nai

Executive Summary

Our research at the University of Washington has centered on three important astrobiological questions: What are the characteristics of planets that can evolve complex organisms? Where might such planets occur? How does biological complexity evolve on a planet, and how might it end?

We are beginning the third year of research into these questions. Below, our results and progress is summarized based on specific research problems defined in our original proposal.

How often, where, and under which conditions do habitable planets form and persist? We define a habitable planet as a solid body capable of supporting life as we know it. The study of extra-solar habitable planets involves a broad interdisciplinary approach that extends from understanding how planets are formed to understanding the conditions that allow such life to originate, survive, and evolve. During the past year (2002-2003), Lucio Mayer and Tom Quinn Mayer and Quinn have continued performing simulations of giant planet formation by fragmentation of a gaseous disk, and have published their results in Science. They are extending previous work by considering the effect of different equations of state on the outcome, and by continuing the calculation for longer periods of time. Lufkin and Quinn have started simulations of giant planet migration in gaseous disks, while Barnes and Quinn in collaboration with Lissauer (Ames) continue their simulations of planetesimal accretion in the terrestrial region. They have made progress in speeding up this computationally challenging calculation. The aim is to determine the mass spectrum of planetesimals coming out of the middle stage of planet formation as a function of distance from the Sun. The information to date suggests that the gravitational instability model appears to be viable, and therefore giant planets should be common. Secondly, planet migration appears to be a chaotic process.

Figure 1. Gasdynamical simulations of planet formation show that giant planets can be formed in hundreds of years. Mayer et al, 2002, Science.

Our team is also examining metallicity gradients in nearby stars. These new observations of recently announced stars with planets continue to confirm their previously reported high metallicities relative to otherwise similar field stars. We have also uncovered preliminary evidence that giant planets being discovered by the Doppler method are less common around K and M dwarfs than they are around G dwarfs.

What caused the Delivery and retention of organics and volatiles through Earth History? The volatile and organic composition of impacting bodies is a key factor in the evolution of habitable planets. What are the relative roles of large comet and asteroid fragments and interplanetary dust particles (IDPs) in bringing these materials to a planet? In collaboration with George Cody of Carnegie NAI lead team, Brownlee and Kress have found that organic material found in bulk meteorites can be released into the gas phase when subjected to atmospheric entry conditions (flash-heating at a rate ~500 K/sec and peak temperature of 1000 K for a few seconds). These compounds may have played important roles in the atmospheric chemistry of early Earth, during a time when the flux of micrometeorites was much higher than today. In particular, all of the compounds named above are greenhouse gases. We also found numerous small polycyclic aromatic hydrocarbons, which would have been excellent absorbers of ultraviolet (UV) radiation during a time when no ozone layer existed. Over 250 200µm diameter unmelted micrometeorites were identified and extracted from sediments recovered from the unique South Pole Water Well sample provided by collaborator Susan Taylor from CRREL in Hannover. Graciela Matrajt (an astrobio-supported Post Doc visiting from France) extracted the particles, crushed them and treated them with HF. AIB was discovered in these samples, the first amino acid detection in micrometeorites. We are studying all three of these processes and their effects on prebiotic evolution.

How do mass extinctions and impacts affect the evolution and survival of complex organisms, i.e., the long-term habitability of planets? Mass extinctions are short-term events that kill off a significant proportion of a planet’s biota, and on Earth have been of greatest consequence to more complex organisms such as metazoans. Surface life is vulnerable to major planetary catastrophes, for example, impact of a large comet or asteroid, radiation and particles from a nearby supernova, or catastrophic climate changes such as intense intervals of greenhouse heating or Snowball-Earth type episodes. It is now known that at least one of the major mass extinctions was caused by large body impact, and we would very much like to know if others as well are related to this. The major line of study involves the study of impact craters and their history. Impact cratering involves evaluation of projectiles, which to first order is a measure of the asteroids and comets passing in near Earth space. More broadly it involves the origin of those objects, the orbital evolution of the time of their existence, and at least in the case of asteroids, a series of collision and fragmentation processes that occurred in the asteroid belt. Once the materials are in near Earth space, it involves the evaluation of the material passing through atmospheres of various densities (and in some cases not passing through the atmosphere, and then the mechanism of the hypervelocity impart event itself. The physics of the impact include the explosive release of the kinetic energy, which produces the vaporization and melting of rocks and excavation of material not only as an eject blanket in the immediate vicinity of the crater, but in the case of large impact events (which can be biologically significant) the ejection of debris through the atmosphere into space where it can then envelope the entire planet. The biological consequence for the sudden release of this energy can occur on many scales from affecting individuals to the death of species and even removal of entire ecosystems or biota. The mechanics, biological and geological consequences of impact cratered is an enormous field in the very sweep of its questions. During the past year, two major mass extinction boundaries were studied: the Permian-Triassic (P/T) boundary in Africa and Canada, and the Triassic-Jurassic (T/J) boundary in the Queen Charlotte Islands, Nevada, Italy, and the Newark Basin. Isotopic and paleontological results from these sites are now either in progress or are finished. Our new work suggests that neither the P/T nor T/J mass extinctions were caused by large body impact

Figure 2. Co-I David Kring on the Triassic/Jurassic boundary, New York Canyon, Nevada. Photo by Peter Ward

What can we learn from the geological and fossil record about the evolution of eukaryotes and metazoans? Though we can extrapolate back from existing organisms or build theoretical biogeochemical models, the only robust empirical data that show how early life on Earth evolved and interacted with its environment comes from the study of early Precambrian rocks. Despite popular misconceptions, globally there is in fact a moderate abundance of well-preserved Archaean (>2.5 Ga) and Paleoproterozoic (2.5-1.6 Ga) rocks, which can serve as our clearest windows on the events that occurred shortly after life’s origin. Further research was performed in 2003 on Archean hydrocarbon biomarker geochemistry, sulfur isotopic fractionation, and Paleoproterozoic hydrocarbon preservation in fluid inclusions. Also, new research was initiated in the areas of metamorphism of Archean biosignatures, Archean paleobarometry, and on the age and origin of controversial “microfossils” putatively of Archean age from the Pilbara Craton, Australia. Field-work was conducted on Hadean supracrustal rocks from Isua, Greenland, the early Archean Apex Basalt near Marble Bar, Australia, and the late Archean Fortescue Group near Tom Price, Australia. This research has shown that cyanobacterial and eukaryotic lipids are present in rocks half a billion years before other fossils of these groups appear in the geologic record and that molecular fossils can survive for much longer under higher thermal regimes than previously expected.

