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Frontiers of Astrobiology
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Frontiers of Astrobiology

A special issue of Life (ISSN 2075-1729). This special issue belongs to the section "Astrobiology".

Deadline for manuscript submissions: closed (15 October 2020) | Viewed by 124854

Special Issue Editor

Prof. Dr. Manasvi Lingam

E-Mail Website
Guest Editor
Department of Aerospace, Physics and Space Sciences, Florida Institute of Technology, Melbourne, FL 32901, USA
Interests: astrobiology; habitability; exoplanets; origin of life; space science; ecology; biophysics; extraterrestrial intelligence

Special Issue Information

Dear Colleagues,

Astrobiology is a famously transdisciplinary field that encompasses the "origin, evolution, distribution, and future of life in the Universe", as described by NASA. This endeavor encompasses fields as diverse as physics and astronomy on the one hand and microbiology, biochemistry, and geobiology on the other. It comes as no surprise, therefore, that astrobiology has witnessed many seminal advances over the past decade, such as the discovery of a temperate planet orbiting the star closest to the Earth, detection of complex organic molecules on several worlds inside our Solar system, breakthroughs in our understanding of prebiotic chemistry and how life may have been initiated on Earth, and noteworthy advances in our understanding of extremophiles and their capacity to survive the harsh environment of outer space.

The next decade appears equally (if not more) promising for astrobiology, with a number of major missions slated to carry out surveys of potentially habitable worlds within and outside the Solar system. Furthermore, the development of more sophisticated laboratory experiments, analytical methods, and numerical models is expected to continue apace. Hence, when viewed collectively, we find ourselves in a better position than ever before to answer that age-old and oft-quoted question, "Are we alone?", and thereby assess the prevalence of life in the Universe.

The objective of this Special Issue is to cover all realms of astrobiology, thus reflecting the inherent diversity of this field. Hence, we welcome all manner of papers—spanning both the physical and biological sciences, and ranging from experimental analyses to theoretical models—that are rigorous, comprehensive, and embody the latest developments in the field. In addition to regular research articles, review papers and hypothesis articles are also solicited for this Special Issue.

Prof. Dr. Manasvi Lingam
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Life is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • origin of life
  • evolution
  • exoplanets
  • extreme environments
  • biogeochemistry
  • prebiotic chemistry
  • habitable worlds
  • limits of life
  • space missions

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Published Papers (17 papers)

