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3. Generalized Stoichiometry and Biogeochemistry for Astrobiological Applications

Author

Kempes, Christopher P., Follows, Michael J., Smith, Hillary, Graham, Heather, House, Christopher H., and Levin, Simon A.

Subjects
Quantitative Biology - Quantitative Methods, Astrophysics - Earth and Planetary Astrophysics
Abstract

A central need in the field of astrobiology is generalized perspectives on life that make it possible to differentiate abiotic and biotic chemical systems. A key component of many past and future astrobiological measurements is the elemental ratio of various samples. Classic work on Earth's oceans has shown that life displays a striking regularity in the ratio of elements as originally characterized by Redfield. The body of work since the original observations has connected this ratio with basic ecological dynamics and cell physiology, while also documenting the range of elemental ratios found in a variety of environments. Several key questions remain in considering how to best apply this knowledge to astrobiological contexts: How can the observed variation of the elemental ratios be more formally systematized using basic biological physiology and ecological or environmental dynamics? How can these elemental ratios be generalized beyond the life that we have observed on our own planet? Here we expand recently developed generalized physiological models to create a simple framework for predicting the variation of elemental ratios found in various environments. We then discuss further generalizing the physiology for astrobiological applications. Much of our theoretical treatment is designed for in situ measurements applicable to future planetary missions. We imagine scenarios where three measurements can be made - particle/cell sizes, particle/cell stoichiometry, and fluid or environmental stoichiometry - and develop our theory in connection with these often deployed measurements., Comment: 18 pages, 5 figures

Published
2020

4. Prebiotic thiol-catalyzed thioamide bond formation.

Author

Hyde, Andrew S. and House, Christopher H.

Subjects
*INTERMEDIATES (Chemistry), *HYDROCYANIC acid, *ORIGIN of life, *GIBBS' free energy, *METHANETHIOL, *THIOAMIDES
Abstract

Thioamide bonds are important intermediates in prebiotic chemistry. In cyanosulfidic prebiotic chemistry, they serve as crucial intermediates in the pathways that lead to the formation of many important biomolecules (e.g., amino acids). They can also serve as purine and pyrimidine precursors, the two classes of heterocycle employed in genetic molecules. Despite their importance, the formation of thioamide bonds from nitriles under prebiotic conditions has required large excesses of sulfide or compounds with unknown prebiotic sources. Here, we describe the thiol-catalyzed formation of thioamide bonds from nitriles. We show that the formation of the simplest of these compounds, thioformamide, forms readily in spark-discharge experiments from hydrogen cyanide, sulfide, and a methanethiol catalyst, suggesting potential accumulation on early Earth. Lastly, we demonstrate that thioformamide has a Gibbs energy of hydrolysis ( Δ G r ∘ ) comparable to other energy-currencies on early Earth such as pyrophosphate and thioester bonds. Overall, our findings imply that thioamides might have been abundant on early Earth and served a variety of functions during chemical evolution. [ABSTRACT FROM AUTHOR]

Published
2024
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8. Genome-wide gene order distances support clustering the gram-positive bacteria

Author

House, Christopher H, Pellegrini, Matteo, and Fitz-Gibbon, Sorel T

Subjects
Microbiology, Biological Sciences, Genetics, Human Genome, tree of life, gene order, evolutionary distance, genomics, Actinobacteria, Firmicutes, Archaea, Environmental Science and Management, Soil Sciences, Medical microbiology
Abstract

