Forthcoming exploration of Mars aims at identifying fossil biosignatures within ancient clay-rich formations. The subsurface of Mars has mostly acted as a giant freezer for the last 4 Gyr, thereby preserving potential remains of martian life. Yet, volcanism and impactors have periodically triggered the circulation of hydrothermal fluids, inevitably causing alteration of potentially fossilised biogenic organic materials. It thus appears crucial to quantify the impact of hydrothermal processes on organic biogeochemical signals in the presence of clay minerals. Here, we submitted RNA to hydrothermal conditions in the presence of Mg-smectites. Results show heterogeneous organo-mineral residues, with sub-micrometric phosphates, carbonates and amorphous silica particles together with Mg-smectites with interlayer spaces saturated by N-rich organic compounds. Although the chemical structure of RNA did not withstand hydrothermal conditions, clay minerals efficiently trapped organic carbon, confirming the relevance of drilling for organic carbon in ancient martian sediments. In addition, the degradation of RNA in the presence of Mg-smectites led to the precipitation of a quite uncommon mineral assemblage that could be seen as a biosignature per se. Martian targets exhibiting this mineral assemblage will thus constitute high priority and highly relevant candidates for sample return.

Interactions between hyperthermophilic archaea and minerals occur in hydrothermal deep-sea vents, one of the most extreme environments for life on Earth. These interactions occur in the internal pores and at surfaces of active hydrothermal chimneys. In this study, we show that, at 85°C, Thermococcales, the predominant hyperthermophilic microorganisms inhabiting hot parts of hydrothermal deep-sea vents, produce greigite nanocrystals (Fe3S4) on extracellular polymeric substances, and that an amorphous iron phosphate acts as a precursor phase. Greigite, although a minor component of chimneys, is a recognized catalyst for CO2 reduction thus implying that Thermococcales may influence the balance of CO2 in hydrothermal ecosystems. We propose that observation of greigite nanocrystals on extracellular polymeric substances could provide a signature of hyperthermophilic life in hydrothermal deep-sea vents.

Expanding our capabilities to unambiguously identify ancient traces of life in ancient rocks requires laboratory experiments to better constrain the evolution of biomolecules during advanced fossilization processes. Here, we submitted RNA to hydrothermal conditions in the presence of a gel of Al-smectite stoichiometry at 200 °C for 20 days. NMR and STXM-XANES investigations revealed that the organic fraction of the residues is no longer RNA, nor the quite homogeneous aromatic-rich residue obtained in the absence of clays, but rather consists of particles of various chemical composition including amide-rich compounds. Rather than the pure clays obtained in the absence of RNA, electron microscopy (SEM and TEM) and diffraction (XRD) data showed that the mineralogy of the experimental residues includes amorphous silica and aluminosilicates mixed together with nanoscales phosphates and clay minerals. In addition to the influence of clay minerals on the degradation of organic compounds, these results evidence the influence of the presence of organic compounds on the nature of the mineral assemblage, highlighting the importance of fine-scale mineralogical investigations when discussing the nature/origin of organo-mineral microstructures found in ancient rocks.

Highlights:• The composition of the gas phase impacts fossilization processes.• Organic compounds are mainly fixed within the interlayer space of the Mg-smectites• Heterogeneous organo-mineral assemblage could potentially be a biosignature

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The most ancient fossil record contains fundamentally important information on both the diversity and disparity of ancient life. Yet this ancient record is not that easy to decode, due to difficulties mainly pertaining to the impact of the geological history. Thus, the convergence of multiple lines of evidence is seen as necessary to build a robust demonstration of the biogenicity of putative traces of life. Yet, we experimentally show here that abiotic organic cell-like microstructures meeting all the criteria of biogenicity may form in cherts under classical conditions of diagenesis. These organic biomorphs produced from a mixture of RNA and quartz in water exposed to temperature and pressure conditions (200°C, ∼15 bars) exhibit morphological, chemical and isotopic signatures typical of organic microfossils. The results of this study exemplify the pitfalls that Archean palaeontologists may encounter when searching for traces of life in ancient rocks.

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