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20 pages, 4401 KiB  
Article
Classification of the Biogenicity of Complex Organic Mixtures for the Detection of Extraterrestrial Life
by Nicholas Guttenberg, Huan Chen, Tomohiro Mochizuki and H. James Cleaves
Life 2021, 11(3), 234; https://doi.org/10.3390/life11030234 - 12 Mar 2021
Cited by 11 | Viewed by 6101
Abstract
Searching for life in the Universe depends on unambiguously distinguishing biological features from background signals, which could take the form of chemical, morphological, or spectral signatures. The discovery and direct measurement of organic compounds unambiguously indicative of extraterrestrial (ET) life is a major [...] Read more.
Searching for life in the Universe depends on unambiguously distinguishing biological features from background signals, which could take the form of chemical, morphological, or spectral signatures. The discovery and direct measurement of organic compounds unambiguously indicative of extraterrestrial (ET) life is a major goal of Solar System exploration. Biology processes matter and energy differently from abiological systems, and materials produced by biological systems may become enriched in planetary environments where biology is operative. However, ET biology might be composed of different components than terrestrial life. As ET sample return is difficult, in situ methods for identifying biology will be useful. Mass spectrometry (MS) is a potentially versatile life detection technique, which will be used to analyze numerous Solar System environments in the near future. We show here that simple algorithmic analysis of MS data from abiotic synthesis (natural and synthetic), microbial cells, and thermally processed biological materials (lab-grown organisms and petroleum) easily identifies relational organic compound distributions that distinguish pristine and aged biological and abiological materials, which likely can be attributed to the types of compounds these processes produce, as well as how they are formed and decompose. This method is independent of the detection of particular masses or molecular species samples may contain. This suggests a general method to agnostically detect evidence of biology using MS given a sufficiently strong signal in which the majority of the material in a sample has either a biological or abiological origin. Such metrics are also likely to be useful for studies of possible emergent living phenomena, and paleobiological samples. Full article
(This article belongs to the Special Issue Frontiers of Astrobiology)
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10 pages, 4248 KiB  
Article
Trace Element Concentrations Associated with Mid-Paleozoic Microfossils as Biosignatures to Aid in the Search for Life
by Andrew Gangidine, Malcolm R. Walter, Jeff R. Havig, Clive Jones, Daniel M. Sturmer and Andrew D. Czaja
Life 2021, 11(2), 142; https://doi.org/10.3390/life11020142 - 13 Feb 2021
Cited by 4 | Viewed by 2454
Abstract
Identifying microbial fossils in the rock record is a difficult task because they are often simple in morphology and can be mimicked by non-biological structures. Biosignatures are essential for identifying putative fossils as being definitively biological in origin, but are often lacking due [...] Read more.
Identifying microbial fossils in the rock record is a difficult task because they are often simple in morphology and can be mimicked by non-biological structures. Biosignatures are essential for identifying putative fossils as being definitively biological in origin, but are often lacking due to geologic effects which can obscure or erase such signs. As such, there is a need for robust biosignature identification techniques. Here we show new evidence for the application of trace elements as biosignatures in microfossils. We found elevated concentrations of magnesium, aluminum, manganese, iron, and strontium colocalized with carbon and sulfur in microfossils from Drummond Basin, a mid-Paleozoic hot spring deposit in Australia. Our results also suggest that trace element sequestrations from modern hot spring deposits persist through substantial host rock alteration. Because some of the oldest fossils on Earth are found in hot spring deposits and ancient hot spring deposits are also thought to occur on Mars, this biosignature technique may be utilized as a valuable tool to aid in the search for extraterrestrial life. Full article
(This article belongs to the Special Issue Frontiers of Astrobiology)
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18 pages, 5697 KiB  
Article
Super-Earths, M Dwarfs, and Photosynthetic Organisms: Habitability in the Lab
by Riccardo Claudi, Eleonora Alei, Mariano Battistuzzi, Lorenzo Cocola, Marco Sergio Erculiani, Anna Caterina Pozzer, Bernardo Salasnich, Diana Simionato, Vito Squicciarini, Luca Poletto and Nicoletta La Rocca
Life 2021, 11(1), 10; https://doi.org/10.3390/life11010010 - 24 Dec 2020
Cited by 14 | Viewed by 4851
Abstract
In a few years, space telescopes will investigate our Galaxy to detect evidence of life, mainly by observing rocky planets. In the last decade, the observation of exoplanet atmospheres and the theoretical works on biosignature gasses have experienced a considerable acceleration. The most [...] Read more.