Initially using 143 genomes, we developed a method for calculating the pair-wise distance between prokaryotic genomes using a Monte Carlo method to estimate the conservation of gene order. The method was based on repeatedly selecting five or six non-adjacent random orthologs from each of two genomes and determining if the chosen orthologs were in the same order. The raw distances were then corrected for gene order convergence using an adaptation of the Jukes-Cantor model, as well as using the common distance correction D' = -ln(1-D). First, we compared the distances found via the order of six orthologs to distances found based on ortholog gene content and small subunit rRNA sequences. The Jukes-Cantor gene order distances are reasonably well correlated with the divergence of rRNA (R (2) = 0.24), especially at rRNA Jukes-Cantor distances of less than 0.2 (R (2) = 0.52). Gene content is only weakly correlated with rRNA divergence (R (2) = 0.04) over all distances, however, it is especially strongly correlated at rRNA Jukes-Cantor distances of less than 0.1 (R (2) = 0.67). This initial work suggests that gene order may be useful in conjunction with other methods to help understand the relatedness of genomes. Using the gene order distances in 143 genomes, the relations of prokaryotes were studied using neighbor joining and agreement subtrees. We then repeated our study of the relations of prokaryotes using gene order in 172 complete genomes better representing a wider-diversity of prokaryotes. Consistently, our trees show the Actinobacteria as a sister group to the bulk of the Firmicutes. In fact, the robustness of gene order support was found to be considerably greater for uniting these two phyla than for uniting any of the proteobacterial classes together. The results are supportive of the idea that Actinobacteria and Firmicutes are closely related, which in turn implies a single origin for the gram-positive cell.

Published
2015

9. Seeding Life on the Moons of the Outer Planets via Lithopanspermia

Author

Worth, R. J., Sigurdsson, Steinn, and House, Christopher H.

Subjects
Astrophysics - Earth and Planetary Astrophysics
Abstract

Material from the surface of a planet can be ejected into space by a large impact, and could carry primitive life forms with it. We performed n-body simulations of such ejecta to determine where in the Solar System rock from Earth and Mars may end up. We find that, in addition to frequent transfer of material among the terrestrial planets, transfer of material from Earth and Mars to the moons of Jupiter and Saturn is also possible, but rare. We expect that such transfer is most likely during the Late Heavy Bombardment or during the next one or two billion years. At this time, the icy moons were warmer and likely had little or no icy shell to prevent meteorites from reaching their liquid interiors. We also note significant rates of re-impact in the first million years after ejection. This could re-seed life on a planet after partial or complete sterilization by a large impact, which would aid the survival of early life during the Late Heavy Bombardment., Comment: Accepted for publication in Astrobiology on Nov. 10, 2013 13 pages, 5 tables, 4 figures, 1 appendix

Published
2013
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11. Genome-wide gene order distances support clustering the gram-positive bacteria.

Author

House, Christopher H, Pellegrini, Matteo, and Fitz-Gibbon, Sorel T

Subjects
Actinobacteria, Archaea, Firmicutes, evolutionary distance, gene order, genomics, tree of life, Environmental Science and Management, Soil Sciences, Microbiology
Abstract

Initially using 143 genomes, we developed a method for calculating the pair-wise distance between prokaryotic genomes using a Monte Carlo method to estimate the conservation of gene order. The method was based on repeatedly selecting five or six non-adjacent random orthologs from each of two genomes and determining if the chosen orthologs were in the same order. The raw distances were then corrected for gene order convergence using an adaptation of the Jukes-Cantor model, as well as using the common distance correction D' = -ln(1-D). First, we compared the distances found via the order of six orthologs to distances found based on ortholog gene content and small subunit rRNA sequences. The Jukes-Cantor gene order distances are reasonably well correlated with the divergence of rRNA (R (2) = 0.24), especially at rRNA Jukes-Cantor distances of less than 0.2 (R (2) = 0.52). Gene content is only weakly correlated with rRNA divergence (R (2) = 0.04) over all distances, however, it is especially strongly correlated at rRNA Jukes-Cantor distances of less than 0.1 (R (2) = 0.67). This initial work suggests that gene order may be useful in conjunction with other methods to help understand the relatedness of genomes. Using the gene order distances in 143 genomes, the relations of prokaryotes were studied using neighbor joining and agreement subtrees. We then repeated our study of the relations of prokaryotes using gene order in 172 complete genomes better representing a wider-diversity of prokaryotes. Consistently, our trees show the Actinobacteria as a sister group to the bulk of the Firmicutes. In fact, the robustness of gene order support was found to be considerably greater for uniting these two phyla than for uniting any of the proteobacterial classes together. The results are supportive of the idea that Actinobacteria and Firmicutes are closely related, which in turn implies a single origin for the gram-positive cell.

