2003 Annual Science Report
University of Colorado, Boulder Reporting | JUL 2002 – JUN 2003
The University of Colorado Center for Astrobiology continued its broad efforts that span the entire range of disciplines within astrobiology. We have substantial components within astrobiology research, teaching, and outreach, and each of these will be described briefly.
Within research, our efforts divide into several overall themes, focussing on the physical sciences, the biological sciences, and the humanities. Each of our nine Co-Investigators fits within one or two of these themes.
In the physical sciences, our efforts emphasize the formation of planetary systems (Co-I John Bally), the earliest environment and potential for life on the Earth (Steve Mojzsis), the nature and evolution of planetary habitability (Brian Toon), and the habitability and potential for life on Mars (Bruce Jakosky).
One of the outstanding questions in planet formation around newly forming stars is under what conditions planets will form and under what conditions they will not. Our new results involve observations in the Carina Nebula. This region is one in which abundant stars are forming. It was anticipated that the high density of massive stars would be destructive to the protoplanetary disks that eventually coalesce into planets. However, evidence for a large number of disks was found, making this nebula host to the largest population of disks outside of the Orion Nebula. Given that most low-mass stars are born in the immediate vicinity of highly destructive massive stars, either planet formation must be very rapid or exo-planets will be relatively rare.
Deciphering the history of the Earth’s own habitability as a planet is central to understanding what makes a planet habitable. Our ongoing geological and geochemical approach to this problem included analysis of the so-called “mass-independent” sulfur isotope anomalies in Precambrian sediments. We have unequivocally demonstrated the existence of non-mass-dependent sulfur isotope fractionation in ancient sedimentary sulfides, using a new ion microprobe multicollector technique. This fractionation is thought to result from atmospheric effects involving various photochemical processes, so that its detection in sulfides tells us something fundamentally important about the nature of the atmosphere, the climate, and the oxidation state at the time of sulfide formation, as well as biogeochemical interactions in the sulfur cycle. This work is now being extended to the Great Oxygenation Event between 1.9 and 2.4 Ga ago.
We are applying our results to understanding the nature of the atmosphere, environment, climate, and habitability of Mars. Through a combination of laboratory studies and numerical modeling, we were able to show that carbon dioxide clouds are unlikely to have provided significant greenhouse warming on early Mars. This calls into question the greenhouse model that is the usual explanation for the early warm environment that allowed liquid water to flow (as reflected in the geomorphology of the surface). We also suggested an alternative model for carving the valley networks that involves transient liquid water at extremely high temperatures following impact events early in Martian history, rather than a steady-state greenhouse warming that would allow temperate liquid water to exist at the surface (Fig. 1). While we cannot yet choose between competing models, defining the range of models is important for understanding planetary habitability and our ability to extend our understanding of these processes to extrasolar planetary systems.
Along the same lines, we are examining potential micro-environments on Mars in which liquid water could exist even at sub-zero temperatures. In particular, we are looking at the ability of thin films of liquid water to form at temperatures as high as -20°C in high-latitude ground ice or polar deposits of water ice. These temperatures can occur at moderate and high obliquity values, meaning that liquid water could have been accessible essentially at the surface as recently as a few million to a few tens of millions of years ago on Mars—essentially at the present epoch. Organisms can survive in these environments, so it is conceivable that evidence for Martian life could be found at relatively shallow depths below the surface (e.g., centimeters to meters, rather than hundreds of meters).
In the biological sciences, our efforts focus on the nature of the ribonucleic acid (RNA) World and the origin of life (Co-I Mike Yarus), gene duplication and the evolution of protein functions (Shelley Copley), microbial ecology and life in extreme environments (Norman Pace), and the evolution and nature of symbiosis (William Friedman).
What is the smallest useful RNA population? This question is key to understanding the accessibility of an RNA World, in that RNA is a very difficult molecule to make under primordial conditions. Therefore, the fewer molecules needed to provide for the evolution of biological functions, the easier it would be for them to occur; this is important both for understanding the origin of life on Earth and for understanding the potential for life to occur elsewhere. In the last year, we have shown theoretically in three different ways (two involving numerical calculations and one computer simulation) that the amount of RNA needed for evolution of an RNA cell (a “ribocyte”) is likely to be ten or twelve orders of magnitude smaller than used in all modern selection experiments. Therefore, the RNA World seems to be immensely more accessible than has been previously supposed, and its existence as a real intermediate in the evolution of life on Earth seems much more likely. Of course, this previously unrealized 1010- to 1012-fold advantage must be tested experimentally.
We have begun a new project to probe the evolution of novel enzyme activities by the recruitment of pre-existing proteins. This will allow us to better understand the evolution of metabolic function in already-existing organisms and the evolution of life beyond the initial organism. While enzymes are highly evolved to perform certain functions, they often have advantageous secondary activities as a consequence of the assemblage of highly reactive groups in their active sites. If these secondary activities become useful to the organism, then the enzyme can be recruited to perform a new function, and subsequent mutations can then lead to improvements in that new function. This process has been profoundly important in the evolution of living organisms, and understanding the ability of organisms to move into new ecological niches requires an understanding of this process. We have begun an experimental program to explore this process. During this past year, we have generated a plasmid-based library containing all of the genes in E. coli that we will use to test the ability to recruit genes to replace ones that have been “knocked out”.
