Catalogue Search | MBRL
Are you sure you want to remove the book from the shelf?
{{itemTitle}}
74
result(s) for
"Sarah Stewart Johnson"
Sort by:
The sirens of Mars : searching for life on another world
Right now, spacecraft are circling Mars, sweeping over Terra Sabaea, Syrtis Major, the dunes of Elysium and Mare Sirenum - on the brink, perhaps, of a discovery that would inspire humankind. With poetic precision, Sarah Stewart Johnson traces the evocative history of our explorations of Mars, interlacing her personal journey as a scientist with tales of other seekers who have scoured the planet for signs of life. Ultimately, she shows how its story is also a story about Earth: it is a mirror, a tell-tale reflection of our own anxieties and yearnings to find - if we're lucky - that we're not alone.
Book
Fate of SO2 in the ancient Martian atmosphere: Implications for transient greenhouse warming
There is increasing evidence that sulfur played an important role on early Mars. Sulfur is distributed ubiquitously on the Martian surface, and sulfur in Martian meteorites carries the signature of atmospheric interactions. Recent work suggests that the radiative properties of sulfur volatiles that were degassed into the Martian atmosphere may have caused a greenhouse effect early in the planet's history. It remains unclear, however, over what timescales warming from sulfur volatiles would have persisted, and consequently how significant this warming may have been. While most photochemistry research to date has concentrated on current Martian conditions, the ancient Martian atmosphere was thicker, warmer, and more reducing than the current regime. Here we investigate sulfur photochemistry in a 500 mb ancient Martian atmosphere. After adapting a model used to study sulfur photochemistry on Earth during the Archean, we find a short lifetime for SO2 in the current Martian atmosphere, similar to results of other photochemical studies. However, our simulations suggest that moderate mixing ratios of SO2 (10−8 ≤ f(SO2) ≤ 10−6) could have persisted in the ancient Martian atmosphere for hundreds of years, generating short but potent warming events following episodes of volcanic activity.
Journal Article
The Martian subsurface as a potential window into the origin of life
Few traces of Earth’s geologic record are preserved from the time of life’s emergence, over 3,800 million years ago. Consequently, what little we understand about abiogenesis — the origin of life on Earth — is based primarily on laboratory experiments and theory. The best geological lens for understanding early Earth might actually come from Mars, a planet with a crust that’s overall far more ancient than our own. On Earth, surface sedimentary environments are thought to best preserve evidence of ancient life, but this is mostly because our planet has been dominated by high photosynthetic biomass production at the surface for the last ~2,500 million years or more. By the time oxygenic photosynthesis evolved on Earth, Mars had been a hyperarid, frozen desert with a surface bombarded by high-energy solar and cosmic radiation for more than a billion years, and as a result, photosynthetic surface life may never have occurred on Mars. Therefore, one must question whether searching for evidence of life in Martian surface sediments is the best strategy. This Perspective explores the possibility that the abundant hydrothermal environments on Mars might provide more valuable insights into life’s origins.
Journal Article
Survival strategies of an anoxic microbial ecosystem in Lake Untersee, a potential analog for Enceladus
Lake Untersee located in Eastern Antarctica, is a perennially ice-covered lake. At the bottom of its southern basin lies 20 m of anoxic, methane rich, stratified water, making it a good analog for Enceladus, a moon of Saturn. Here we present the first metagenomic study of this basin and detail the community composition and functional potential of the microbial communities at 92 m, 99 m depths and within the anoxic sediment. A diverse and well-populated microbial community was found, presenting the potential for Enceladus to have a diverse and abundant community. We also explored methanogenesis, sulfur metabolism, and nitrogen metabolism, given the potential presence of these compounds on Enceladus. We found an abundance of these pathways offering a variety of metabolic strategies. Additionally, the extreme conditions of the anoxic basin make it optimal for testing spaceflight technology and life detection methods for future Enceladus exploration.
Journal Article
Microbial communities and their predicted metabolic functions in a desiccating acid salt lake
The waters of Lake Magic in Western Australia are among the most geochemically extreme on Earth. This ephemeral saline lake is characterized by pH as low as 1.6 salinity as high as 32% total dissolved solids, and unusually complex geochemistry, including extremely high concentrations of aluminum, silica, and iron. We examined the microbial composition and putative function in this extreme acid brine environment by analyzing lake water, groundwater, and sediment samples collected during the austral summer near peak evapoconcentration. Our results reveal that the lake water metagenome, surprisingly, was comprised of mostly eukaryote sequences, particularly fungi and to a lesser extent, green algae. Groundwater and sediment samples were dominated by acidophilic Firmicutes, with eukaryotic community members only detected at low abundances. The lake water bacterial community was less diverse than that in groundwater and sediment, and was overwhelmingly represented by a single OTU affiliated with Salinisphaera. Pathways associated with halotolerance were found in the metagenomes, as were genes associated with biosynthesis of protective carotenoids. During periods of complete desiccation of the lake, we hypothesize that dormancy and entrapment in fluid inclusions in halite crystals may increase long-term survival, leading to the resilience of complex eukaryotes in this extreme environment.
