NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the steering group responsible for the report were chosen for their special competences and with regard for appropriate balance.
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STEERING GROUP FOR THE WORKSHOP ON SIZE LIMITS OF VERY SMALL MICROORGANISMS
ANDREW KNOLL,
Harvard University,
Co-chair
MARY JANE OSBORN,
University of Connecticut Health Center,
Co-chair
JOHN BAROSS,
University of Washington
HOWARD C. BERG,
Harvard University
NORMAN R. PACE,
University of California at Berkeley
MITCHELL SOGIN,
Marine Biological Laboratory
Staff
SANDRA I. GRAHAM, Study Director (from October 17, 1998)
JOSEPH L. ZELIBOR, Jr., Study Director (through October 16, 1998)
ERIN C. HATCH, Research Associate
JACQUELINE D. ALLEN, Senior Program Assistant (through February 1999)
THERESA M. FISHER, Senior Program Assistant (from April 1999)
LAURA OST, Consultant
SPACE STUDIES BOARD
CLAUDE R. CANIZARES,
Massachusetts Institute of Technology,
Chair
MARK R. ABBOTT,
Oregon State University
FRAN BAGENAL,
University of Colorado
DANIEL N. BAKER,
University of Colorado
LAWRENCE BOGORAD,
Harvard University*
DONALD E. BROWNLEE,
University of Washington*
ROBERT E. CLELAND,
University of Washington
GERARD W. ELVERUM, JR.,
TRW Space and Technology Group
ANTHONY W. ENGLAND,
University of Michigan*
MARILYN L. FOGEL,
Carnegie Institution of Washington
RONALD GREELEY,
Arizona State University*
BILL GREEN, former member,
U.S. House of Representatives
JOHN H. HOPPS, JR.,
Morehouse College
CHRIS I. JOHANNSEN,
Purdue University
ANDREW H. KNOLL,
Harvard University
RICHARD G. KRON,
University of Chicago
JONATHAN I. LUNINE,
University of Arizona
ROBERTA BALSTAD MILLER,
Columbia University
BERRIEN MOORE HI,
University of New Hampshire*
GARY J. OLSEN,
University of Illinois at Urbana-Champaign
MARY JANE OSBORN,
University of Connecticut Health Center
SIMON OSTRACH,
Case Western Reserve University*
MORTON B. PANISH,
AT&T Bell Laboratories (ret.)*
CARLÉ M. PIETERS,
Brown University*
THOMAS A. PRINCE,
California Institute of Technology
PEDRO L. RUSTAN, JR.,
U.S. Air Force (ret.)
JOHN A. SIMPSON,
University of Chicago*
GEORGE L. SISCOE,
Boston University
EUGENE B. SKOLNIKOFF,
Massachusetts Institute of Technology
EDWARD M. STOLPER,
California Institute of Technology*
NORMAN E. THAGARD,
Florida State University
ALAN M. TITLE,
Lockheed Martin Advanced Technology Center
RAYMOND VISKANTA,
Purdue University
PETER W. VOORHEES,
Northwestern University
ROBERT E. WILLIAMS,
Space Telescope Science Institute*
JOHN A. WOOD,
Harvard-Smithsonian Center for Astrophysics
JOSEPH K. ALEXANDER, Director
COMMISSION ON PHYSICAL SCIENCES, MATHEMATICS, AND APPLICATIONS
PETER M. BANKS,
ERIM International, Inc.,
Co-chair
W. CARL LINEBERGER,
University of Colorado,
Co-chair
WILLIAM BROWDER,
Princeton University
LAWRENCE D. BROWN,
University of Pennsylvania
MARSHALL H. COHEN,
California Institute of Technology
RONALD G. DOUGLAS,
Texas A&M University
JOHN E. ESTES,
University of California at Santa Barbara
JERRY P. GOLLUB,
Haverford College
MARTHA P. HAYNES,
Cornell University
JOHN L. HENNESSY,
Stanford University
CAROL M. JANTZEN,
Westinghouse Savannah River Company
PAUL G. KAMINSKI,
Technovation, Inc.
KENNETH H. KELLER,
University of Minnesota
MARGARET G. KIVELSON,
University of California at Los Angeles
DANIEL KLEPPNER,
Massachusetts Institute of Technology
JOHN KREICK,
Sanders, a Lockheed Martin Company
MARSHA I. LESTER,
University of Pennsylvania
M. ELISABETH PATÉ-CORNELL,
Stanford University
NICHOLAS P. SAMIOS,
Brookhaven National Laboratory
CHANG-LIN TIEN,
University of California at Berkeley
NORMAN METZGER, Executive Director
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FOREWORD
The world was galvanized in August 1996 by the announcement of possible evidence for relic biogenic activity in the martian meteorite ALH84001. Prominent among the five features cited in support of this startling hypothesis was the observation of "carbonate globules and features resembling terrestrial microorganisms, terrestrial biogenic carbonate structures, or microfossils."1 The structures, revealed in electron micrographs, range in length from 10 to 200 nm. One reason for skepticism about the claim that these have biologic origin is that the martian structures are generally much smaller than the terrestrial objects to which they were compared.