Published
2014

12. Background levels of methane in Mars’ atmosphere show strong seasonal variations

Author

Webster, Christopher R., Mahaffy, Paul R., Atreya, Sushil K., Moores, John E., Flesch, Gregory J., Malespin, Charles, McKay, Christopher P., Martinez, German, Smith, Christina L., Martin-Torres, Javier, Gomez-Elvira, Javier, Zorzano, Maria-Paz, Wong, Michael H., Trainer, Melissa G., Steele, Andrew, Archer, Doug, Sutter, Brad, Coll, Patrice J., Freissinet, Caroline, Meslin, Pierre-Yves, Gough, Raina V., House, Christopher H., Pavlov, Alexander, Eigenbrode, Jennifer L., Glavin, Daniel P., Pearson, John C., Keymeulen, Didier, Christensen, Lance E., Schwenzer, Susanne P., Navarro-Gonzalez, Rafael, Pla-García, Jorge, Rafkin, Scot C. R., Vicente-Retortillo, Álvaro, Kahanpää, Henrik, Viudez-Moreiras, Daniel, Smith, Michael D., Harri, Ari-Matti, Genzer, Maria, Hassler, Donald M., Lemmon, Mark, Crisp, Joy, Sander, Stanley P., Zurek, Richard W., and Vasavada, Ashwin R.

Published
2018

13. Distributions of Microbial Activities in Deep Subseafloor Sediments

Author

D'Hondt, Steven, Jørgensen, Bo Barker, Miller, D. Jay, Batzke, Anja, Blake, Ruth, Cragg, Barry A., Cypionka, Heribert, Dickens, Gerald R., Ferdelman, Timothy, Hinrichs, Kai-Uwe, Holm, Nils G., Mitterer, Richard, Spivack, Arthur, Wang, Guizhi, Bekins, Barbara, Engelen, Bert, Ford, Kathryn, Gettemy, Glen, Rutherford, Scott D., Sass, Henrik, Skilbeck, C. Gregory, Aiello, Ivano W., Guèrin, Gilles, House, Christopher H., Inagaki, Fumio, Meister, Patrick, Naehr, Thomas, Niitsuma, Sachiko, Parkes, R. John, Schippers, Axel, Smith, David C., Teske, Andreas, Wiegel, Juergen, and Padilla, Christian Naranjo

Published
2004

17. Relict Ocean Worlds: Ceres

Author

De Sanctis, Maria Cristina, Mitri, Giuseppe, Castillo-Rogez, Julie, House, Christopher H., Marchi, Simome, Raymond, Carol A., and Sekine, Yasuhito

Published
2020
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19. Distinctive microfossil supports early Paleoproterozoic rise in complex cellular organisation.

Author

Barlow, Erica V., House, Christopher H., Liu, Ming‐Chang, Wetherington, Maxwell T., and Van Kranendonk, Martin J.

Subjects
*FOSSIL microorganisms, *GREAT Oxidation Event, *SURFACE of the earth, *FOSSILS, *CARBON isotopes, *ISOTOPIC analysis
Abstract

The great oxidation event (GOE), ~2.4 billion years ago, caused fundamental changes to the chemistry of Earth's surface environments. However, the effect of these changes on the biosphere is unknown, due to a worldwide lack of well‐preserved fossils from this time. Here, we investigate exceptionally preserved, large spherical aggregate (SA) microfossils permineralised in chert from the c. 2.4 Ga Turee Creek Group in Western Australia. Field and petrographic observations, Raman spectroscopic mapping, and in situ carbon isotopic analyses uncover insights into the morphology, habitat, reproduction and metabolism of this unusual form, whose distinctive, SA morphology has no known counterpart in the fossil record. Comparative analysis with microfossils from before the GOE reveals the large SA microfossils represent a step‐up in cellular organisation. Morphological comparison to extant micro‐organisms indicates the SAs have more in common with coenobial algae than coccoidal bacteria, emphasising the complexity of this microfossil form. The remarkable preservation here provides a unique window into the biosphere, revealing an increase in the complexity of life coinciding with the GOE. [ABSTRACT FROM AUTHOR]