Also, we discovered that microbial sulfate reduction existed in ~3.5 billion year old oceans and this reduction shows that peripherally branching bacterial phyla had already evolved. Finally, and perhaps most significantly, the work of our group, based on mapping of the ~3.5 billion year old Apex Basalt, indicates that the “microfossils” previously reported from this unit are not as old as the surrounding rocks.

What can we learn from the physiology and molecular characteristics of extant life about the evolutionary pathways by which microbes and their communities evolve, and by which complex organisms originate? The 0.5 Byr period prior to 3.5 Ga probably experienced extensive evolutionary experimentation and very limited physiological diversity. Recent evidence also points to lateral gene transfer between Bacteria and Archaea as the main mechanism involved in the formation of eukaryotes. Subsequently, symbioses between specific kinds of bacteria and eukaryotes contributed to the rise of oxygen-respiring and oxygen-producing multicellular biota. Today, both lateral gene transfer and interdependence of different species living in communities are ancient processes that continue in the microbial world.

The invention of multicellularity was a major biological innovation contributing to new states of biocomplexity. This was preceded, however, by the invention of eukaryotic cellular organization. The genome sequences from Bacteria have yielded many surprises, including the presence of genes thought only to be present in eukaryotes. We have found genes for tubulin in four species of Prosthecobacter, a genus of the bacterial division, Verrucomicrobia. The discovery of tubulin in bacteria raises questions as to their origin. They could have been transferred horizontally from a eukaryotic organism to these bacteria or vice versa. The goal of our current research is to better understand the origin of the bacterial tubulin genes that are very distantly related to those from eukaryotes, indicating that the transfer, if it occurred, must have happened a long time ago. Based upon partial genome sequences that are available, we are currently trying to determine which of the three hypotheses seems most reasonable. The most readily tested hypothesis is the last one. If it is correct, then we would expect that a substantial number of eukaryotic genes would be found in the genomes of the Verrucomicrobia. Preliminary results based upon Blast searches of genes of the Verrucomicrobia indicate, however, that fewer other eukaryotic genes have been found in the Prosthecobacter genome than expected based on the simplest variant of this model, that the Verrucomicrobia are the founding member of the Eucarya.

What was the nature of early Earth communities? A second way to examine the ancient Earth is through study of microbial communities that likely resemble those of the Precambrian. These communities include those found in (a) anaerobic and photosynthetic microbial mats and biofilms, (b) the sub-seafloor associated with deep-sea hydrothermal vents, and© water ice. It is possible that hidden in the presently unknown diversity of these ecosystems there exist organisms with metabolic pathways that are relics of common metabolisms of the past. We participated in one Early Microbial Ecosystem Research Group (EMERG) field trip in 9/2002 to the Guerrero Negro evaporation ponds to study microbial mat populations in these ponds. These mat populations are visually homogeneous over kilometers of extent and display steep geochemical gradients (e.g. light, oxygen) with depth. We are assessing bacterial diversity using Terminal Restriction Fragment Length Polymorphism (TRFLP), a rapid method for determining total community structure and composition. We have assessed diversity over two scales: horizontally, at varying spatial scales over a kilometer distance, and vertically, using cores sliced at submillimetric scales with depth. Results indicate that these bacterial communities are remarkably stable across even large spatial scales (~1 km); however some variation even at fine scales (cm) was detected. Highly significant differences have been seen with depth in the mat over the upper 9 mm of the mat and significant differences between day and night samples were detected, suggesting significant migration of microorganisms may be occurring.

The second project is investigating a specific functional assemblage of microorganisms, the sulfate-respiring prokaryotes (SRP). High levels of activity for this group have been measured in this site. A wide diversity of SRP has been identified and significant differences in SRP community composition with depth were revealed. Significant correlations of SRP community structure with geochemical gradients were detected.

What is organismal metabolism in extreme environments? Another approach to understanding the habitability of planets is to consider the range of extreme environmental conditions on Earth that support life. For example, the detection of water ice and/or submarine hydrothermal vent systems on another planetary body would satisfy some of the key criteria for habitability. Laboratory observations of bacterial motility and field observations of bacterial attachment led to the testable hypothesis that a temperature threshold exists in ice formations (-10°C in sea ice), below which bacteria cease moving as a means to locate optimal resources and conditions and instead become attached to a surface (first-stage biofilm formation), allowing for continued activity down to -20°C (lowest temperature tested yet). We have also introduced a new aspect to first-stage biofilm formation in the cold: the role of bacterial viruses in altering bacterial behavior and possibly triggering attachment. Building on our finding of considerable morphological diversity of bacteriophage in subzero Arctic seawater and on field work last fall (CASES 2002 expedition), novel phage-host systems were established in the laboratory, using the obligately psychrophilic (cold-loving) bacterium, Colwellia psychrerythraea strain 34H (whole genome sequence available), as the host. Initial experiments have revealed a potential link between phage infection of a bacterium and its production of exopolymers that promote attachment.