Life | Special Issue : Planetary Exploration: Habitats and Terrestrial Analogs

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1263 KiB

Open AccessArticle

Photosynthesis in Hydrogen-Dominated Atmospheres

by William Bains, Sara Seager and Andras Zsom

Life 2014, 4(4), 716-744; https://doi.org/10.3390/life4040716 - 18 Nov 2014

Cited by 27 | Viewed by 12718

Abstract

The diversity of extrasolar planets discovered in the last decade shows that we should not be constrained to look for life in environments similar to early or present-day Earth. Super-Earth exoplanets are being discovered with increasing frequency, and some will be able to [...] Read more.

The diversity of extrasolar planets discovered in the last decade shows that we should not be constrained to look for life in environments similar to early or present-day Earth. Super-Earth exoplanets are being discovered with increasing frequency, and some will be able to retain a stable, hydrogen-dominated atmosphere. We explore the possibilities for photosynthesis on a rocky planet with a thin H₂-dominated atmosphere. If a rocky, H₂-dominated planet harbors life, then that life is likely to convert atmospheric carbon into methane. Outgassing may also build an atmosphere in which methane is the principal carbon species. We describe the possible chemical routes for photosynthesis starting from methane and show that less energy and lower energy photons could drive CH₄-based photosynthesis as compared with CO₂-based photosynthesis. We find that a by-product biosignature gas is likely to be H₂, which is not distinct from the hydrogen already present in the environment. Ammonia is a potential biosignature gas of hydrogenic photosynthesis that is unlikely to be generated abiologically. We suggest that the evolution of methane-based photosynthesis is at least as likely as the evolution of anoxygenic photosynthesis on Earth and may support the evolution of complex life. Full article

(This article belongs to the Special Issue Planetary Exploration: Habitats and Terrestrial Analogs)

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Figure 1

Figure 1
Energy of the synthesis of sample compounds. Comparison of Gibbs free energy of the synthesis of 49 terrestrial metabolites from CO2, H2O, SO42− and N2 (X-axis) or CH4, H2O, H2S and N2 (Y-axis). Free energy is for unionized compounds in aqueous solution, at 25 °C, except for octane, nonane, decane, undecane and hexadecane, which are calculated as liquids, because of their very low solubility in water. Data from [<a href="#B65-life-04-00716" class="html-bibr">65</a>]. Metabolites (with coloring to identify outliers) are formic acid (black point), acetic acid, glycolic acid, propanoic acid, lactic acid, butanoic acid, pentanoic acid, benzoic acid, oxalic acid (red point), malonic acid, succinic acid, glutaric acid, methanol (purple point), ethanol, propanol, 2-propanol, butanol, pentanol, ethane, propane, butane, pentane, octane, nonane, decane, undecane, hexadecane, toluene, ethylbenzene, alanine, arginine, asparagine, aspartic acid, cysteine (green point), glutamic acid, glutamine (yellow point), glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine and valine. Full article ">Figure 2
Chemical diversity accessible as a function of free energy. The fraction of the chemical space that can be captured for a given expenditure of energy. X-axis: input free energy. Y-axis: fraction of the 1,987,593 structures of nine or less non-H atoms generated by COMBIMOL that can be generated with no more than the free energy input on the X-axis. Shown are reactions where CH4 is oxidized, generating free H2 (blue), and CO2 is reduced in reactions generating free O2 (red) and Fe3+ (green). Full article ">Figure 3
Photon energies for photosynthesis. Illustration of the maximum wavelengths that might be required for different types of photosynthetic reactions. Y-axis, volts. X-axis, the wavelength of light (nm). The black curve shows the standard electrode potential of a single electron reaction that consumes (or in the reverse direction, generates) energy equivalent to the energy in a mole of photons of a particular wavelength. Horizontal bars show the standard electrode potential needed to drive the generation of free oxygen from water and free hydrogen from CH4 + H2O. The point where each horizontal bar crosses the black curve illustrates the likely maximum wavelength that could be used to power the relevant reaction. Full article ">Figure 4
Pressure vs. orbital parameters for planets around different stars. Plot of the surface pressure (Y-axis) needed to maintain a surface temperature of 25 °C on a planet with a 20-m/s2 surface gravity, 90% H2 atmosphere, orbiting around different mass stars in a roughly circular orbit with a specific semi-major axis (X-axis). See the text for other conditions. The different color lines represent different stellar masses: higher masses of atmosphere (i.e., higher surface pressure) mean that a planet must orbit further from its star to have a surface temperature of 25 °C, and a higher mass star also means that a wider orbit is required. Full article ">Figure 5
Surface photon flux as a function of surface pressure and stellar mass. For a combination of stellar mass (Y-axis) and surface pressure (X-axis), the semi-major axis of the planet was calculated as per <a href="#life-04-00716-f004" class="html-fig">Figure 4</a>. From the stellar photon flux, distance and atmospheric absorption, the surface flux of photons was calculated (color scale on the right of the graph). Stellar mass has a minimal effect, because a higher stellar mass requires the planet to orbit further from the star to maintain a clement surface environment. Full article ">