Journal Article
Insights from the Metagenome of an Acid Salt Lake: The Role of Biology in an Extreme Depositional Environment
The extremely acidic brine lakes of the Yilgarn Craton of Western Australia are home to some of the most biologically challenging waters on Earth. In this study, we employed metagenomic shotgun sequencing to generate a microbial profile of the depositional environment associated with the sulfur-rich sediments of one such lake. Of the 1.5 M high-quality reads generated, 0.25 M were mapped to protein features, which in turn provide new insights into the metabolic function of this community. In particular, 45 diverse genes associated with sulfur metabolism were identified, the majority of which were linked to either the conversion of sulfate to adenylylsulfate and the subsequent production of sulfide from sulfite or the oxidation of sulfide, elemental sulfur, and thiosulfate via the sulfur oxidation (Sox) system. This is the first metagenomic study of an acidic, hypersaline depositional environment, and we present evidence for a surprisingly high level of microbial diversity. Our findings also illuminate the possibility that we may be meaningfully underestimating the effects of biology on the chemistry of these sulfur-rich sediments, thereby influencing our understanding of past geobiological conditions that may have been present on Earth as well as early Mars.
Journal Article
Geological diversity and microbiological potential of lakes on Mars
Hundreds of ancient lake basins detected on Mars via orbital remote sensing represent rare oases of hydrosphere–atmosphere–lithosphere interactions with great astrobiological potential. These palaeolake basins, and associated lacustrine deposits, could preserve evidence of biogenesis on Mars, and their geology, mineralogy and geochemistry place strong constraints on past climate. Most Martian palaeolakes date to the Noachian (>3.7 Gyr ago (Ga)) and probably lasted ~10
2
–10
6
years, representing only a small fraction of the ~400 Myr of Noachian time. However, some palaeolakes occurred during the Hesperian (3–3.7 Ga), and it is likely that many shallow thermokarst lakes occurred in the Amazonian (<3 Ga) but left few traces. Noachian lacustrine deposits contain detrital Fe/Mg-rich clay minerals as well as authigenic Fe/Mg carbonates, sulfates, silica, chlorides and clay minerals that potentially preserve the characteristics of the ancient atmosphere and climate. While Martian palaeolakes are undeniably among the top targets for future surface exploration and sample return, many questions surrounding prospects for biogenesis and biological productivity in short-lived lakes and transient warm climates on an otherwise cold planet remain. Martian lakes also provide tremendous comparative value for reconstructing the geology and geobiology of inland waters on the Archaean Earth.
Mars hosted hundreds of lakes, most of which formed earlier than 3.7 billion years ago and lasted only a limited amount of time. This overview of their characteristics and mineralogy highlights the importance of the Martian lakes as a record of ancient climate and potential for biogenesis.
Journal Article
Radiation Tolerance of Nanopore Sequencing Technology for Life Detection on Mars and Europa
The search for life beyond Earth is a key motivator in space exploration. Informational polymers, like DNA and RNA, are key biosignatures for life as we know it. The MinION is a miniature DNA sequencer based on versatile nanopore technology that could be implemented on future planetary missions. A critical unanswered question is whether the MinION and its protein-based nanopores can withstand increased radiation exposure outside Earth’s shielding magnetic field. We evaluated the effects of ionizing radiation on the MinION platform – including flow cells, reagents, and hardware – and discovered limited performance loss when exposed to ionizing doses comparable to a mission to Mars. Targets with harsher radiation environments, like Europa, would require improved radiation resistance via additional shielding or design refinements.
Journal Article
Ancient bacteria show evidence of DNA repair
Recent claims of cultivable ancient bacteria within sealed environments highlight our limited understanding of the mechanisms behind long-term cell survival. It remains unclear how dormancy, a favored explanation for extended cellular persistence, can cope with spontaneous genomic decay over geological timescales. There has been no direct evidence in ancient microbes for the most likely mechanism, active DNA repair, or for the metabolic activity necessary to sustain it. In this paper, we couple PCR and enzymatic treatment of DNA with direct respiration measurements to investigate long-term survival of bacteria sealed in frozen conditions for up to one million years. Our results show evidence of bacterial survival in samples up to half a million years in age, making this the oldest independently authenticated DNA to date obtained from viable cells. Additionally, we find strong evidence that this long-term survival is closely tied to cellular metabolic activity and DNA repair that over time proves to be superior to dormancy as a mechanism in sustaining bacteria viability.
Journal Article