Microbe-mineral interactions in the microstromatolitic crusts of the lacustrine chimneys and volcanic bedrock of Lake Abhe, Republic of Djibouti

Utilising a multi-analytical approach, we investigated centimetre-scale domical and tabular microstromatolitic encrustations from Lake Abhe, a hyperalkaline and hypersaline environment in the Afar Rift, Djibouti. We explored the complex interactions between microorganisms and sediments, alongside the preservation potential of microfossils in this extreme setting. Using optical and scanning electron microscopy, we found that the microstromatolitic crusts exhibit both abiogenic and biogenic characteristics and fabrics, influenced by the mixing of lacustrine and hydrothermal fluids and microbial activity. Microcolumnar fabrics, composed of micrite and microsparite laminae, likely formed in calm, supersaturated water conditions and were mediated by microbial mineralisation. Crystalline fabrics, on the other hand, appear to originate from inorganic processes, followed by early diagenetic growth of larger crystals of Mg-calcite. Filamentous cyanobacterial sheaths, preserved within the crusts and often perpendicular to laminations, suggest a photosynthetic growth mode. Extracellular polymeric substances played a key role in microbial carbonate formation by providing nucleation sites for mineral precipitation and anchoring microbes to their substrates. Raman microspectroscopy reveals a concentration of carbonaceous materials within the carbonate matrix; these are closely associated with filamentous sheaths, suggesting their biological origin. The preservation of these biogenic components is also linked to the presence of Mg-bearing silicates, which are likely tied to microbial activity and high pH conditions, consistent with observations in other alkaline lacustrine systems. This study offers insights into microbial carbonate formation and biosignature preservation in extreme alkaline environments, contributing to our understanding of early Earth microbial ecosystems