In a few years, space telescopes will investigate our Galaxy to detect evidence of life, mainly by observing rocky planets. In the last decade, the observation of exoplanet atmospheres and the theoretical works on biosignature gasses have experienced a considerable acceleration. The most attractive feature of the realm of exoplanets is that 40% of M dwarfs host super-Earths with a minimum mass between 1 and 30 Earth masses, orbital periods shorter than 50 days, and radii between those of the Earth and Neptune (1–3.8 R⊕). Moreover, the recent finding of cyanobacteria able to use far-red (FR) light for oxygenic photosynthesis due to the synthesis of chlorophylls d and f, extending in vivo light absorption up to 750 nm, suggests the possibility of exotic photosynthesis in planets around M dwarfs. Using innovative laboratory instrumentation, we exposed different cyanobacteria to an M dwarf star simulated irradiation, comparing their responses to those under solar and FR simulated lights. As expected, in FR light, only the cyanobacteria able to synthesize chlorophyll d and f could grow. Surprisingly, all strains, both able or unable to use FR light, grew and photosynthesized under the M dwarf generated spectrum in a similar way to the solar light and much more efficiently than under the FR one. Our findings highlight the importance of simulating both the visible and FR light components of an M dwarf spectrum to correctly evaluate the photosynthetic performances of oxygenic organisms exposed under such an exotic light condition. Full article
(This article belongs to the Special Issue Frontiers of Astrobiology)
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19 pages, 11176 KiB  
Article
In Situ Growth of Halophilic Bacteria in Saline Fracture Fluids from 2.4 km below Surface in the Deep Canadian Shield
by Regina L. Wilpiszeski, Barbara Sherwood Lollar, Oliver Warr and Christopher H. House
Life 2020, 10(12), 307; https://doi.org/10.3390/life10120307 - 24 Nov 2020
Cited by 5 | Viewed by 2923
Abstract
Energy derived from water-rock interactions such as serpentinization and radiolysis, among others, can sustain microbial ecosystems deep within the continental crust, expanding the habitable biosphere kilometers below the earth’s surface. Here, we describe a viable microbial community including sulfate-reducing microorganisms from one such [...] Read more.
Energy derived from water-rock interactions such as serpentinization and radiolysis, among others, can sustain microbial ecosystems deep within the continental crust, expanding the habitable biosphere kilometers below the earth’s surface. Here, we describe a viable microbial community including sulfate-reducing microorganisms from one such subsurface lithoautotrophic ecosystem hosted in fracture waters in the Canadian Shield, 2.4 km below the surface in the Kidd Creek Observatory in Timmins, Ontario. The ancient groundwater housed in fractures in this system was previously shown to be rich in abiotically produced hydrogen, sulfate, methane, and short-chain hydrocarbons. We have further investigated this system by collecting filtered water samples and deploying sterile in situ biosampler units into boreholes to provide an attachment surface for the actively growing fraction of the microbial community. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, and DNA sequencing analyses were undertaken to classify the recovered microorganisms. Moderately halophilic taxa (e.g., Marinobacter, Idiomarina, Chromohalobacter, Thiobacillus, Hyphomonas, Seohaeicola) were recovered from all sampled boreholes, and those boreholes that had previously been sealed to equilibrate with the fracture water contained taxa consistent with sulfate reduction (e.g., Desulfotomaculum) and hydrogen-driven homoacetogenesis (e.g., Fuchsiella). In contrast to this “corked” borehole that has been isolated from the mine environment for approximately 7 years at the time of sampling, we sampled additional open boreholes. The waters flowing freely from these open boreholes differ from those of the long-sealed borehole. This work complements ongoing efforts to describe the microbial diversity in fracture waters at Kidd Creek in order to better understand the processes shaping life in the deep terrestrial subsurface. In particular, this work demonstrates that anaerobic bacteria and known halophilic taxa are present and viable in the fracture waters presently outflowing from existing boreholes. Major cations and anions found in the fracture waters at the 2.4 km level of the mine are also reported. Full article
(This article belongs to the Special Issue Frontiers of Astrobiology)
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24 pages, 3295 KiB  
Article
Six ‘Must-Have’ Minerals for Life’s Emergence: Olivine, Pyrrhotite, Bridgmanite, Serpentine, Fougerite and Mackinawite
by Michael J. Russell and Adrian Ponce
Life 2020, 10(11), 291; https://doi.org/10.3390/life10110291 - 19 Nov 2020
Cited by 22 | Viewed by 7872
Abstract
