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2003 Annual Science Report

Carnegie Institution of Washington Reporting  |  JUL 2002 – JUN 2003

Executive Summary

The team led by the Carnegie Institution of Washington is studying the physical, chemical, and biological evolution of hydrothermal systems, including vent complexes associated with ocean ridges, deep aquifers, and other subsurface aqueous environments, both on Earth and on other Solar System and extrasolar bodies. Such diverse systems are important environments for life on Earth and possibly elsewhere in the cosmos.

The traditional view of life’s origin on Earth has focused on processes near the photic zone at the ocean-atmosphere interface, where ionizing radiation provides energy for prebiotic organic synthesis. In the context of astrobiology, this origin paradigm restricts the initial “habitable zone” around stars to planets and moons with surface water. According to this view, subsequent adaptations on Earth, and possibly elsewhere, led to expansion of the biosphere into subsurface habitats.

An alternative hypothesis is that life-forming processes may also occur in subsurface hydrothermal environments at the water-mineral interface. This hypothesis, that life on Earth originated from oxidation-reduction reactions in deep hydrothermal zones, perhaps at or near ocean ridge systems (Figure 1), opens exciting possibilities for astrobiological research. If a subsurface, high-pressure origin of life is possible, then the initial habitable zone around stars is greatly expanded to aqueous environments where redox reactions can be driven along thermal and chemical gradients.

The black smoker sulfide chimney Finn (A) was recovered from the Mothra
Vent Field on the Endeavour Segment of the Juan de Fuca Ridge in July 1998. Finn was venting 302°C fluid upon recovery (B, close-up of Finn) and contained complex mineralogical gradients within its walls. The structure was sampled by Co-I Baross for co-registered microbiological and petrological studies. Attached microbial communities, some of which formed 10 μm-thick biofilms©, were observed, by use of fluorescent probes, throughout the structure, including high-temperature regions near the central vent conduit.

Several lines of evidence lend credibility to the hydrothermal origins hypothesis. Numerous recent discoveries of high-pressure life, especially lithotrophic prokaryotes, suggest that hydrothermal environments support abundant life. Models of the Earth’s formation postulate large, surface-sterilizing impacts as recently as 3.8 billion years ago, but deep hydrothermal zones may have insulated life from these traumas. Studies of molecular phylogeny reveal that thermophilic microbes are perhaps the closest living relatives of the last universal common ancestor. Finally, hydrothermal organic synthesis experiments reveal unexpectedly facile synthetic pathways. Whether or not life originated in a subsurface hydrothermal zone, these lines of evidence, coupled with the assumed widespread distribution of such environments in our Solar System and elsewhere, point to the need and opportunity for an intense study of the characteristics of hydrothermal systems.

Our consortium’s research activities explore the physical, chemical, and biological evolution of hydrothermal systems from these complementary fronts:

  • We model planetary formation, and we detect and characterize extrasolar planets, in an effort to understand the range of objects that develop hydrothermal systems as well as the distribution of volatiles, especially water, within those objects.
  • We investigate the circumstances under which hydrothermal systems form on planets and other bodies and the expected physical and chemical characteristics of those systems as they evolve.
  • We study geochemical processes in hydrothermal systems, especially those that lead to abiotic organic synthesis. A particular focus is the role of mineral catalysis in these systems.
  • We consider the origin and evolution of biological entities in hydrothermal systems through studies of the biochemistry of contemporary hydrothermal organisms.

A complete understanding of hydrothermal systems and their role in life’s origins requires substantial advances on all of these fronts, as well as an extensive and challenging integration of these topics. During the past year we achieved significant progress in each of these research areas, as well as increased attention to the interfaces among these theoretical, experimental, and field approaches.

Among the highlights from the past year’s research in the area of planetary formation and evolution were the following:

  • Ten new planets were discovered by high-precision radial velocity surveys, including the closest known analogue to our Solar System.
  • A transit-planet search yielded three promising candidates for new extrasolar planets.
  • Orbital stability of Earth-like, habitable planets in known extrasolar planetary systems has been constrained by theoretical modeling.
  • Observations of suspected protoplanetary disks imply that planet formation must occur faster than previously thought.
  • A new scenario for Solar System formation raises the estimated frequency of habitable planetary systems by a factor of five.
  • A new hypothesis to account for the pattern of crustal magnetic anomalies on Mars suggests that these features may provide a basis for assessing early hydrothermal activity and hence possibilities for early life on Mars.

