Search

This contribution reviews microbial microfacies recognisable in vertical sections through modern or fossil matovergrown sediment. Microfacies are products of endobenthic or epibenthic microbial mats interacting with sediment dynamics. Laminae of such mats form during sediment dynamic quiescence by organisation of filaments to an interwoven mat fabrics (binding), and biomass enrichment (growth) by cell replication and the production of extracellular polymeric substances (EPS). Vertical sections through mat-covered sediment may show buried stacks of subrecent mat laminae (biolaminites) that rose from alternating periods of mat development and sediment deposition. Mat laminae that drape ripple marks are visible as sinoidal structures in sediment core or sections. If the sediment-stabilising properties of a microbial mat is overcome by currents or waves of high strengths, cmsize mat fragments (mat chips and roll-ups) are ripped off and redeposited. Intrasedimenary gases, trapped beneath mat layers, may cause a high secondary porosity (sponge pore sand) in the sandy substrates. The internal build-up (microfabrics) of mat laminae is investigated under high magnification. Endobenthic microbial mat fabrics include filamentous and coccoid cells forming a network, EPS, and sedimentary grains. The grains derived from interaction of the endobenthic mats with bed load. Epibenthic microbial mat fabrics may include also silt-size particles syndepositionally enriched by baffling and trapping of suspension load. The fabrics of these mats commonly also include oriented sedimentary grains. These grains (now aligned bedding parallel) were dragged upward from the substrate during the development and growth of the mat. Overall, microbial microfacies provide insight into the sedimentation pattern of the (paleo-)environment and into the types of the substrate-colonising microbial mats. [ABSTRACT FROM AUTHOR]

Venus today is inhospitable at the surface, its average temperature of 750 K being incompatible to the existence of life as we know it. However, the potential for past surface habitability and upper atmosphere (cloud) habitability at the present day is hotly debated, as the ongoing discussion regarding a possible phosphine signature coming from the clouds shows. We review current understanding about the evolution of Venus with special attention to scenarios where the planet may have been capable of hosting microbial life. We compare the possibility of past habitability on Venus to the case of Earth by reviewing the various hypotheses put forth concerning the origin of habitable conditions and the emergence and evolution of plate tectonics on both planets. Life emerged on Earth during the Hadean when the planet was dominated by higher mantle temperatures (by about 200 ∘ C ), an uncertain tectonic regime that likely included squishy lid/plume-lid and plate tectonics, and proto continents. Despite the lack of well-preserved crust dating from the Hadean and Paleoarchean, we attempt to review current understanding of the environmental conditions during this critical period based on zircon crystals and geochemical signatures from this period, as well as studies of younger, relatively well-preserved rocks from the Paleoarchean. For these early, primitive life forms, the tectonic regime was not critical but it became an important means of nutrient recycling, with possible consequences on the global environment in the long-term, that was essential to the continuation of habitability and the evolution of life. For early Venus, the question of stable surface water is closely related to tectonics. We discuss potential transitions between stagnant lid and (episodic) tectonics with crustal recycling, as well as consequences for volatile cycling between Venus' interior and atmosphere. In particular, we review insights into Venus' early climate and examine critical questions about early rotation speed, reflective clouds, and silicate weathering, and summarize implications for Venus' long-term habitability. Finally, the state of knowledge of the Venusian clouds and the proposed detection of phosphine is covered. [ABSTRACT FROM AUTHOR]

The Kaapvaal and Zimbabwe cratons host some of the earliest evidence for life. When compared to the contemporaneous East Pilbara craton, cherts and other metasedimentary horizons in southern Africa preserve traces of life with far greater morphological and geochemical fidelity. In spite of this, most fossiliferous horizons of southern Africa have received relatively limited attention. This review summarises current knowledge regarding the nature of early life and its distribution with respect to environments and ecosystems in the Archaean (>2.5 Ga) of the region, correlating stratigraphic, sedimentological, geochemical and palaeontological understanding. There is abundant and compelling evidence for both anoxygenic photosynthetic and chemosynthetic biomes dominating Palaeoarchaean-Mesoarchaean strata dating back to around 3.5 Ga, and the prevalence of each is tied to palaeoenvironmental parameters deducible from the rock record. Well-developed, large stromatolites characteristic of younger Mesoarchaean-Neoarchaean sequences were probably constructed by oxygenic photosynthesisers. Isotopic evidence from the Belingwe greenstone belt and the Transvaal Supergroup indicates that both a full sulphur cycle and complex nitrogen cycling were in operation by the Mesoarchaean-Neoarchaean. The Archaean geological record of southern Africa is thus a rich repository of information regarding the co-evolving geosphere and biosphere in deep time. [ABSTRACT FROM AUTHOR]

