Biosignatures from extreme environments as targets for astrobiological exploration

This PhD thesis focused on a comparative study of stromatolite occurrences in three highly alkaline, basalt-hosted lacustrine environments: Lake Ashenge (Ethiopia), Lake Abbe (Djibouti) and Carri Laufquen Lakes (Argentina). These ecosystems present occurrences of stromatolites observed to grow along the shorelines of (palaeo)lakes and around basaltic substrates, and may have formed in analogous environmental conditions to Jezero crater, a palaeolacustrine system on Mars. A multi-analytical approach was employed to characterise stromatolite formations. Although each site displays distinct macroscale morphologies, they exhibit comparable micromorphologies and microstructures on a microscale, suggesting shared formation process. We proposed two mechanisms of mineralisation: 1) primary precipitation via nanocrystal aggregation; and 2) early diagenetic replacement. Microscopy and spectroscopy analysis revealed biomineralised remnants of stromatolitic biomass, including microbial mats, filamentous cyanobacterial microfossils, and extracellular polymeric substances (EPS) preserved as Mg-bearing carbonates. EPS played a role in both mechanisms of mineralization. Raman and FTIR combined with 13C-NMR demonstrated the preservation of aromatic and aliphatic organic material within filamentous microstructures, likely derived from primary microbial communities. Morphological and geochemical data suggest that photosynthetic organisms dominated these ecosystems, forming microbial mats and palisade structures, preserved in situ. Mineralogical characterisation indicates that Mg-Al-bearing silicates, including phyllosilicates, are associated with filamentous structures and play crucial role to their preservation. This study introduces a novel analytical approach as the first to apply 13C-NMR to lacustrine stromatolites and, to our knowledge, providing the first detailed description of Lake Ashenge stromatolites. Key findings include insights into the organomineralization mechanisms within EPS biofilm, the relationship between sheaths preservation and Mg-Al-silicates, the influence of basaltic substrates on stromatolite formation, and the evaluation of the study areas as planetary field analogues. Studying terrestrial Martian palaeolake analogues is valuable for understanding microbial ecosystems and their fossilization models, as Martian carbonates are particularly promising for preserving ancient life evidence.