Life cannot emerge on a planet or moon without the appropriate electrochemical disequilibria and the minerals that mediate energy-dissipative processes. Here, it is argued that four minerals, olivine ([Mg>Fe]2SiO4), bridgmanite ([Mg,Fe]SiO3), serpentine ([Mg,Fe,]2-3Si2O [...] Read more.
Life cannot emerge on a planet or moon without the appropriate electrochemical disequilibria and the minerals that mediate energy-dissipative processes. Here, it is argued that four minerals, olivine ([Mg>Fe]2SiO4), bridgmanite ([Mg,Fe]SiO3), serpentine ([Mg,Fe,]2-3Si2O5[OH)]4), and pyrrhotite (Fe(1−x)S), are an essential requirement in planetary bodies to produce such disequilibria and, thereby, life. Yet only two minerals, fougerite ([Fe2+6xFe3+6(x−1)O12H2(7−3x)]2+·[(CO2−)·3H2O]2−) and mackinawite (Fe[Ni]S), are vital—comprising precipitate membranes—as initial “free energy” conductors and converters of such disequilibria, i.e., as the initiators of a CO2-reducing metabolism. The fact that wet and rocky bodies in the solar system much smaller than Earth or Venus do not reach the internal pressure (≥23 GPa) requirements in their mantles sufficient for producing bridgmanite and, therefore, are too reduced to stabilize and emit CO2—the staple of life—may explain the apparent absence or negligible concentrations of that gas on these bodies, and thereby serves as a constraint in the search for extraterrestrial life. The astrobiological challenge then is to search for worlds that (i) are large enough to generate internal pressures such as to produce bridgmanite or (ii) boast electron acceptors, including imported CO2, from extraterrestrial sources in their hydrospheres. Full article
(This article belongs to the Special Issue Frontiers of Astrobiology)
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60 pages, 7560 KiB  
Article
Macrobiont: Cradle for the Origin of Life and Creation of a Biosphere
by Benton C. Clark and Vera M. Kolb
Life 2020, 10(11), 278; https://doi.org/10.3390/life10110278 - 12 Nov 2020
Cited by 13 | Viewed by 3948
Abstract
Although the cellular microorganism is the fundamental unit of biology, the origin of life (OoL) itself is unlikely to have occurred in a microscale environment. The macrobiont (MB) is the macro-scale setting where life originated. Guided by the methodologies of Systems Analysis, we [...] Read more.
Although the cellular microorganism is the fundamental unit of biology, the origin of life (OoL) itself is unlikely to have occurred in a microscale environment. The macrobiont (MB) is the macro-scale setting where life originated. Guided by the methodologies of Systems Analysis, we focus on subaerial ponds of scale 3 to 300 m diameter. Within such ponds, there can be substantial heterogeneity, on the vertical, horizontal, and temporal scales, which enable multi-pot prebiotic chemical evolution. Pond size-sensitivities for several figures of merit are mathematically formulated, leading to the expectation that the optimum pond size for the OoL is intermediate, but biased toward smaller sizes. Sensitivities include relative access to nutrients, energy sources, and catalysts, as sourced from geological, atmospheric, hydrospheric, and astronomical contributors. Foreshores, especially with mudcracks, are identified as a favorable component for the success of the macrobiont. To bridge the gap between inanimate matter and a planetary-scale biosphere, five stages of evolution within the macrobiont are hypothesized: prebiotic chemistry → molecular replicator → protocell → macrobiont cell → colonizer cell. Comparison of ponds with other macrobionts, including hydrothermal and meteorite settings, allows a conclusion that more than one possible macrobiont locale could enable an OoL. Full article
(This article belongs to the Special Issue Frontiers of Astrobiology)
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20 pages, 5903 KiB  
Article
Effects of Heavy Ion Particle Irradiation on Spore Germination of Bacillus spp. from Extremely Hot and Cold Environments
by Vincenzo Zammuto, Maria G. Rizzo, Laura M. De Plano, Domenico Franco, Salvatore Guglielmino, Maria T. Caccamo, Salvatore Magazù, Akira Fujimori, Angelina Lo Giudice, Mauro Guglielmin, Kevin Roderick McAlpin, Ralf Moeller and Concetta Gugliandolo
Life 2020, 10(11), 264; https://doi.org/10.3390/life10110264 - 30 Oct 2020
Cited by 10 | Viewed by 2620
Abstract
Extremophiles are optimal models in experimentally addressing questions about the effects of cosmic radiation on biological systems. The resistance to high charge energy (HZE) particles, and helium (He) ions and iron (Fe) ions (LET at 2.2 and 200 keV/µm, respectively, until 1000 Gy), [...] Read more.