2556 KiB

Open AccessArticle

Mud Volcanoes of Trinidad as Astrobiological Analogs for Martian Environments

by Riad Hosein, Shirin Haque and Denise M. Beckles

Life 2014, 4(4), 566-585; https://doi.org/10.3390/life4040566 - 13 Oct 2014

Cited by 14 | Viewed by 17159

Abstract

Eleven onshore mud volcanoes in the southern region of Trinidad have been studied as analog habitats for possible microbial life on Mars. The profiles of the 11 mud volcanoes are presented in terms of their physical, chemical, mineralogical, and soil properties. The mud [...] Read more.

Eleven onshore mud volcanoes in the southern region of Trinidad have been studied as analog habitats for possible microbial life on Mars. The profiles of the 11 mud volcanoes are presented in terms of their physical, chemical, mineralogical, and soil properties. The mud volcanoes sampled all emitted methane gas consistently at 3% volume. The average pH for the mud volcanic soil was 7.98. The average Cation Exchange Capacity (CEC) was found to be 2.16 kg/mol, and the average Percentage Water Content was 34.5%. Samples from three of the volcanoes, (i) Digity; (ii) Piparo and (iii) Devil’s Woodyard were used to culture bacterial colonies under anaerobic conditions indicating possible presence of methanogenic microorganisms. The Trinidad mud volcanoes can serve as analogs for the Martian environment due to similar geological features found extensively on Mars in Acidalia Planitia and the Arabia Terra region. Full article

(This article belongs to the Special Issue Planetary Exploration: Habitats and Terrestrial Analogs)

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14286 KiB

Open AccessArticle

Models of Formation and Activity of Spring Mounds in the Mechertate-Chrita-Sidi El Hani System, Eastern Tunisia: Implications for the Habitability of Mars

by Elhoucine Essefi, Goro Komatsu, Alberto G. Fairén, Marjorie A. Chan and Chokri Yaich

Life 2014, 4(3), 386-432; https://doi.org/10.3390/life4030386 - 28 Aug 2014

Cited by 8 | Viewed by 9405

Abstract

Spring mounds on Earth and on Mars could represent optimal niches of life development. If life ever occurred on Mars, ancient spring deposits would be excellent localities to search for morphological or chemical remnants of an ancient biosphere. In this work, we investigate [...] Read more.

