerals and their internal structures (lattice defects, zoning). For example, terrestrial calcite, either of sedimentary or biogenic origin, often display a bright orange CL, as a result of the incorporation of trace Mn2+ in its lattice. Aragonite can also be discriminated from calcite by its green CL. Carbonates are a major target for the search of life on Mars, and CL imagery could contribute to reveal carbonates in situ. Thomas et al. [1] have validated the concept of an electron lamp to make CL imagery of a rock surface placed in a martian CO2 atmosphere. We present 2 examples of terrestrial bacterial microstructures that are revealed by CL. (1) In Sinemurian sediments from the Montmiral borehole (Valence Basin, France), banded wavy calcite in contact with pyrite represents fossilized biofilms of sulfato-reducing bacteria, as confirmed by the sulfur isotopic composition of pyrite ~+36 %0 PDB. (2) At l'Ile Crémieux, north of the Valence basin, a dense filamentous microbial/fungal community with a bright orange CL signature is embedded in vuggy calcite from a tectonic vein. The mat is anchored 1-2 mm deep in the oolitic veinwall and emerges at right angle in the 'open' fracture space. Finally, carbonate vesicles and exhalite crusts from the Svalbard basalt in Groendland, with orange CL, are shown as analogues to carbonates from the martian ALH84001 igneous meteorite. [1]Thomas et al. (2009) in A. Gucsik (Ed.) "Cathodoluminescence and Its Application in the Planetary Sciences