Extremophiles are optimal models in experimentally addressing questions about the effects of cosmic radiation on biological systems. The resistance to high charge energy (HZE) particles, and helium (He) ions and iron (Fe) ions (LET at 2.2 and 200 keV/µm, respectively, until 1000 Gy), of spores from two thermophiles, Bacillushorneckiae SBP3 and Bacilluslicheniformis T14, and two psychrotolerants, Bacillus sp. A34 and A43, was investigated. Spores survived He irradiation better, whereas they were more sensitive to Fe irradiation (until 500 Gy), with spores from thermophiles being more resistant to irradiations than psychrotolerants. The survived spores showed different germination kinetics, depending on the type/dose of irradiation and the germinant used. After exposure to He 1000 Gy, D-glucose increased the lag time of thermophilic spores and induced germination of psychrotolerants, whereas L-alanine and L-valine increased the germination efficiency, except alanine for A43. FTIR spectra showed important modifications to the structural components of spores after Fe irradiation at 250 Gy, which could explain the block in spore germination, whereas minor changes were observed after He radiation that could be related to the increased permeability of the inner membranes and alterations of receptor complex structures. Our results give new insights on HZE resistance of extremophiles that are useful in different contexts, including astrobiology. Full article
(This article belongs to the Special Issue Frontiers of Astrobiology)
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12 pages, 506 KiB  
Article
The Habitability of the Galactic Bulge
by Amedeo Balbi, Maryam Hami and Andjelka Kovačević
Life 2020, 10(8), 132; https://doi.org/10.3390/life10080132 - 03 Aug 2020
Cited by 11 | Viewed by 3873
Abstract
We present a new investigation of the habitability of the Milky Way bulge, that expands previous studies on the Galactic Habitable Zone. We discuss existing knowledge on the abundance of planets in the bulge, metallicity and the possible frequency of rocky planets, orbital [...] Read more.
We present a new investigation of the habitability of the Milky Way bulge, that expands previous studies on the Galactic Habitable Zone. We discuss existing knowledge on the abundance of planets in the bulge, metallicity and the possible frequency of rocky planets, orbital stability and encounters, and the possibility of planets around the central supermassive black hole. We focus on two aspects that can present substantial differences with respect to the environment in the disk: (i) the ionizing radiation environment, due to the presence of the central black hole and to the highest rate of supernovae explosions and (ii) the efficiency of putative lithopanspermia mechanism for the diffusion of life between stellar systems. We use analytical models of the star density in the bulge to provide estimates of the rate of catastrophic events and of the diffusion timescales for life over interstellar distances. Full article
(This article belongs to the Special Issue Frontiers of Astrobiology)
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12 pages, 1157 KiB  
Article
Silicate-, Magnesium Ion-, and Urea-Induced Prebiotic Phosphorylation of Uridine via Pyrophosphate; Revisiting the Hot Drying Water Pool Scenario
by Maheen Gull, Arthur Omran, Tian Feng and Matthew A. Pasek
Life 2020, 10(8), 122; https://doi.org/10.3390/life10080122 - 25 Jul 2020
Cited by 15 | Viewed by 3532
Abstract
The availability of nucleotides on the early Earth is of great significance for the origin of a self-replicating system capable of undergoing evolution. We hereby report the successful phosphorylation reactions of the nucleoside uridine under heating in the “drying pool” prebiotic model at [...] Read more.
The availability of nucleotides on the early Earth is of great significance for the origin of a self-replicating system capable of undergoing evolution. We hereby report the successful phosphorylation reactions of the nucleoside uridine under heating in the “drying pool” prebiotic model at temperatures ranging from 60–75 °C, and by using pyrophosphate as a phosphorylation agent. Uridine monophosphates (UMP) such as uridine-5′-monophosphate (5′-UMP), 2′-UMP, and 3′-UMP, as well as cyclic 2′-3′-UMP, were identified by 31P-NMR. In addition to the above-mentioned products, a dimer of uridine-phosphate-uridine (U-P-U) was also observed. The reactions were promoted by white quartz sand, Mg2+, and by using urea as a condensation agent. The reactions also proceeded without this mixture; however, the yields increased remarkably with the presence of the above-mentioned materials. The results suggest that a hot/evaporating-drying pool of water containing organics, salts, and reactive phosphorus could be sufficient to form significant phosphate esters. Full article
(This article belongs to the Special Issue Frontiers of Astrobiology)
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47 pages, 6096 KiB  
Article
On the Potential of Silicon as a Building Block for Life
by Janusz Jurand Petkowski, William Bains and Sara Seager
Life 2020, 10(6), 84; https://doi.org/10.3390/life10060084 - 10 Jun 2020
Cited by 44 | Viewed by 29657
Abstract
Despite more than one hundred years of work on organosilicon chemistry, the basis for the plausibility of silicon-based life has never been systematically addressed nor objectively reviewed. We provide a comprehensive assessment of the possibility of silicon-based biochemistry, based on a review of [...] Read more.
