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Critical to the origin of life are the ingredients of life, of course, but also the physical and chemical conditions in which prebiotic chemical reactions can take place. These factors place constraints on the types of Hadean environment in which life could have emerged. Many locations, ranging from hydrothermal vents and pumice rafts, through volcanic-hosted splash pools to continental springs and rivers, have been proposed for the emergence of life on Earth, each with respective advantages and certain disadvantages. However, there is another, hitherto unrecognized environment that, on the Hadean Earth (4.5–4.0 Ga), would have been more important than any other in terms of spatial and temporal scale: the sedimentary layer between oceanic crust and seawater. Using as an example sediments from the 3.5–3.33 Ga Barberton Greenstone Belt, South Africa, analogous at least on a local scale to those of the Hadean eon, we document constant permeation of the porous, carbonaceous, and reactive sedimentary layer by hydrothermal fluids emanating from the crust. This partially UV-protected, subaqueous sedimentary environment, characterized by physical and chemical gradients, represented a widespread system of miniature chemical reactors in which the production and complexification of prebiotic molecules could have led to the origin of life. Key Words: Origin of life—Hadean environment—Mineral surface reactions—Hydrothermal fluids—Archean volcanic sediments. Astrobiology 18, 259–293.

International audience; Arsenic is one of the more widespread toxic trace elements, whose presence in environment is linked either to geological background or human activities. The fate of arsenic in environmental compartments is closely linked to the microbial transformations of the inorganic species AsIII and AsV. In order to monitor the evolution of microbial As-related global activities, a simple batch test has been designed and applied. The principle of the test is based on the monitoring of oxidation of 1 mM AsIII in a basal medium inoculated with environmental samples. Results are interpreted considering of oxidation rate or rate constant, and lapse time. Several phenomena are likely to influence the global oxidation rate, such as the relative activity of diverse oxidizing microbes and the competition between oxidizing and reducing processes, in relation to organic matter bioavailability. AsIII oxidizing activities of microorganisms in eight surface soils from polluted sites were quantified with and without addition of organic substrates to the basal medium. Results suggested that AsIII oxidation rate constant was limited by the low concentration of organic substrate, this limitation being removed by supplying 0.08 g/L of organic carbon. Higher organic carbon input negatively affected AsIII oxidation rate constant. Then, the AsIII oxidizing test was applied to a soil highly polluted by the destruction of chemical weapons, simultaneously with the enumeration of AsIII-oxidizing microbes using the Most Probable Number method. Results suggested that the concentration of AsIII-oxidizing microbes was correlated with the lapse time and not with the oxidation rate. Experiments performed with a pure AsIII oxidizing bacterium confirmed a correlation between the lapse time and initial concentration of active cells, AsIII oxidation being detected when the bacterial concentration was close to 107 cells ml-1. In these conditions, the oxidation rate was independent from bacterial concentration. In a next step, the influence of microbial AsV reduction parameters will be considered.

In the context of astrobiological missions to Mars, the key question is what biosignatures to search for and how? lndigenous Martian organisms, if they existed or still exist, can be classified as extremophile per se. Following this precept the FP7-funded European MASE project (Mars Analogues for Space Exploration} is investigating various aspects of anaerobic life under Mars' extreme envrionmental conditions, including the potential for preservation over long geological time periods of certain strains. In this contribution, we report on the mineralisation and preservation of Yersinia sp. in silica and gypsum, two minerals that have been reported on Mars, in cold and anaerobic conditions, similar to Martian conditions. The organism, polyextremotolerant bacterium Yersinia sp. MASE-LG-1 (hereafter named Yersinia. sp.) was isolated from the lcelandic Graenavatn Lake, an acidic (pH3), cold and oligotrophic volcanic crater lake. These organisms have a strong tolerance to diverse Mars-like stresses (Rettberg et al., 2015). We also studied the effect of physiological status on mineralisation by exposing Yersinia to two common stresses thought to have increased du ring Mars history, desiccation and radiation. The mineralisation process has been studied using microbiological (microbial viability), morphological (scanning and transmission electron microscopy), biochemical (GC-MS, Rock-Eval) and spectroscopic (FTIR and RAMAN spectroscopy) methodologies. Based on these approaches, the potential of mineralised Yersinia sp. cells to be preserved over geological time scales is also discussed. Salient results include the fact that fossilisation in gypsum solutions is slower than in silica; not all cells were mineralised, even after 6-months in the fossilising solutions, although the FTIR, Raman and SOLID biomarker signatures were lost by this time period; Rock-Eval analysis suggests that the kerogen in the fossilised strain may not survive preservation over long geological periods, although carbon molecules preserved in fossil microbial traces up to ~3.45 Ga have been detected in the rock record.

