Abstract
The microbial communities inferred in silica sinter rocks, based on multiscale morphological features (fabrics and textures) and the presence of lipid biomarkers and their carbon isotopic composition, are evaluated in the Krýsuvík geothermal area of Iceland. Close to vent environments (T > 75 °C and pH 1.7‒3), stream floors are capped with homogeneous vitreous crusts and breccia levels, with no distinct recognizable silicified microbes. About 4 m far from the vents (T 75‒60 °C and pH 3‒6) and beyond (T < 60 °C and pH 6‒7.6), microbial sinters, including wavy and palisade laminated and bubble fabrics, differ between abandoned meanders and desiccated ponds. Fabric and texture variances are related to changes in the ratio of filament/coccoid silicified microbes and associated porosity. Coatings of epicellular silica, less than 2 µm thick, favor identification of individual microbial filaments, whereas coalescence of opal spheres into agglomerates precludes recognition of original microbial textures and silicified microbes. Episodic fluctuations in the physico-chemical conditions of surface waters controlled the acidic hydrolysis of biomarkers. Wavy laminated fabrics from pond margins comprise fatty acids, mono- and dialkyl glycerol, mono- and diethers, monoalkyl glycerol esters and small traces of 10-methyl branched C16 and C18 fatty acids and archaeol, indicative of intergrowths of cyanobacteria, Aquificales, and sulfate reducing bacteria and methanogenic archaea. In contrast, wavy laminated fabrics from abandoned meanders and palisade laminated fabrics from ponds differ in their branched fatty acids and the presence vs. absence of bacteriohopanetetrol, reflecting different cyanobacterial contributions. δ13C values of biomarkers range from −22.7 to −32.9‰, but their values in the wavy (pond) and bubble fabrics have much wider ranges than those of the wavy (meander), palisade, and vitreous fabrics, reflecting dissolved inorganic carbon (DIC) sources and a decrease in 13C downstream outflow channels, with heavier values closer to vents and depleted values in ponds.
Highlights
IntroductionThe Archean microbial record provides physical and biochemical evidence for life on early Earth, but the relationship between microbial morphological taxonomy and microbial molecular composition remains poorly constrained [1,2,3]
This work provides a detailed description of petrographic textures, silicified microbes, lipid biomarkers, and their carbon isotopes of several siliceous sinter fabrics from the Krýsuvík geothermal field, Iceland
The extremely fast silica precipitation rates in the near vent environments, with T > 75 ◦ C and pH 1.7–3, controlled precipitation of stream floors with homogeneous vitreous crusts that prevented the taxonomic identification of microbial morphotypes
Summary
The Archean microbial record provides physical and biochemical evidence for life on early Earth, but the relationship between microbial morphological taxonomy and microbial molecular composition remains poorly constrained [1,2,3]. Silicified microbial communities in present-day hydrothermal systems have been studied as analogues to recognize filamentous and coccoid casts attributed to microbial life, including microbially related structures in terrestrial hot springs that date back as far as 3.5 Ga [4,5]. Methods of interpreting microbial signatures in the rock record mainly rely on the recovery of biosignatures and isotopic datasets. The occurrence and stable carbon isotopic composition (δ13 C) of lipid biomarkers, such as n-alkanes, fatty acids, and n-alcohols, are (if well preserved) potentially geologically long-lived and can be extracted from and analyzed in the geological record [14,15,16,17,18]
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