A new direction involves “ecogenomics”, the goal of which is to understand the organismal makeup of extremophile communities and how the individual kinds of organisms contribute to the overall concentration of biomass. We have been investigating hypersaline microbial mats, mainly at Guerrero Negro, Baja California. The results contribute to our knowledge of the diversity of life in extreme environments (Fig 2). Although substantial effort has gone into study of chemical aspects of the Guerrero Negro system, relatively little is known about the organisms that comprise these communities. Using molecular survey methods in which ribosomal RNA genes are obtained directly from the natural environment, our results show promise of revolutionizing our view of the makeup of such communities. For example, we find that cyanobacteria, while conspicuously present in these mats, are only one component, and generally a minor component, of the numerically dominant organisms. The generally more abundant organisms are representative of the “Green Nonsulfur” phylogenetic division of bacteria (Fig. 3). This was an unexpected result that changes fundamentally the way that the community needs to be modeled.
Symbioses, the mutually beneficial relationships between two organisms, have evolved numerous times over the course of evolutionary history of life on Earth. Different forms of symbiosis have resulted in some of the most profound evolutionary radiations, such as the origin of the mitochondrion from a symbiosis between a proteobacterium and early eukaryotes, or the establishment of multicellular photosynthetic organisms (i.e., plants) from a symbiosis of fungus and green algae. We are characterizing the fungal symbiotic partners that may have been critical to the colonization of terrestrial (that is, land) environments by photosynthetic organisms. During the last year, we have discovered a complex relationship between early lineages of multicellular land plants and their fungal partners. During one part of their life cycle, the fungus provides nutrients to the host plant in exchange for a supply of fixed carbon; in another part, the symbiosis involves an entirely different fungal partner, and the plant is essentially a parasite on the fungus. This will allow us to determine whether plants radiated into terrestrial environments alone or whether fungi in association with plants were the key to the single colonization of land by photosynthetic organisms. This will have important implications for the evolution of life both on Earth and potentially on other planets.
In the humanities, Co-I Carol Cleland works on the nature of astrobiology as a (predominantly) historical science and the definition of life. Her results on the nature of definition and the ability to define life are having an impact both nationally and internationally. They suggest that we do not have, at present, the ability to formulate a cogent definition for life, given the difficulty of determining which characteristics of life are specific to terrestrial life and which might be more generally applicable. In this context, attempts to define life are analogous to attempts to define “water” prior to the development of molecular theory—they just couldn’t work.
In our teaching efforts, we offer both undergraduate and graduate courses in astrobiology, and we have recently instituted a graduate certificate in astrobiology that can be earned as an adjunct to a related graduate degree. Our mainstay undergraduate course is “Extraterrestrial Life”, an upper-level non-majors course that provides an overview of the entire discipline of astrobiology; it regularly draws the maximum of 75 students each semester, and has been offered every semester for a half-dozen years. At the graduate level, we taught our graduate course in astrobiology; this course is geared toward providing a broad overview of the discipline for graduate students in either the physical or the biological sciences, and the course this year had about 15 students enrolled. In addition, a wide variety of more-specialized courses are taught within the individual disciplines of astrobiology; these include, for example, planetary atmospheres and surfaces, microbiology, and evolution.
Our outreach activities this past year involved two main efforts. First, we continued our very successful series of public symposia with one entitled “Life on Earth—and Elsewhere?” This forum provided an opportunity in our fifth year of NAI funding to spotlight our own program, with several of the Co-Is presenting their research results across the entire spectrum of topics. Second, we began what we hope will be a long series of outreach symposia in which we take the excitement of astrobiology to other institutions. We sponsored a symposium on “Life in the Universe” at Ft. Lewis College in southern Colorado, with presentations by several of our Co-Is. This allowed us to talk about cutting-edge research and exciting directions to students who don’t regularly have a chance to see this type of work, and to interact with both students and faculty in informal settings. Our intent was to use this as a “tune up”, and now to begin a regular series of taking the program to other institutions at no cost to them. We have plans for a second symposium, and will be taking it to a variety of institutions that are attended in large part by students from backgrounds that are not well represented in the sciences.
In summary, we have a vigorous and first-rate astrobiology program at the University of Colorado, and we are making major progress in many of the different areas of astrobiology. Through our Center for Astrobiology, we can bring all of the components together into a broader understanding of life on Earth and the potential for life to exist elsewhere.
Figure 1. This figure is a good summary of the work presented in the paper by T.L. Segura, O.B. Toon, A. Colaprete, and K. Zahnle Science 298, 1977-1980 on the environmental effects of large impacts on Mars. Panel A shows the amount of water from vaporization of the impactor (dotted curve), target material (dash-one dot curve), and polar caps (dash-two dot curve), from melting the subsurface (dashed curve) and the total (solid curve) as a
function of impactor size. Panel B shows the time that the planet’s regolith is above 273 K as a function of impactor size. The modeled objects (100-, 200-, 250- km diameter) are shown as data points.
Figure 2. Microbes in the hypersaline microbial mats of Guerrero Negro. CU Astrobiology Institute researchers, with other members of the NAI Ecogenomics Team, study these photosynthetic microbial mats as an example of extreme ecosystems. Molecular analyses have revealed that the genetic diversity of the communities is enormous.
Figure 3. Photosynthetic microbial communities in brine-saturated halite-gypsum crust from Guerrero Negro, Mexico. The Colorado Astrobiology Team studies these and other unusual microbial communities as models of life in extreme settings.