The aim of this article is to provide the reader with an overview of the different possible scenarios for the emergence of life, to critically assess them and, according to the conclusions we reach, to analyze whether similar processes could have been conducive to independent origins of life on the several icy moons of the Solar System. Instead of directly proposing a concrete and unequivocal cradle of life on Earth, we focus on describing the different requirements that are arguably needed for the transition between non-life to life. We approach this topic from geological, biological, and chemical perspectives with the aim of providing answers in an integrative manner. We reflect upon the most prominent origins hypotheses and assess whether they match the aforementioned abiogenic requirements. Based on the conclusions extracted, we address whether the conditions for abiogenesis are/were met in any of the oceanic icy moons. [ABSTRACT FROM AUTHOR]

Critical to the origin of life are the ingredients of life, of course, but also the physical and chemical conditions in which prebiotic chemical reactions can take place. These factors place constraints on the types of Hadean environment in which life could have emerged. Many locations, ranging from hydrothermal vents and pumice rafts, through volcanic-hosted splash pools to continental springs and rivers, have been proposed for the emergence of life on Earth, each with respective advantages and certain disadvantages. However, there is another, hitherto unrecognized environment that, on the Hadean Earth (4.5-4.0 Ga), would have been more important than any other in terms of spatial and temporal scale: the sedimentary layer between oceanic crust and seawater. Using as an example sediments from the 3.5-3.33 Ga Barberton Greenstone Belt, South Africa, analogous at least on a local scale to those of the Hadean eon, we document constant permeation of the porous, carbonaceous, and reactive sedimentary layer by hydrothermal fluids emanating from the crust. This partially UV-protected, subaqueous sedimentary environment, characterized by physical and chemical gradients, represented a widespread system of miniature chemical reactors in which the production and complexification of prebiotic molecules could have led to the origin of life. Key Words: Origin of life-Hadean environment-Mineral surface reactions-Hydrothermal fluids-Archean volcanic sediments. Astrobiology 18, 259-293. [ABSTRACT FROM AUTHOR]

We model the fluids involved in the alteration processes recorded in the Sheepbed Member mudstones of Yellowknife Bay ( YKB), Gale crater, Mars, as revealed by the Mars Science Laboratory Curiosity rover investigations. We compare the Gale crater waters with fluids modeled for shergottites, nakhlites, and the ancient meteorite ALH 84001, as well as rocks analyzed by the Mars Exploration rovers, and with terrestrial ground and surface waters. The aqueous solution present during sediment alteration associated with phyllosilicate formation at Gale was high in Na, K, and Si; had low Mg, Fe, and Al concentrations-relative to terrestrial groundwaters such as the Deccan Traps and other modeled Mars fluids; and had near neutral to alkaline pH. Ca and S species were present in the 10−3 to 10−2 concentration range. A fluid local to Gale crater strata produced the alteration products observed by Curiosity and subsequent evaporation of this groundwater-type fluid formed impure sulfate- and silica-rich deposits-veins or horizons. In a second, separate stage of alteration, partial dissolution of this sulfate-rich layer in Yellowknife Bay, or beyond, led to the pure sulfate veins observed in YKB. This scenario is analogous to similar processes identified at a terrestrial site in Triassic sediments with gypsum veins of the Mercia Mudstone Group in Watchet Bay, UK. [ABSTRACT FROM AUTHOR]