Published
2024
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25. Curiosity at Vera Rubin Ridge: Major Findings and Implications for Habitability

Author

Fraeman, Abigail A, Sun, Vivian Zheng, Edgar, Lauren A, Fedo, Christopher, Fox, Valerie Kristen, Grotzinger, John P, Hardgrove, Craig, Horgan, Briony H. N, House, Christopher H, Jacobs, Samantha, Johnson, Jeffery Roy, Johnson, Sarah Stewart, Rampe, Elizabeth B, Salvatore, Mark R, Stack, Kathryn, Thopmson, Lucy M, Wellington, Danika F, Wiens, Roger C, Williams, Amy J, and Vasavada, Ashwin R

Subjects
Lunar And Planetary Science And Exploration
Published
2018

26. Oxidized and Reduced Sulfur Observed by the Sample Analysis at Mars (SAM) Instrument Suite on the Curiosity Rover Within the Glen Torridon Region at Gale Crater, Mars

Author

Wong, Gregory M., primary, Franz, Heather B., additional, Clark, Joanna V., additional, McAdam, Amy C., additional, Lewis, James M. T., additional, Millan, Maëva, additional, Ming, Douglas W., additional, Gomez, Felipe, additional, Clark, Benton, additional, Eigenbrode, Jennifer L., additional, Navarro‐González, Rafael, additional, and House, Christopher H., additional

Published
2022
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29. Organic carbon concentrations in 3.5-billion-year-old lacustrine mudstones of Mars

Author

Stern, Jennifer C., primary, Malespin, Charles A., additional, Eigenbrode, Jennifer L., additional, Webster, Christopher R., additional, Flesch, Greg, additional, Franz, Heather B., additional, Graham, Heather V., additional, House, Christopher H., additional, Sutter, Brad, additional, Archer, Paul Douglas, additional, Hofmann, Amy E., additional, McAdam, Amy C., additional, Ming, Douglas W., additional, Navarro-Gonzalez, Rafael, additional, Steele, Andrew, additional, Freissinet, Caroline, additional, and Mahaffy, Paul R., additional

Published
2022
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30. Generalized Stoichiometry and Biogeochemistry for Astrobiological Applications

Author

Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences, Kempes, Christopher P., Follows, Michael J, Smith, Hillary, Graham, Heather, House, Christopher H., Levin, Simon A., Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences, Kempes, Christopher P., Follows, Michael J, Smith, Hillary, Graham, Heather, House, Christopher H., and Levin, Simon A.

Abstract

A central need in the field of astrobiology is generalized perspectives on life that make it possible to differentiate abiotic and biotic chemical systems McKay (2008). A key component of many past and future astrobiological measurements is the elemental ratio of various samples. Classic work on Earth’s oceans has shown that life displays a striking regularity in the ratio of elements as originally characterized by Redfield (Redfield 1958; Geider and La Roche 2002; Eighty years of Redfield 2014). The body of work since the original observations has connected this ratio with basic ecological dynamics and cell physiology, while also documenting the range of elemental ratios found in a variety of environments. Several key questions remain in considering how to best apply this knowledge to astrobiological contexts: How can the observed variation of the elemental ratios be more formally systematized using basic biological physiology and ecological or environmental dynamics? How can these elemental ratios be generalized beyond the life that we have observed on our own planet? Here, we expand recently developed generalized physiological models (Kempes et al. 2012, 2016, 2017, 2019) to create a simple framework for predicting the variation of elemental ratios found in various environments. We then discuss further generalizing the physiology for astrobiological applications. Much of our theoretical treatment is designed for in situ measurements applicable to future planetary missions. We imagine scenarios where three measurements can be made—particle/cell sizes, particle/cell stoichiometry, and fluid or environmental stoichiometry—and develop our theory in connection with these often deployed measurements.