Despite more than one hundred years of work on organosilicon chemistry, the basis for the plausibility of silicon-based life has never been systematically addressed nor objectively reviewed. We provide a comprehensive assessment of the possibility of silicon-based biochemistry, based on a review of what is known and what has been modeled, even including speculative work. We assess whether or not silicon chemistry meets the requirements for chemical diversity and reactivity as compared to carbon. To expand the possibility of plausible silicon biochemistry, we explore silicon’s chemical complexity in diverse solvents found in planetary environments, including water, cryosolvents, and sulfuric acid. In no environment is a life based primarily around silicon chemistry a plausible option. We find that in a water-rich environment silicon’s chemical capacity is highly limited due to ubiquitous silica formation; silicon can likely only be used as a rare and specialized heteroatom. Cryosolvents (e.g., liquid N2) provide extremely low solubility of all molecules, including organosilicons. Sulfuric acid, surprisingly, appears to be able to support a much larger diversity of organosilicon chemistry than water. Full article
(This article belongs to the Special Issue Frontiers of Astrobiology)
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25 pages, 2822 KiB  
Article
Rocket Science: The Effect of Spaceflight on Germination Physiology, Ageing, and Transcriptome of Eruca sativa Seeds
by Jake O. Chandler, Fabian B. Haas, Safina Khan, Laura Bowden, Michael Ignatz, Eugenia M. A. Enfissi, Frances Gawthrop, Alistair Griffiths, Paul D. Fraser, Stefan A. Rensing and Gerhard Leubner-Metzger
Life 2020, 10(4), 49; https://doi.org/10.3390/life10040049 - 24 Apr 2020
Cited by 18 | Viewed by 8080
Abstract
In the ‘Rocket Science’ project, storage of Eruca sativa (salad rocket) seeds for six months on board the International Space Station resulted in delayed seedling establishment. Here we investigated the physiological and molecular mechanisms underpinning the spaceflight effects on dry seeds. We found [...] Read more.
In the ‘Rocket Science’ project, storage of Eruca sativa (salad rocket) seeds for six months on board the International Space Station resulted in delayed seedling establishment. Here we investigated the physiological and molecular mechanisms underpinning the spaceflight effects on dry seeds. We found that ‘Space’ seed germination vigor was reduced, and ageing sensitivity increased, but the spaceflight did not compromise seed viability and the development of normal seedlings. Comparative analysis of the transcriptomes (using RNAseq) in dry seeds and upon controlled artificial ageing treatment (CAAT) revealed differentially expressed genes (DEGs) associated with spaceflight and ageing. DEG categories enriched by spaceflight and CAAT included transcription and translation with reduced transcript abundances for 40S and 60S ribosomal subunit genes. Among the ‘spaceflight-up’ DEGs were heat shock proteins (HSPs), DNAJ-related chaperones, a heat shock factor (HSFA7a-like), and components of several DNA repair pathways (e.g., ATM, DNA ligase 1). The ‘response to radiation’ category was especially enriched in ‘spaceflight-up’ DEGs including HSPs, catalases, and the transcription factor HY5. The major finding from the physiological and transcriptome analysis is that spaceflight causes vigor loss and partial ageing during air-dry seed storage, for which space environmental factors and consequences for seed storage during spaceflights are discussed. Full article
(This article belongs to the Special Issue Frontiers of Astrobiology)
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6 pages, 235 KiB  
Article
Possible Transfer of Life by Earth-Grazing Objects to Exoplanetary Systems
by Amir Siraj and Abraham Loeb
Life 2020, 10(4), 44; https://doi.org/10.3390/life10040044 - 17 Apr 2020
Cited by 4 | Viewed by 3463
Abstract
Recently, a 30-cm object was discovered to graze the Earth’s atmosphere and shift into a Jupiter-crossing orbit. We use the related survey parameters to calibrate the total number of such objects. The number of objects that could have exported terrestrial microbes out of [...] Read more.
Recently, a 30-cm object was discovered to graze the Earth’s atmosphere and shift into a Jupiter-crossing orbit. We use the related survey parameters to calibrate the total number of such objects. The number of objects that could have exported terrestrial microbes out of the Solar System is in the range 2 × 10 9 – 3 × 10 11 . We find that 10 7 – 10 9 such objects could have been captured by binary star systems over the lifetime of the Solar System. Adopting the fiducial assumption that one polyextremophile colony is picked up by each object, the total number of objects carrying living colonies on them upon capture could be 10– 10 3 . Full article
(This article belongs to the Special Issue Frontiers of Astrobiology)
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22 pages, 2509 KiB  
Article
Defining Lyfe in the Universe: From Three Privileged Functions to Four Pillars
by Stuart Bartlett and Michael L. Wong
Life 2020, 10(4), 42; https://doi.org/10.3390/life10040042 - 16 Apr 2020
