Multiscale Microbial Preservation and Biogeochemical Signals in a Modern Hot-Spring Siliceous Sinter Rich in CO2 Emissions, Krýsuvík Geothermal Field, Iceland

The microbial communities inferred in silica sinter rocks, based on multiscale morphological features (fabrics and textures) the presence of lipid biomarkers their carbon isotopic composition, are evaluated Krýsuvík geothermal area Iceland. Close to vent environments (T > 75 °C pH 1.7‒3), stream floors capped with homogeneous vitreous crusts breccia levels, no distinct recognizable silicified microbes. About 4 m far from vents 75‒60 3‒6) beyond < 60 6‒7.6), sinters, including wavy palisade laminated bubble fabrics, differ between abandoned meanders desiccated ponds. Fabric texture variances related changes ratio filament/coccoid microbes associated porosity. Coatings epicellular silica, less than 2 µm thick, favor identification individual filaments, whereas coalescence opal spheres into agglomerates precludes recognition original textures Episodic fluctuations physico-chemical conditions surface waters controlled acidic hydrolysis biomarkers. Wavy fabrics pond margins comprise fatty acids, mono- dialkyl glycerol, diethers, monoalkyl glycerol esters small traces 10-methyl branched C16 C18 acids archaeol, indicative intergrowths cyanobacteria, Aquificales, sulfate reducing bacteria methanogenic archaea. In contrast, ponds vs. absence bacteriohopanetetrol, reflecting different cyanobacterial contributions. δ13C values range −22.7 −32.9‰, but (pond) have much wider ranges those (meander), palisade, dissolved inorganic (DIC) sources a decrease 13C downstream outflow channels, heavier closer depleted