Published
2022

31. Orbital and In‐Situ Investigation of Periodic Bedrock Ridges in Glen Torridon, Gale Crater, Mars

Author

Stack, Kathryn M., primary, Dietrich, William E., additional, Lamb, Michael P., additional, Sullivan, Robert J., additional, Christian, John R., additional, Newman, Claire E., additional, O’Connell‐Cooper, Catherine D., additional, Sneed, Jonathan W., additional, Day, Mackenzie, additional, Baker, Mariah, additional, Arvidson, Raymond E., additional, Fedo, Christopher M., additional, Khan, Sabrina, additional, Williams, Rebecca M. E., additional, Bennett, Kristen A., additional, Bryk, Alexander B., additional, Cofield, Shannon, additional, Edgar, Lauren A., additional, Fox, Valerie K., additional, Fraeman, Abigail A., additional, House, Christopher H., additional, Rubin, David M., additional, Sun, Vivian Z., additional, and Van Beek, Jason K., additional

Published
2022
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36. Depleted carbon isotope compositions observed at Gale crater, Mars

Author

House, Christopher H., primary, Wong, Gregory M., additional, Webster, Christopher R., additional, Flesch, Gregory J., additional, Franz, Heather B., additional, Stern, Jennifer C., additional, Pavlov, Alex, additional, Atreya, Sushil K., additional, Eigenbrode, Jennifer L., additional, Gilbert, Alexis, additional, Hofmann, Amy E., additional, Millan, Maëva, additional, Steele, Andrew, additional, Glavin, Daniel P., additional, Malespin, Charles A., additional, and Mahaffy, Paul R., additional

Published
2022
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40. Spontaneous Oligomerization of Nucleotide Alternatives in Aqueous Solutions

Author

Smith, Karen E, House, Christopher H, Dworkin, Jason P, and Callahan, Michael P

Subjects
Space Sciences (General)
Abstract

On early Earth, a primitive polymer that could spontaneously form from likely available precursors may have preceded both RNA and DNA as the first genetic material. Here, we report that heated aqueous solutions containing 5-hydroxymethyluracil (HMU) result in oligomers of uracil, heated solutions containing 5-hydroxymethylcytosine (HMC) result in oligomers of cytosine, and heated solutions containing both HMU and HMC result in mixed oligomers of uracil and cytosine. Oligomerization of hydroxymethylated pyrimidines, which may have been abundant on the primitive Earth, might have been important in the development of simple informational polymers.

Published
2016
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42. Origin of Life on Mars: Suitability and Opportunities

Author

Clark, Benton C., primary, Kolb, Vera M., additional, Steele, Andrew, additional, House, Christopher H., additional, Lanza, Nina L., additional, Gasda, Patrick J., additional, VanBommel, Scott J., additional, Newsom, Horton E., additional, and Martínez-Frías, Jesús, additional

Published
2021
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43. The grayness of the origin of life

Author

Smith, Hillary H., Hyde, Andrew S., Simkus, Danielle N., Libby, Eric, Maurer, Sarah E., Graham, Heather V., Kempes, Christopher P., Lollar, Barbara Sherwood, Chou, Luoth, Ellington, Andrew D., Fricke, G. Matthew, Girguis, Peter R., Grefenstette, Natalie M., Pozarycki, Chad I., House, Christopher H., Johnson, Sarah Stewart, Smith, Hillary H., Hyde, Andrew S., Simkus, Danielle N., Libby, Eric, Maurer, Sarah E., Graham, Heather V., Kempes, Christopher P., Lollar, Barbara Sherwood, Chou, Luoth, Ellington, Andrew D., Fricke, G. Matthew, Girguis, Peter R., Grefenstette, Natalie M., Pozarycki, Chad I., House, Christopher H., and Johnson, Sarah Stewart

Abstract

In the search for life beyond Earth, distinguishing the living from the non-living is paramount. However, this distinction is often elusive, as the origin of life is likely a stepwise evolutionary process, not a singular event. Regardless of the favored origin of life model, an inherent “grayness” blurs the theorized threshold defining life. Here, we explore the ambiguities between the biotic and the abiotic at the origin of life. The role of grayness extends into later transitions as well. By recognizing the limitations posed by grayness, life detection researchers will be better able to develop methods sensitive to prebiotic chemical systems and life with alternative biochemistries.