Spring mounds on Earth and on Mars could represent optimal niches of life development. If life ever occurred on Mars, ancient spring deposits would be excellent localities to search for morphological or chemical remnants of an ancient biosphere. In this work, we investigate models of formation and activity of well-exposed spring mounds in the Mechertate-Chrita-Sidi El Hani (MCSH) system, eastern Tunisia. We then use these models to explore possible spring mound formation on Mars. In the MCSH system, the genesis of the spring mounds is a direct consequence of groundwater upwelling, triggered by tectonics and/or hydraulics. As they are oriented preferentially along faults, they can be considered as fault spring mounds, implying a tectonic influence in their formation process. However, the hydraulic pressure generated by the convergence of aquifers towards the surface of the system also allows consideration of an origin as artesian spring mounds. In the case of the MCSH system, our geologic data presented here show that both models are valid, and we propose a combined hydro-tectonic model as the likely formation mechanism of artesian-fault spring mounds. During their evolution from the embryonic (early) to the islet (“island”) stages, spring mounds are also shaped by eolian accumulations and induration processes. Similarly, spring mounds have been suggested to be relatively common in certain provinces on the Martian surface, but their mode of formation is still a matter of debate. We propose that the tectonic, hydraulic, and combined hydro-tectonic models describing the spring mounds at MCSH could be relevant as Martian analogs because: (i) the Martian subsurface may be over pressured, potentially expelling mineral-enriched waters as spring mounds on the surface; (ii) the Martian subsurface may be fractured, causing alignment of the spring mounds in preferential orientations; and (iii) indurated eolian sedimentation and erosional remnants are common features on Mars. The spring mounds further bear diagnostic mineralogic and magnetic properties, in comparison with their immediate surroundings. Consequently, remote sensing techniques can be very useful to identify similar spring mounds on Mars. The mechanisms (tectonic and/or hydraulic) of formation and evolution of spring mounds at the MCSH system are suitable for the proliferation and protection of life respectively. Similarly, life or its resulting biomarkers on Mars may have been protected or preserved under the spring mounds. Full article

(This article belongs to the Special Issue Planetary Exploration: Habitats and Terrestrial Analogs)

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1655 KiB

Open AccessCommunication

Fluorine-Rich Planetary Environments as Possible Habitats for Life

by Nediljko Budisa, Vladimir Kubyshkin and Dirk Schulze-Makuch

Life 2014, 4(3), 374-385; https://doi.org/10.3390/life4030374 - 18 Aug 2014

Cited by 23 | Viewed by 10806

Abstract

In polar aprotic organic solvents, fluorine might be an element of choice for life that uses selected fluorinated building blocks as monomers of choice for self-assembling of its catalytic polymers. Organofluorine compounds are extremely rare in the chemistry of life as we know [...] Read more.

In polar aprotic organic solvents, fluorine might be an element of choice for life that uses selected fluorinated building blocks as monomers of choice for self-assembling of its catalytic polymers. Organofluorine compounds are extremely rare in the chemistry of life as we know it. Biomolecules, when fluorinated such as peptides or proteins, exhibit a “fluorous effect”, i.e., they are fluorophilic (neither hydrophilic nor lipophilic). Such polymers, capable of creating self-sorting assemblies, resist denaturation by organic solvents by exclusion of fluorocarbon side chains from the organic phase. Fluorous cores consist of a compact interior, which is shielded from the surrounding solvent. Thus, we can anticipate that fluorine-containing “teflon”-like or “non-sticking” building blocks might be monomers of choice for the synthesis of organized polymeric structures in fluorine-rich planetary environments. Although no fluorine-rich planetary environment is known, theoretical considerations might help us to define chemistries that might support life in such environments. For example, one scenario is that all molecular oxygen may be used up by oxidation reactions on a planetary surface and fluorine gas could be released from F-rich magma later in the history of a planetary body to result in a fluorine-rich planetary environment. Full article

(This article belongs to the Special Issue Planetary Exploration: Habitats and Terrestrial Analogs)