Cited by 36 | Viewed by 28193
Abstract
Motivated by the need to paint a more general picture of what life is—and could be—with respect to the rest of the phenomena of the universe, we propose a new vocabulary for astrobiological research. Lyfe is defined as any system that fulfills all [...] Read more.
Motivated by the need to paint a more general picture of what life is—and could be—with respect to the rest of the phenomena of the universe, we propose a new vocabulary for astrobiological research. Lyfe is defined as any system that fulfills all four processes of the living state, namely: dissipation, autocatalysis, homeostasis, and learning. Life is defined as the instance of lyfe that we are familiar with on Earth, one that uses a specific organometallic molecular toolbox to record information about its environment and achieve dynamical order by dissipating certain planetary disequilibria. This new classification system allows the astrobiological community to more clearly define the questions that propel their research—e.g., whether they are developing a historical narrative to explain the origin of life (on Earth), or a universal narrative for the emergence of lyfe, or whether they are seeking signs of life specifically, or lyfe at large across the universe. While the concept of “life as we don’t know it” is not new, the four pillars of lyfe offer a novel perspective on the living state that is indifferent to the particular components that might produce it. Full article
(This article belongs to the Special Issue Frontiers of Astrobiology)
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10 pages, 1897 KiB  
Article
Semipermeable Mixed Phospholipid-Fatty Acid Membranes Exhibit K+/Na+ Selectivity in the Absence of Proteins
by Xianfeng Zhou, Punam Dalai and Nita Sahai
Life 2020, 10(4), 39; https://doi.org/10.3390/life10040039 - 14 Apr 2020
Cited by 12 | Viewed by 3786
Abstract
Two important ions, K+ and Na+, are unequally distributed across the contemporary phospholipid-based cell membrane because modern cells evolved a series of sophisticated protein channels and pumps to maintain ion gradients. The earliest life-like entities or protocells did not possess [...] Read more.
Two important ions, K+ and Na+, are unequally distributed across the contemporary phospholipid-based cell membrane because modern cells evolved a series of sophisticated protein channels and pumps to maintain ion gradients. The earliest life-like entities or protocells did not possess either ion-tight membranes or ion pumps, which would result in the equilibration of the intra-protocellular K+/Na+ ratio with that in the external environment. Here, we show that the most primitive protocell membranes composed of fatty acids, that were initially leaky, would eventually become less ion permeable as their membranes evolved towards having increasing phospholipid contents. Furthermore, these mixed fatty acid-phospholipid membranes selectively retain K+ but allow the passage of Na+ out of the cell. The K+/Na+ selectivity of these mixed fatty acid-phospholipid semipermeable membranes suggests that protocells at intermediate stages of evolution could have acquired electrochemical K+/Na+ ion gradients in the absence of any macromolecular transport machinery or pumps, thus potentially facilitating rudimentary protometabolism. Full article
(This article belongs to the Special Issue Frontiers of Astrobiology)
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8 pages, 908 KiB  
Article
A Possible Primordial Acetyleno/Carboxydotrophic Core Metabolism
by Jessica Sobotta, Thomas Geisberger, Carolin Moosmann, Christopher M. Scheidler, Wolfgang Eisenreich, Günter Wächtershäuser and Claudia Huber
Life 2020, 10(4), 35; https://doi.org/10.3390/life10040035 - 07 Apr 2020
Cited by 13 | Viewed by 3961
Abstract
Carbon fixation, in addition to the evolution of metabolism, is a main requirement for the evolution of life. Here, we report a one-pot carbon fixation of acetylene (C2H2) and carbon monoxide (CO) by aqueous nickel sulfide (NiS) under hydrothermal [...] Read more.
Carbon fixation, in addition to the evolution of metabolism, is a main requirement for the evolution of life. Here, we report a one-pot carbon fixation of acetylene (C2H2) and carbon monoxide (CO) by aqueous nickel sulfide (NiS) under hydrothermal (>100 °C) conditions. A slurry of precipitated NiS converts acetylene and carbon monoxide into a set of C2–4-products that are surprisingly representative for C2–4-segments of all four central CO2-fixation cycles of the domains Bacteria and Archaea, whereby some of the products engage in the same interconversions, as seen in the central CO2-fixation cycles. The results suggest a primordial, chemically predetermined, non-cyclic acetyleno/carboxydotrophic core metabolism. This metabolism is based on aqueous organo–metal chemistry, from which the extant central CO2-fixation cycles based on thioester chemistry would have evolved by piecemeal modifications. Full article
(This article belongs to the Special Issue Frontiers of Astrobiology)
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Review