Published
2021
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45. The Grayness of the Origin of Life

Author

Smith, Hillary H., primary, Hyde, Andrew S., additional, Simkus, Danielle N., additional, Libby, Eric, additional, Maurer, Sarah E., additional, Graham, Heather V., additional, Kempes, Christopher P., additional, Sherwood Lollar, Barbara, additional, Chou, Luoth, additional, Ellington, Andrew D., additional, Fricke, G. Matthew, additional, Girguis, Peter R., additional, Grefenstette, Natalie M., additional, Pozarycki, Chad I., additional, House, Christopher H., additional, and Johnson, Sarah Stewart, additional

Published
2021
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46. The value of CHONS isotopic measurements of major compounds as probes of planetary origin, evolution, and habitability

Author

Miller, Kelly E., primary, Theiling, Bethany, additional, Hofmann, Amy E., additional, Castillo-Rogez, Julie, additional, Neveu, Marc, additional, Hosseini, Sona, additional, Barnes, Jessica, additional, Kleer, Katherine de, additional, Barrett, Thomas J., additional, Franz, Heather B., additional, Glein, Christopher R., additional, House, Christopher H., additional, Blase, Ryan C., additional, Libardoni, Mark J., additional, Spilker, Linda J., additional, Choukroun, Mathieu, additional, and Drouin, Brian J., additional

Published
2021
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47. On the Past, Present, and Future Role of Biology in NASA’s Exploration of our Solar System

Author

Hand, Kevin, primary, Phillips, Cynthia B., additional, Chyba, Christopher F., additional, Toner, Brandy, additional, Katija, Kakani, additional, Orphan, Victoria, additional, Huber, Julie, additional, Cavanaugh, Colleen M., additional, Carlson, Marian, additional, Christner, Brent, additional, Templeton, Alexis, additional, Seewald, Jeffrey, additional, Hofgartner, Jason D., additional, Amend, Jan P., additional, Orcutt, Beth N., additional, Bartlett, Douglas H., additional, Falkowski, Paul, additional, Anderson, Rika, additional, Spear, John R., additional, Shank, Tim, additional, Fischer, Woodward W., additional, Hazen, Robert M., additional, Hoehler, Tori, additional, D'Hondt, Steven, additional, Pitesky, Jo, additional, Lynch, Kennda, additional, Shock, Everett L., additional, Craft, Kate, additional, Boyd, Eric, additional, House, Christopher H., additional, Reysenbach, Anna-Louise, additional, Glass, Jennifer, additional, Fike, David, additional, Baross, John A., additional, Gogarten, Johann Peter, additional, Kaçar, Betül, additional, El-Naggar, Moh, additional, Murray, Alison E., additional, Dupont, Christopher, additional, Scully, Jennifer, additional, Rothschild, Lynn, additional, Trembath-Reichert, Elizabeth, additional, Klein, Frieder, additional, Cohen, Phoebe A., additional, Gile, Gillian H., additional, Lloyd, Karen, additional, Dekas, Anne, additional, Delaney, John R., additional, Skidmore, Mark, additional, Buongiorno, Joy, additional, Rogers, Karyn, additional, Hofmann, Amy, additional, Brazelton, William J., additional, Anbar, Ariel, additional, Manalang, Dana Ann, additional, Stevenson, Bradley, additional, Neuer, Susanne, additional, Hara, Ellie, additional, Nordheim, Thomas, additional, Shapiro, Russell, additional, Bradley, Alexander S., additional, Mikucki, Jill, additional, Brown, Michael E., additional, Glamoclija, Mihaela, additional, Reyes, Carolina, additional, Sánchez-Román, Mónica, additional, Farmer, Jack D., additional, Giovannelli, Donato, additional, Suel, Gurol, additional, Trumbo, Samantha, additional, Cameron, Marissa, additional, Osburn, Magdalena, additional, Bradley, James Andrew, additional, Garcia-Pichel, Ferran, additional, Steen, Andrew D., additional, Marlow, Jeffrey, additional, Trubl, Gareth, additional, Robinson, Kirtland, additional, Caro, Tristan, additional, Fulfer, Victoria, additional, Parker, Ceth W., additional, Feyhl-Buska, Jayme, additional, and Roussel, Anaïs, additional