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Graphical abstract

Graphical abstract
Full article ">Figure 1
Physiochemical nature of fluorous effect. Left: phase separation of perfluorinated compounds/solvents which are immiscible with both organic and aqueous phases. In the contemporary literature, the term “fluorous” is used for highly fluorinated (i.e., perfluorinated) solvents, in an analogy “aqueous” for water-based systems [<a href="#B30-life-04-00374" class="html-bibr">30</a>]. Right: hydrocarbon versus fluorous protein core (dark structures—water solvent; faint spots—organic solvent). The hydrocarbon core of a polymer (Top) opens up upon addition of an organic solvent leading to denaturation, whereas a fluorous core (Bottom) remains intact. The fluorous effect is very similar to the effect responsible for the non-sticking properties of Teflon [<a href="#B31-life-04-00374" class="html-bibr">31</a>]. In general, polymeric materials modified by multiple fluorinated carbons possess elevated hydrophobicity and stability. These considerations might be of particular importance in assessing whether life might be able to exist in hydrophobic solvents [<a href="#B9-life-04-00374" class="html-bibr">9</a>,<a href="#B32-life-04-00374" class="html-bibr">32</a>] under “reverse phase” conditions. Figure modified from [<a href="#B33-life-04-00374" class="html-bibr">33</a>]. Full article ">Figure 2
Hydrophobic canonical (Ala, Val, Leu, Ile, Met) and noncanonical amino acids (Norvaline, Nva; α-aminobutyrate, Abu; norleucine, Nle) used for building of protein cores. At least traces of Nle, Nva and Abu have been found in the interstellar medium and carbonaceous meteorites, in addition to some canonical amino acids [<a href="#B28-life-04-00374" class="html-bibr">28</a>]. These amino acids have perfluorinated counterparts of anthropogenic origin which have special properties. Frequently used trifluorinated amino acids are: TFMet (6,6,6-trifluoromethionine), TFLeu (5,5,5-trifluoroleucine), HFLeu—(5,5,5,5’,5’,5’-hexafluoroleucine), TFVal—(2S,3R)-4,4,4-trifluorovaline; 5TFIle [(2S,3S)-5,5,5-trifluoroisoleucine] and 3TFIle—[(2S,3S)-3,3,3-trifluoroisoleucine]. The asterisk in the structure indicates an undefined stereogenic (i.e., chiral) carbon center. Full article ">

903 KiB

Open AccessCommunication

Supercritical Carbon Dioxide and Its Potential as a Life-Sustaining Solvent in a Planetary Environment

by Nediljko Budisa and Dirk Schulze-Makuch

Life 2014, 4(3), 331-340; https://doi.org/10.3390/life4030331 - 8 Aug 2014

Cited by 99 | Viewed by 30253

Abstract

Supercritical fluids have different properties compared to regular fluids and could play a role as life-sustaining solvents on other worlds. Even on Earth, some bacterial species have been shown to be tolerant to supercritical fluids. The special properties of supercritical fluids, which include [...] Read more.

Supercritical fluids have different properties compared to regular fluids and could play a role as life-sustaining solvents on other worlds. Even on Earth, some bacterial species have been shown to be tolerant to supercritical fluids. The special properties of supercritical fluids, which include various types of selectivities (e.g., stereo-, regio-, and chemo-selectivity) have recently been recognized in biotechnology and used to catalyze reactions that do not occur in water. One suitable example is enzymes when they are exposed to supercritical fluids such as supercritical carbon dioxide: enzymes become even more stable, because they are conformationally rigid in the dehydrated state. Furthermore, enzymes in anhydrous organic solvents exhibit a “molecular memory”, i.e., the capacity to “remember” a conformational or pH state from being exposed to a previous solvent. Planetary environments with supercritical fluids, particularly supercritical carbon dioxide, exist, even on Earth (below the ocean floor), on Venus, and likely on Super-Earth type exoplanets. These planetary environments may present a possible habitat for exotic life. Full article

(This article belongs to the Special Issue Planetary Exploration: Habitats and Terrestrial Analogs)