Raman spectroscopy can be used for analysing both mineral and organic phases, thus allowing characterisation of the microbial-scale geological context as well as the search for possible traces of life. This method is therefore very useful for in situ planetary exploration missions. Compared with the myriad of sample preparation techniques available in terrestrial laboratories, the possibilities for sample preparation during in situ missions on other planetary bodies are extremely limited and are generally restricted to abrasion of rock surfaces or crushing of the target samples. Whereas certain techniques need samples to be prepared in powder form, such as X-ray diffraction, this kind of preparation is not particularly suitable for optical microscopy and/or Raman spectroscopy. In this contribution, we examine the effects of powdering rock and mineral samples on optical observations and Raman analyses. We used a commercial Raman spectrometer, as well as a Raman laser spectrometer that simulates the instrument being developed for the future ExoMars 2018 mission. The commercial Raman spectrometer documents significant modifications to the spectra of the powdered samples, including broadening of the peaks and shifts in their position, as well as the appearance of new peaks. These effects are caused by localised heating of the sample under the laser beam and amplification of nominal surface effects due to the increase in surface area in finer grain sizes. However, most changes observed in the Raman spectra using the Raman laser spectrometer system are negligible because the relatively large (50 µm diameter) laser spot size produces lower irradiance. Furthermore, minor phases were more easily detectable in the powdered samples. Most importantly, however, this sample preparation method results in the loss of the textural features and context, making identification of potential fossilized microbial remains more problematic. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The role of viruses at different stages of the origin of life has recently been reconsidered. It appears that viruses may have accompanied the earliest forms of life, allowing the transition from an RNA to a DNA world and possibly being involved in the shaping of tree of life in the three domains that we know presently. In addition, a large variety of viruses has been recently identified in extreme environments, hosted by extremophilic microorganisms, in ecosystems considered as analogues to those of the early Earth. Traces of life on the early Earth were preserved by the precipitation of silica on the organic structures. We present the results of the first experimental fossilisation by silica of viruses from extremophilic Archaea (SIRV2 - Sulfolobus islandicus rod-shaped virus 2, TPV1 - Thermococcus prieurii virus 1, and PAV1 - Pyrococcus abyssi virus 1). Our results confirm that viruses can be fossilised, with silica precipitating on the different viral structures (proteins, envelope) over several months in a manner similar to that of other experimentally and naturally fossilised microorganisms. This study thus suggests that viral remains or traces could be preserved in the rock record although their identification may be challenging due to the small size of the viral particles. [ABSTRACT FROM AUTHOR]

Hydrothermal activity was common on the early Earth and associated micro-organisms would most likely have included thermophilic to hyperthermophilic species. 3.5–3.3 billion-year-old, hydrothermally influenced rocks contain silicified microbial mats and colonies that must have been bathed in warm to hot hydrothermal emanations. Could they represent thermophilic or hyperthermophilic micro-organisms and if so, how were they preserved? We present the results of an experiment to silicify anaerobic, hyperthermophilic micro-organisms from the Archaea Domain Pyrococcus abyssi and Methanocaldococcus jannaschii, that could have lived on the early Earth. The micro-organisms were placed in a silica-saturated medium for periods up to 1 year. Pyrococcus abyssi cells were fossilized but the M. jannaschii cells lysed naturally after the exponential growth phase, apart from a few cells and cell remains, and were not silicified although their extracellular polymeric substances were. In this first simulated fossilization of archaeal strains, our results suggest that differences between species have a strong influence on the potential for different micro-organisms to be preserved by fossilization and that those found in the fossil record represent probably only a part of the original diversity. Our results have important consequences for biosignatures in hydrothermal or hydrothermally influenced deposits on Earth, as well as on early Mars, as environmental conditions were similar on the young terrestrial planets and traces of early Martian life may have been similarly preserved as silicified microfossils. [ABSTRACT FROM AUTHOR]

Secondary minerals near and within fractures in Columbia River basalts contain objects the size and shape of bacteria. These bacteriomorphs are most commonly rods or ellipses but also include cocci and diplococci forms, vibrioids and club-shaped rods, and associated pairs of objects that suggest cellular division by binary fission. Secondary minerals associated with, enclosing, and making up bacteriomorphs include iron oxyhydroxides, sulfides, and smectites containing ferrous iron. The secondary minerals are intimately intermixed with kerogen. Moreover, bacteriomorphs in the pyrite consist of kerogen. Careful consideration of mineral associations, the occurrence of organic carbon, and the spatial context of bacteriomorphs indicate that they are microfossils. The association of microfossils with minerals formed in reducing environments suggests an ancient ecosystem dominated at least in part by sulfate-reducing bacteria, similar to communities within these basalts today. [ABSTRACT FROM AUTHOR]