Published
2021
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49. Investigation of Pyridine Carboxylic Acids in CM2 Carbonaceous Chondrites: Potential Precursor Molecules for Ancient Coenzymes

Author

Smith, Karen E, Callahan, Michael P, Gerakines, Perry A, Dworkin, Jason P, and House, Christopher H

Subjects
Lunar And Planetary Science And Exploration
Abstract

The distribution and abundances of pyridine carboxylic acids (including nicotinic acid) in eight CM2 carbonaceous chondrites (ALH 85013, DOM 03183, DOM 08003, EET 96016, LAP 02333, LAP 02336, LEW 85311, and WIS 91600) were investigated by liquid chromatography coupled to UV detection and high resolution Orbitrap mass spectrometry. We find that pyridine monocarboxylic acids are prevalent in CM2-type chondrites and their abundance negatively correlates with the degree of pre-terrestrial aqueous alteration that the meteorite parent body experienced. We also report the first detection of pyridine dicarboxylic acids in carbonaceous chondrites. Additionally, we carried out laboratory studies of proton-irradiated pyridine in carbon dioxide-rich ices (a 1:1 mixture) to serve as a model of the interstellar ice chemistry that may have led to the synthesis of pyridine carboxylic acids. Analysis of the irradiated ice residue shows that a comparable suite of pyridine mono- and dicarboxylic acids was produced, although aqueous alteration may still play a role in the synthesis (and ultimate yield) of these compounds in carbonaceous meteorites. Nicotinic acid is a precursor to nicotinamide adenine dinucleotide, a likely ancient molecule used in cellular metabolism in all of life, and its common occurrence in CM2 chondrites may indicate that meteorites may have been a source of molecules for the emergence of more complex coenzymes on the early Earth.

Published
2014
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50. Spindle-shaped Microstructures: Potential Models for Planktonic Life Forms on Other Worlds

Author

Oehler, Dorothy Z, Walsh, Maud M, Sugitani, Kenichiro, and House, Christopher H

Subjects
Exobiology
Abstract

Spindle-shaped, organic microstructures ("spindles") are now known from Archean cherts in three localities (Figs. 1-4): The ~3 Ga Farrel Quartzite from the Pilbara of Australia [1]; the older, ~3.3-3.4 Ga Strelley Pool Formation, also from the Pilbara of Australia [2]; and the ~3.4 Ga Kromberg Formation of the Barberton Mountain Land of South Africa [3]. Though the spindles were previously speculated to be pseudofossils or epigenetic organic contaminants, a growing body of data suggests that these structures are bona fide microfossils and further, that they are syngenetic with the Archean cherts in which they occur [1-2, 4-10]. As such, the spindles are among some of the oldest-known organically preserved microfossils on Earth. Moreover, recent delta C-13 study of individual spindles from the Farrel Quartzite (using Secondary Ion Mass Spectrometry [SIMS]) suggests that the spindles may have been planktonic (living in open water), as opposed to benthic (living as bottom dwellers in contact with muds or sediments) [9]. Since most Precambrian microbiotas have been described from benthic, matforming communities, a planktonic lifestyle for the spindles suggests that these structures could represent a segment of the Archean biosphere that is poorly known. Here we synthesize the recent work on the spindles, and we add new observations regarding their geographic distribution, robustness, planktonic habit, and long-lived success. We then discuss their potential evolutionary and astrobiological significance.

Published
2014

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