Highlights in the area of the evolution of organic matter and water in meteorites included the following:

  • Chondrule formation may produce observable chemical signatures in a disk.
  • CR chondrites contain the most primitive organic matter yet studied from meteorites. The range of compositions seen in chondrites result from aqueous alteration and/or thermal metamorphism under a variety of conditions on the different chondrite parent bodies.
  • Pyrolysis of organic matter in interplanetary dust particles during atmospheric entry may have had an important influence on atmospheric chemistry early in Earth’s history.
  • Ion imaging has revealed for the first time clear evidence for a D-rich, but C-poor, component in interplanetary dust particles. This material most likely consists of hydrated minerals such as phyllosilicates and has a D/H ratio similar to that observed in cometary water.
  • There is a hint that Martian meteorites preserve a record of the evolution of the Martian atmosphere, but the potential influences of shock and/or terrestrial contamination in the meteorites need to be better understood.
  • Measurements have revealed isotopically heavy Fe (δ56Fe ranging up to +0.5‰) in Fe-rich sediments associated with Fe-metabolizing bacterial colonies around hydrothermal vents on the Loihi seamount. This result holds out the promise that Fe isotopes will be a useful biomarker.

Highlights in the area of experimental tests of proposed hydrothermal organic synthesis reactions included the following:

  • A robust abiotic chemical pathway from citrate to aspartate has been demonstrated. This pathway requires that citrate undergoes a retro-aldol reaction to form oxalacetate and that the oxalacetate has sufficient (albeit low) stability to remain long enough to be “trapped” as aspartate. Aspartate lies on the path toward pyrimidine synthesis and hence is a threshold into the ribonucleic acid (RNA) world.
  • The facile conversion of nitrate and nitrite to ammonia at elevated pressures (to 0.2 GPa) and temperatures (to 250°C) has been documented in the presence of 20 different transition-metal oxide and sulfide minerals.
  • A surprisingly facile, straightforward course to chirality and functional polypeptides has been proposed. The pathway stems from the creation and survival of polyamino acids in impacting cometary bodies, a statistical demonstration of large fractions of chiral segments in the racemic polymers, and hydrolysis kinetics analyses showing the sole survival of the chiral segments.
  • A reversal reaction has been demonstrated in the system FeO + CaCO3 + water by producing methane at 5 GPa pressure and temperatures greater than 1800 K. Such a result may have important implications for the generation of hydrocarbons in the hot, deep interiors of terrestrial planets.

In the area of supporting theoretical studies of hydrothermal synthesis reactions, a highlight was the following:

  • Thermodynamic calculations support the possibility that sulfate reduction may be metabolically viable on Europa given the complex chemical processes thought to occur there.

Highlights in the area of isotopic and molecular tracers of life during the past year included the following:

  • There is chemical evidence that the ancient (400-My-old), now extinct, vascular plant Asteroxylon Milleri had the biochemical capability for lignin synthesis whereas the slightly more primitive Aglaophyton did not. Identifying the point in the evolution of plants when lignin synthesis arose may provide insight into the vital evolutionary step that aided colonization of the continents by plants.
  • The trophic interactions that characterized the ancient ecosystem preserved in the now-fossilized Enspel Formation in Germany have been reconstructed from the stable isotopic abundances of carbon and nitrogen.
  • Preliminary data on five microbial species show mass-dependent fractionation of sulfur isotopes.
  • Mass-independent sulfur isotope anomalies correlate with depositional environments of 2.5- to 2.7-Ga black shale and dolomite deposits from the Hamersley and Fortescue Groups, Western Australia.

Highlights in the area of biological studies of hydrothermal systems included the following:

  • A characterization of microbes in subsurface fluids from Axial Volcano, Juan de Fuca Ridge, has demonstrated that there is a highly diverse community of bacteria and archaea that are unique to the subsurface of mid-ocean ridges.
  • A community of potentially nitrogen-fixing archaea and bacteria that occupies the hot, anaerobic habitats of the mid-ocean ridge subseafloor is not dependent on electron acceptors and nitrogen compounds produced by photosynthetic organisms.
  • Studies of microorganisms within sulfide chimneys at active hydrothermal vents suggest that biofilm formation on minerals may be associated with microbial growth or survival at extremely high temperatures.
  • The first phylogenetic characterization of a microbial community associated with a peridotite-hosted hydrothermal vent field has been completed.
  • Deoxyribonucleic acid (DNA) microarrays for E. coli have been optimized to screen for changes in gene expression at high pressures.
  • Isotopic analysis indicates that lithotrophic Fe-oxidizing bacteria appear to be heterotrophic, i.e., they obtain their energy from an inorganic energy source (Fe2+) and their C from organic matter.

In the area of new molecular recognition instruments for astrobiological applications, highlights of the past year included the following:

  • Antihopane antibodies have been developed for rapid screening of the presence of hopanes in extracts of soil samples.
  • Optimization of organic and aqueous extraction protocols have been developed for the extraction of viable and fossil biomarkers for future use in robotic systems.
  • Initial testing protocols have been designed to place these extraction techniques onto a lab-on-a-chip format.
  • Microfluidic devices for the handling of sample extraction and inoculation of microarrays are in the design stage as part of a collaboration with Marshall Space Flight Center.

In summary, our team’s recent research, including discoveries of new planetary systems, exploration of possible hydrothermal regimes on other worlds, elucidation of robust hydrothermal synthetic pathways, documentation of novel microbial metabolic strategies, and finding unexpected high-pressure environments for life, inform the central questions of astrobiology. Taken together, these discoveries are changing our views of life’s origin and its distribution in the universe.