Modern Subsurface Bacteria in Pristine 2.7 Ga-Old Fossil Stromatolite Drillcore Samples from the Fortescue Group, Western Australia

Several abiotic processes leading to the formation of life-like signatures or later contamination with actual biogenic traces can blur the interpretation of the earliest fossil record. In recent years, a large body of evidence showing the occurrence of diverse and active microbial communities in the terrestrial subsurface has accumulated. Considering the time elapsed since Archaean sedimentation, the contribution of subsurface microbial communities postdating the rock formation to the fossil biomarker pool and other biogenic remains in Archaean rocks may be far from negligible.In order to evaluate the degree of potential contamination of Archean rocks by modern microorganisms, we looked for the presence of living indigenous bacteria in fresh diamond drillcores through 2,724 Myr-old stromatolites (Tumbiana Formation, Fortescue Group, Western Australia) using molecular methods based on the amplification of small subunit ribosomal RNA genes (SSU rDNAs). We analyzed drillcore samples from 4.3 m and 66.2 m depth, showing signs of meteoritic alteration, and also from deeper "fresh" samples showing no apparent evidence for late stage alteration (68 m, 78.8 m, and 99.3 m). We also analyzed control samples from drilling and sawing fluids and a series of laboratory controls to establish a list of potential contaminants introduced during sample manipulation and PCR experiments. We identified in this way the presence of indigenous bacteria belonging to Firmicutes, Actinobacteria, and Alpha-, Beta-, and Gammaproteobacteria in aseptically-sawed inner parts of drillcores down to at least 78.8 m depth.The presence of modern bacterial communities in subsurface fossil stromatolite layers opens the possibility that a continuous microbial colonization had existed in the past and contributed to the accumulation of biogenic traces over geological timescales. This finding casts shadow on bulk analyses of early life remains and makes claims for morphological, chemical, isotopic, and biomarker traces syngenetic with the rock unreliable in the absence of detailed contextual analyses at microscale

Prokaryotic and Eukaryotic Community Structure in Field and Cultured Microbialites from the Alkaline Lake Alchichica (Mexico)

The geomicrobiology of crater lake microbialites remains largely unknown despite their evolutionary interest due to their resemblance to some Archaean analogs in the dominance of in situ carbonate precipitation over accretion. Here, we studied the diversity of archaea, bacteria and protists in microbialites of the alkaline Lake Alchichica from both field samples collected along a depth gradient (0–14 m depth) and long-term-maintained laboratory aquaria. Using small subunit (SSU) rRNA gene libraries and fingerprinting methods, we detected a wide diversity of bacteria and protists contrasting with a minor fraction of archaea. Oxygenic photosynthesizers were dominated by cyanobacteria, green algae and diatoms. Cyanobacterial diversity varied with depth, Oscillatoriales dominating shallow and intermediate microbialites and Pleurocapsales the deepest samples. The early-branching Gloeobacterales represented significant proportions in aquaria microbialites. Anoxygenic photosynthesizers were also diverse, comprising members of Alphaproteobacteria and Chloroflexi. Although photosynthetic microorganisms dominated in biomass, heterotrophic lineages were more diverse. We detected members of up to 21 bacterial phyla or candidate divisions, including lineages possibly involved in microbialite formation, such as sulfate-reducing Deltaproteobacteria but also Firmicutes and very diverse taxa likely able to degrade complex polymeric substances, such as Planctomycetales, Bacteroidetes and Verrucomicrobia. Heterotrophic eukaryotes were dominated by Fungi (including members of the basal Rozellida or Cryptomycota), Choanoflagellida, Nucleariida, Amoebozoa, Alveolata and Stramenopiles. The diversity and relative abundance of many eukaryotic lineages suggest an unforeseen role for protists in microbialite ecology. Many lineages from lake microbialites were successfully maintained in aquaria. Interestingly, the diversity detected in aquarium microbialites was higher than in field samples, possibly due to more stable and favorable laboratory conditions. The maintenance of highly diverse natural microbialites in laboratory aquaria holds promise to study the role of different metabolisms in the formation of these structures under controlled conditions

A trace element study of siderite-jasper banded iron formation in the 3.45 Ga Warrawoona Group, Pilbara Craton - Formation from hydrothermal fluids and shallow seawater

Shale-normalised rare earth element and yttrium (REE + Y) patterns for siderite-jasper couples in a banded iron formation of the 3.45 Ga Panorama Formation, Warrawoona Group, eastern Pilbara Craton, display distinct positive Y and Eu anomalies and weak positive La and Gd anomalies, combined with depleted light REE relative to middle and heavy REE. Ambient seawater and hydrothermal fluids are identified as major sources of REE + Y for the BIF. In the case of siderites, strong correlations between incompatible trace elements and trace element ratios diagnostic of seawater indicate variable input from a terrigenous source (e.g. volcanic ash). We propose a volcanic caldera setting as a likely depositional environment where jasper and siderite precipitated as alternating bands in response to episodic changes in ambient water chemistry. The episodicity was either driven by fluctuations in the intensity of hydrothermal activity or changes in magma chamber activity, which in turn controlled relative sea level. In this context, precipitation of jasper probably reflects background conditions during which seawater was saturated in silica due to evaporative conditions, while siderites were deposited most likely during intermittent periods of enhanced volcanic activity when seawater was more acidic due to the release of exhalative phases (e.g. CO2). © 2005 Elsevier B.V. All rights reserved

Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures

Biological activity is a major factor in Earth\u27s chemical cycles, including facilitating CO2 sequestration and providing climate feedbacks. Thus a key question in Earth\u27s evolution is when did life arise and impact hydrosphere-atmosphere-lithosphere chemical cycles? Until now, evidence for the oldest life on Earth focused on debated stable isotopic signatures of 3,800-3,700 million year (Myr)-old metamorphosed sedimentary rocks and minerals1,2 from the Isua supracrustal belt (ISB), southwest Greenland3. Here we report evidence for ancient life from a newly exposed outcrop of 3,700-Myr-old metacarbonate rocks in the ISB that contain 1-4-cm-high stromatolites-macroscopically layered structures produced by microbial communities. The ISB stromatolites grew in a shallow marine environment, as indicated by seawater-like rare-earth element plus yttrium trace element signatures of the metacarbonates, and by interlayered detrital sedimentary rocks with cross-lamination and storm-wave generated breccias. The ISB stromatolites predate by 220 Myr the previous most convincing and generally accepted multidisciplinary evidence for oldest life remains in the 3,480-Myr-old Dresser Formation of the Pilbara Craton, Australia4,5. The presence of the ISB stromatolites demonstrates the establishment of shallow marine carbonate production with biotic CO2 sequestration by 3,700 million years ago (Ma), near the start of Earth\u27s sedimentary record. A sophistication of life by 3,700 Ma is in accord with genetic molecular clock studies placing life\u27s origin in the Hadean eon (\u3e4,000 Ma)6

Questioning the evidence for Earth's oldest fossils

Structures resembling remarkably preserved bacterial and cyanobacterial microfossils from about 3,465-million-year-old Apex cherts of the Warrawoona Group in Western Australia currently provide the oldest morphological evidence for life on Earth and have been taken to support an early beginning for oxygen-producing photosynthesis. Eleven species of filamentous prokaryote, distinguished by shape and geometry, have been put forward as meeting the criteria required of authentic Archaean microfossils, and contrast with other microfossils dismissed as either unreliable or unreproducible. These structures are nearly a billion years older than putative cyanobacterial biomarkers, genomic arguments for cyanobacteria, an oxygenic atmosphere and any comparably diverse suite of microfossils. Here we report new research on the type and re-collected material, involving mapping, optical and electron microscopy, digital image analysis, micro-Raman spectroscopy and other geochemical techniques. We reinterpret the purported microfossil-like structure as secondary artefacts formed from amorphous graphite within multiple generations of metalliferous hydrothermal vein chert and volcanic glass. Although there is no support for primary biological morphology, a Fischer--Tropsch-type synthesis of carbon compounds and carbon isotopic fractionation is inferred for one of the oldest known hydrothermal systems on Earth

Sulfidization of 3.48 billion-year-old stromatolites of the Dresser Formation, Pilbara Craton: Constraints from in-situ sulfur isotope analysis of pyrite

This study reports in–situ sulfur isotope analyses (32S, 33S, 34S and 36S) of pyrite in strongly sulfidized stromatolites from the ~3.48 billion–year–old Dresser Formation, Pilbara Craton, Australia. These data shed light on sulfur reservoirs and sulfide precipitation processes and provide clues for the contribution of sulfur–cycling microbes to sulfidization. Sulfur isotope signatures derived from mass dependent fractionation (MDF; monitored by δ34S) and mass independent fractionation (MIF; here Δ33S and Δ36S) of pyrite in stromatolites, and of microscopic pyrite within associated barite, allow for the identification of distinctive sulfur sources: i) magmatic–hydrothermal sulfide (H2S) with δ34S and Δ33S ~ 0%; ii) magmatic–hydrothermal sulfate (SO42−) with a MDF signature (MDF–SO42−; δ34S ~ 10‰ and Δ33S ~ 0‰; iii) photochemically–derived sulfate with a MIF signature (MIF–SO4; δ34S ~ −6‰ and Δ33S ~ −3.0‰); iv) photochemically–derived elemental sulfur (S0) with δ34S ≪ 0 and Δ33S ≫ 0‰. The sulfur isotope data suggest that sulfidization was largely driven by reduction of intermixed MDF–SO42− and MIF–SO42− (bulk signature of δ34S ~ 5‰ and Δ33S ~ −1.4‰), and dilution of produced H2S (δ34S ~ −12‰ and Δ33S ~ −1.4‰) by native H2S in magmatic–hydrothermal fluids. The δ34S shifts (up to ~17‰) generated by sulfate reduction are consistent with both thermochemical reactions and influence of sulfate–cycling microbes, the latter which may have facilitated rapid pyrite precipitation and preservation of microbial remains that are entombed within the petrogenetically earliest pyrite generation of stromatolites. Collectively, our data are consistent with ancient stromatolite growth in proximity to shallow marine hydrothermal vents, where hydrothermal fluids contributed to sulfidization that may have been further influenced by sulfur–cycling microbes

Influence of Hadean crust evident in basalts and cherts from the Pilbara Craton

Application of the 147Sm–143Nd and 146Sm–142Nd chronometers has suggested that the initial differentiation of Earth’s mantle into enriched and depleted reservoirs may have begun within the first 100–200 million years of Earth’s history1. However, little is known about the differentiation of the early crust; although evidence has suggested the presence of enriched crustal material2, 3, 4, 5, data regarding the nature and composition of this crust are limited. Here we present 147Sm–143Nd data from the weakly metamorphosed basalt and layered chert–barite successions from the Dresser Formation of the Pilbara Craton, Western Australia. The Sm–Nd isochron indicates an age of 3.49±0.10 billion years, in agreement with previous estimates from Pb–Pb (ref. 6) and U–Pb (ref. 7) dating, which indicates that the Sm–Nd system has not been reset. Our measured εNd value of −3.3±1.0 for the rocks at this site is consistent with formation from an older protolith. On the basis of our modelling of trace element and isotopic compositions from these rocks, we suggest that the older component was crustal in nature, and differentiated from the convective mantle more than 4.3 billion years ago