Introduction: Several decades dedicated to the study of Mars has enabled scientists to understand that, during its history, environmental conditions on early Mars strongly contrasted with the present-day conditions, hostile for life. Indeed, previous (Mars Express, Viking…) and more recent (MSL) missions confirmed that liquid water, heat (volcan-ism, hydrothermalism), organic matter, and redox conditions probably occurred on the planet, thus enabling scientists to seriously consider early Mars as being habitable ans suitable for the emergence of Martian life [1]. However, the detection of past life on Mars, if it existed, also requires that biomarkers (i) be preserved over geological time scales and that (ii) they remained detectable. Therefore, as terrestrial analogues for Mars, astrobiologists are addressing questions related to microbial adaptation, lifestyles and survival in extraterrestrial environments [2]. In this context, the European MASE project (Mars Ana-logues for Space Exploration) aims at better understand-ing habitability, microbial lifestyles and biomarker preservation in such environmental analogues. To do this, one of the goals of MASE is to better characterize the evolution and preservation of diverse biomarkers during the microbial fossilization process [3].

International audience; Physico-chemical and biological micro-scale environmental parameters within microbial mats formed in hypersaline conditions favour the precipitation of minerals, such as carbonates. We used optical microscopy and the technique "Fluorescence Induction Relaxation " (FIRe) to differentiate the photosynthetic activity of oxygenic photosynthesisers (cyanobacteria) from anoxygenic photosynthesisers (Chloroflexus-like bacteria, CFB) in samples obtained in 2011. After this preliminary investigation, we characterised the elemental composition of the different species of microorganisms, their extracellular substances (EPS), and the minerals precipitated on their surface. This study was made in-situ by µ-PIXE using the nuclear microprobe of the AIFIRA platform (CEN Bordeaux-Gradignan ; protons of 1.5 or 3MeV). With this microprobe it is possible to map the distribution of elements occurring in quantities down to several ppm, a resolution that is particularly favourable for studying microorganisms. SEM observation of the same zones allowed us to localise exactly the microbial structures (cells, EPS) and minerals analysed by nuclear probe. We were thus able to document the differential S and P concentrations in the different microbial species, the CLB being richer in P. Note that the CLB filaments are < 1 µm in diameter. We were also able to demonstrate the anti-correlation of Ca and Mg in the minerals precipitated directly on the microorganisms and on their EPS. Thus we have shown the utility of these in situ, nano-scale methods in studying microbial structures consisting of different species with different metabolic activitie, and different functional groups on their cell walls and EPS implicated in the bioprecipitation of different kinds of minerals. Such features in ancient microbial mats could aid their interpretation and possibly the distinction between ancient oxygenic and anoxygenic mats.

The main biocalcification events in the phylogenetically distinct taxa Astrosc/era, S. (Acanthochaetetes) and Vace/ etia are described. Each taxon constructs its secondary calcareous skeleton in its own highly specialized way and provides therefore insight in the biocalcification processes of ancient reef constructors like stromatoporoids, chaetetids, and sphinctozoans. peerReviewed