Sediments and diatoms from the mudflats of the Bay of Borgneuf in western France were examined in an electron microscope study of biofilm and microbial mats. The sediments biofilm was composed of exopolymeric substances (EPS), incorporated clay particles and, rarely, bacteria. This film coated all particles at the sediment-water interface. Its surface morphology reflected its composition and internal structure. Thin films were smooth, whilst a lumpy structure of incorporated fibrils produced either a mammillated or ropy surface, and clays in the structure gave rise to a flaky morphology. AT shallow depths in the sediment column (0·5 cm) the biofilm was already degraded. The biofilm coating degraded diatom frustules in the benethic diatom culture consisted of EPS and bacteria and presented a ragged appearance. Microbial mats occurred on the surface of the fresh littoral sediments as well as those in the aquarium, and on the wall of the aquarium. The mat on the surface of the aquarium sediments had an open structure with webs of fibrils and bacteria in the pore space. It formed in a relatively quiet environment. Pore space was more limited in the mat from the surface of the fresh litoral sediments, in which direct contact between biofilm coated particles was common. In the exposed environment of the aquarium wall there was a thick, resistant coating of EPS. In addition to binding particles together, the presence of mats and biofilms in sediments affects sediment physical properties such as porosity and permeability, the flux of dissolved substances in pore waters and the dissolution of particles and can, therefore, influence early diagenesis. Mats and biofilms seem to be more readily preserved in the geological record than the micro-organisms, such as bacteria, which produce them. Their identification in the sedimentary record would greatly aid interpretation of sediment genesis and evaluation of the microbial role in sediment formation.

We describe three space experiments designed to expose to space conditions, and more specifically to solar UV radiation, selected samples of organic and mineral material. [ABSTRACT FROM PUBLISHER]

Paleoarchean stromatolites are among the oldest compelling evidence for life. We present advanced two- and three-dimensional (2-D and 3-D) reconstructions of the morphology, mineralogy, trace element geochemistry, and taphonomy of permineralized stromatolites from the lowermost horizons of the ca. 3.5 Ga Dresser Formation, Pilbara, Western Australia. Rare earth element plus yttrium compositions suggest a restricted paleodepositional setting influenced by marine influxes; this contrasts with other Dresser stromatolites, which developed around terrestrial hot springs. Mineral phase relationships and positive Eu anomalies denote syndepositional hydrothermal influence and silicification promoting high-fidelity microstructural preservation. Although no primary kerogen is preserved, numerous 2-D and 3-D morphological characteristics denote a biogenic origin, including the onlap of sedimentary layers onto stromatolitic topography, fine-scale undulatory laminations, non-isopachous laminations with crestal thickening, laminoid fenestrae, and subvertical pillar-like fabrics interpreted as microbial palisade structure; these features suggest that the stromatolite ecosystem was dominantly phototrophic. The deep iron-rich weathering profile of the Dresser stromatolites makes them pertinent analogues for potential microbialites in altered carbonates on Mars. Were similar putative biogenic macro-, meso- and micromorphologies identified in habitable Martian settings by rover imaging systems, such materials would be compelling targets for sample return. [ABSTRACT FROM AUTHOR]

The fossilised soft tissues of a tadpole and an associated coprolitic structure from the organic-rich volcanoclastic lacustrine Upper Oligocene Enspel sediments (Germany) were investigated using high-resolution imaging techniques and nondestructive in situ surface analysis. Total organic carbon analysis of the coprolite and the sediment revealed values of 28.9 and 8.9% respectively. The soft tissues from the tadpole and the coprolite were found to be composed of 0.5 to 1 μm-sized spheres and rod shapes. These features are interpreted as the fossil remains of bacterial biofilms consisting probably of heterotrophic bacteria and fossilised extracellular polymeric substances. They became fossilised while in the process of degrading the organic matter of the organism and the coprolite. Comparison with a modern marine biofilm revealed morphologic details identical to those observed in the fossil bacterial biofilms. Although the fossil biofilms on both macrofossils exhibited identical microtextures, their mode of preservation was inhomogeneous and varied between calcium phosphate and an as yet unidentified mineral phase consisting mainly of Si, Ca, Ti, P, and S, but also showing the presence of Mg, Al, and Fe. The coprolite consists purely of fossilised bacterial cells in a densely packed arrangement and associated fossilised extracellular polymeric substances.In addition to preliminary imaging and energy-dispersive X-ray analysis, both the fossil biofilms and the sediment were investigated by nondestructive in situ analysis using time of flight-secondary ion mass spectroscopy (ToF-SIMS). The mass spectra obtained on the coprolite in mass-resolved chemical mapping mode allowed the tentative identification of a number of organic secondary ion species. Some spectra appear to indicate the presence of bacterial hopanoids, but further work using standard techniques such as gas chromatography mass spectroscopy is needed to conclusively verify the presence of these substances. Nevertheless, ToF-SIMS chemical maps were successfully correlated with electron microscopy images, allowing the correlation of molecular spectra, the spatial distribution of individual organic species, and specific morphologic features to demonstrate the potential of this approach in the analysis of microfossils. [Copyright &y& Elsevier]