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Graphical abstract

Graphical abstract
Full article ">Figure 1
Schematic p-T phase diagram of CO2. Note if the temperature and pressure of a substance are both higher than Tc and Pc for a particular substance, the substance is defined as a supercritical fluid. Carbon dioxide has four distinct phases; the standard solid, liquid and gas phase as well as the supercritical phase. Carbon dioxide transitions to supercritical phase occur relatively readily at the critical points of 7.38 MPa, 304 K/31.1 °C and 73.8 bar. Full article ">Figure 2
Effect of scCO2 as an anhydrous solvent on biochemical systems, specifically in regard to “enzyme memory”. Lyophilization of enzymes (freeze-drying or cryo-desiccation) in organic solvents can enable enzymes to “remember” the exposure to a ligand—a phenomenon known as ‘ligand imprinting” or “ligand-induced enzyme memory” [<a href="#B12-life-04-00331" class="html-bibr">12</a>]. The induction of enzyme memory requires considerable conformational flexibility in the protein. High conformational rigidity present in nearly all anhydrous environments is the basic requirement for appearance and retention of “memory”. For that reason, simple addition of smaller amounts of water is sufficient enough to erase the “memory” effect. Full article ">Figure 3
Carbamate synthesis in reaction between CO2 and lysine side chains on the surface of an enzyme and the formation of carbonic acid and its dissociation to the bicarbonic anion in scCO2 [<a href="#B9-life-04-00331" class="html-bibr">9</a>]. Full article ">

Review

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1163 KiB

Open AccessReview

Volcanogenic Fluvial-Lacustrine Environments in Iceland and Their Utility for Identifying Past Habitability on Mars

by Claire Cousins

Life 2015, 5(1), 568-586; https://doi.org/10.3390/life5010568 - 16 Feb 2015

Cited by 18 | Viewed by 9032

Abstract

The search for once-habitable locations on Mars is increasingly focused on environments dominated by fluvial and lacustrine processes, such as those investigated by the Mars Science Laboratory Curiosity rover. The availability of liquid water coupled with the potential longevity of such systems renders [...] Read more.

The search for once-habitable locations on Mars is increasingly focused on environments dominated by fluvial and lacustrine processes, such as those investigated by the Mars Science Laboratory Curiosity rover. The availability of liquid water coupled with the potential longevity of such systems renders these localities prime targets for the future exploration of Martian biosignatures. Fluvial-lacustrine environments associated with basaltic volcanism are highly relevant to Mars, but their terrestrial counterparts have been largely overlooked as a field analogue. Such environments are common in Iceland, where basaltic volcanism interacts with glacial ice and surface snow to produce large volumes of meltwater within an otherwise cold and dry environment. This meltwater can be stored to create subglacial, englacial, and proglacial lakes, or be released as catastrophic floods and proglacial fluvial systems. Sedimentary deposits produced by the resulting fluvial-lacustrine activity are extensive, with lithologies dominated by basaltic minerals, low-temperature alteration assemblages (e.g., smectite clays, calcite), and amorphous, poorly crystalline phases (basaltic glass, palagonite, nanophase iron oxides). This paper reviews examples of these environments, including their sedimentary deposits and microbiology, within the context of utilising these localities for future Mars analogue studies and instrument testing. Full article

(This article belongs to the Special Issue Planetary Exploration: Habitats and Terrestrial Analogs)

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1535 KiB

Open AccessReview

Biota and Biomolecules in Extreme Environments on Earth: Implications for Life Detection on Mars

by Joost W. Aerts, Wilfred F.M. Röling, Andreas Elsaesser and Pascale Ehrenfreund

Life 2014, 4(4), 535-565; https://doi.org/10.3390/life4040535 - 13 Oct 2014

Cited by 36 | Viewed by 14580

Abstract

The three main requirements for life as we know it are the presence of organic compounds, liquid water, and free energy. Several groups of organic compounds (e.g., amino acids, nucleobases, lipids) occur in all life forms on Earth and are used as diagnostic [...] Read more.