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29 pages, 4441 KiB  
Review
Prebiotic Reaction Networks in Water
by Quoc Phuong Tran, Zachary R. Adam and Albert C. Fahrenbach
Life 2020, 10(12), 352; https://doi.org/10.3390/life10120352 - 16 Dec 2020
Cited by 13 | Viewed by 5345
Abstract
A prevailing strategy in origins of life studies is to explore how chemistry constrained by hypothetical prebiotic conditions could have led to molecules and system level processes proposed to be important for life’s beginnings. This strategy has yielded model prebiotic reaction networks that [...] Read more.
A prevailing strategy in origins of life studies is to explore how chemistry constrained by hypothetical prebiotic conditions could have led to molecules and system level processes proposed to be important for life’s beginnings. This strategy has yielded model prebiotic reaction networks that elucidate pathways by which relevant compounds can be generated, in some cases, autocatalytically. These prebiotic reaction networks provide a rich platform for further understanding and development of emergent “life-like” behaviours. In this review, recent advances in experimental and analytical procedures associated with classical prebiotic reaction networks, like formose and Miller-Urey, as well as more recent ones are highlighted. Instead of polymeric networks, i.e., those based on nucleic acids or peptides, the focus is on small molecules. The future of prebiotic chemistry lies in better understanding the genuine complexity that can result from reaction networks and the construction of a centralised database of reactions useful for predicting potential network evolution is emphasised. Full article
(This article belongs to the Special Issue Frontiers of Astrobiology)
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Other

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1 pages, 155 KiB  
Erratum
Erratum: Guttenberg et al. Classification of the Biogenicity of Complex Organic Mixtures for the Detection of Extraterrestrial Life. Life 2021, 11, 234
by Nicholas Guttenberg, Huan Chen, Tomohiro Mochizuki and H. James Cleaves II
Life 2021, 11(6), 461; https://doi.org/10.3390/life11060461 - 21 May 2021
Cited by 1 | Viewed by 1357
Abstract
There was an error in the original article [...] Full article
(This article belongs to the Special Issue Frontiers of Astrobiology)
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