The three main requirements for life as we know it are the presence of organic compounds, liquid water, and free energy. Several groups of organic compounds (e.g., amino acids, nucleobases, lipids) occur in all life forms on Earth and are used as diagnostic molecules, i.e., biomarkers, for the characterization of extant or extinct life. Due to their indispensability for life on Earth, these biomarkers are also prime targets in the search for life on Mars. Biomarkers degrade over time; in situ environmental conditions influence the preservation of those molecules. Nonetheless, upon shielding (e.g., by mineral surfaces), particular biomarkers can persist for billions of years, making them of vital importance in answering questions about the origins and limits of life on early Earth and Mars. The search for organic material and biosignatures on Mars is particularly challenging due to the hostile environment and its effect on organic compounds near the surface. In support of life detection on Mars, it is crucial to investigate analogue environments on Earth that resemble best past and present Mars conditions. Terrestrial extreme environments offer a rich source of information allowing us to determine how extreme conditions affect life and molecules associated with it. Extremophilic organisms have adapted to the most stunning conditions on Earth in environments with often unique geological and chemical features. One challenge in detecting biomarkers is to optimize extraction, since organic molecules can be low in abundance and can strongly adsorb to mineral surfaces. Methods and analytical tools in the field of life science are continuously improving. Amplification methods are very useful for the detection of low concentrations of genomic material but most other organic molecules are not prone to amplification methods. Therefore, a great deal depends on the extraction efficiency. The questions “what to look for”, “where to look”, and “how to look for it” require more of our attention to ensure the success of future life detection missions on Mars. Full article

(This article belongs to the Special Issue Planetary Exploration: Habitats and Terrestrial Analogs)

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2068 KiB

Open AccessReview

Río Tinto: A Geochemical and Mineralogical Terrestrial Analogue of Mars

by Ricardo Amils, David Fernández-Remolar and The IPBSL Team

Life 2014, 4(3), 511-534; https://doi.org/10.3390/life4030511 - 15 Sep 2014

Cited by 69 | Viewed by 16439

Abstract

The geomicrobiological characterization of the water column and sediments of Río Tinto (Huelva, Southwestern Spain) have proven the importance of the iron and the sulfur cycles, not only in generating the extreme conditions of the habitat (low pH, high concentration of toxic heavy [...] Read more.

The geomicrobiological characterization of the water column and sediments of Río Tinto (Huelva, Southwestern Spain) have proven the importance of the iron and the sulfur cycles, not only in generating the extreme conditions of the habitat (low pH, high concentration of toxic heavy metals), but also in maintaining the high level of microbial diversity detected in the basin. It has been proven that the extreme acidic conditions of Río Tinto basin are not the product of 5000 years of mining activity in the area, but the consequence of an active underground bioreactor that obtains its energy from the massive sulfidic minerals existing in the Iberian Pyrite Belt. Two drilling projects, MARTE (Mars Astrobiology Research and Technology Experiment) (2003–2006) and IPBSL (Iberian Pyrite Belt Subsurface Life Detection) (2011–2015), were developed and carried out to provide evidence of subsurface microbial activity and the potential resources that support these activities. The reduced substrates and the oxidants that drive the system appear to come from the rock matrix. These resources need only groundwater to launch diverse microbial metabolisms. The similarities between the vast sulfate and iron oxide deposits on Mars and the main sulfide bioleaching products found in the Tinto basin have given Río Tinto the status of a geochemical and mineralogical Mars terrestrial analogue. Full article

(This article belongs to the Special Issue Planetary Exploration: Habitats and Terrestrial Analogs)

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Figure 1
Río Tinto basin at Berrocal (J. Segura). Full article ">Figure 2
Alto de la Mesa old terrace. Full article ">Figure 3
MARTE (Mars Astrobiology Research and Technology Experiment) project, borehole (BH) BH4 drilling site. Full article ">Figure 4
Electric resistivity tomography profile of Peña de Hierro showing the location of the selected drilling sites. Full article ">Figure 5
Mössbauer spectrum of jarosite at Meridiani Planum (Courtesy of NASA/JPL-Caltech). Full article ">Figure 6
Suite of instruments in Curiosity’s mobile arm (Courtesy NASA/JPL-Caltech). Full article ">Figure 7
Methane detection on Mars [<a href="#B81-life-04-00511" class="html-bibr">81</a>] (Courtesy of NASA/Goddard). Full article ">Figure 8
Iron bioformations along the coastline of Cardozo Cove at King George Island [<a href="#B110-life-04-00511" class="html-bibr">110</a>]. Full article ">

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