Regardless of one's conclusions about the meteoric evidence for life on Mars, the public fanfare was very effective in focusing attention on scientific questions central to understanding if and how we can recognize extraterrestrial life. The topic of the workshop whose findings are reported here, the size limits of very small microorganisms, is an important one for the interpretation of the carbonate structures on ALH84001, as preparation for future investigations of other samples carried to Earth naturally or in spacecraft and as a litmus test of how well we understand biological organization in general. An indicator of the interest and excitement catalyzed by the Mars announcement is that nearly all the panelists invited to participate accepted with alacrity.
Nearly two dozen researchers applied their diverse expertise to the problem of extrapolating from what we know about Earth's abundant microbial population and the laws of physics and chemistry to draw conclusions about size limits for putative extraterrestrial life forms. Extrapolation is necessary because the only thing we can be reasonably confident about is that extraterrestrial organisms will differ in significant ways from those we find around us. Our life forms have been described by a Nobel Prizewinning biologist as Rube Goldberg contraptions assembled over eons by the stochastic processes of evolution—one can hardly expect that these would be exactly reproduced in other environments.
Part of the legacy of the ALH84001 meteorite is a significant increase in the vigor of NASA's programs in astrobiology, the exploration of the context and possible evidence for life elsewhere in the
solar system, and the search for other planetary systems that might harbor life. These are pursuits that stimulate scientists and the public alike. They also demand the highest standards of scientific rigor—as stated by the late Carl Sagan, extraordinary claims demand extraordinary evidence. This volume lives up to those standards.
CLAUDE R. CANIZARES
CHAIR, SPACE STUDIES BOARD
PREFACE
The question of minimal microbial size continues to be a subject of debate within the scientific community. There is no widely accepted theoretical minimum size for microorganisms. In examining samples from Mars and elsewhere for signs of life, scientists need to know what to look for and how to interpret the results. To help guide its activities in this area, NASA's Office of Space Science (OSS) requested that the Space Studies Board organize a workshop to provide a forum for discussions of the theoretical minimum size for microorganisms. The Board formed the Steering Group for the Workshop on Size Limits of Very Small Microorganisms, which organized a workshop of leading experts in fields relevant to this question.
The researchers who participated in the two-day workshop, convened on October 22-23, 1998, at the facilities of the National Academy of Sciences in Washington, D.C., addressed the following questions:
1. |
What features of biology characterize microorganisms at or near nanometer scale? Is there a theoretical size limit below which free-living organisms cannot be viable? If we relax the requirement that cells have the biochemical complexity of modem cells, can we model primordial cells well enough to estimate their likely sizes? |
2. |
Is there a relationship between minimum cell size and environment? Is there a continuum of size and complexity that links conventional bacteria to viruses? What is the phylogenetic distribution of very small bacteria? |
3. |
Can we understand the processes of fossilization and non-biological processes sufficiently well to differentiate fossils from artifacts in an extraterrestrial rock sample? |
4. |
Does our current understanding of the processes that led from chemical to biological evolution place constraints on the size of early organisms? If size is not constrained, are there chemical signatures that might record the transition to living systems? |
Workshop participants were organized into four panels. Each panel was coordinated by a moderator, who provided a brief introduction to the panel topic and set the stage for the panel discussion. The
moderator ensured that the panelists debated issues in a constructive and scientific manner and that all sides of the issues were explored. Prior to the workshop, each panelist submitted a short paper with a critical assessment of issues. Each panelist made a presentation during the workshop and was later given the opportunity to add to his or her paper any additional points made during that presentation. The content and views expressed in these papers are solely the responsibility of the individual authors.
Although the workshop did not provide definitive answers to the questions addressed, this proceedings document—which describes the workshop findings and identifies issues and opportunities for future research areas to improve our understanding of the size limits of microorganisms—offers novel perspectives and insights, and it provides an intellectual framework for further exploration of key issues discussed by workshop participants.
Comprising eighteen invited papers and a summary of each of the four panel discussions, this volume points out areas in which participants reached general consensus. It does not offer conclusions and recommendations. It is anticipated that this material will provide a valuable reference for astrobiology research and efforts related to the examination of samples returned from Mars and planetary satellites.
ACKNOWLEDGMENT OF REVIEWERS
This report has been reviewed by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the National Research Council's (NRC's) Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the authors and the NRC in making the published report as sound as possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The contents of the review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We wish to thank the following individuals for their participation in the review of this report:
Jack Farmer, Arizona State University,
Marilyn L. Fogel, Carnegie Institution of Washington,
Jeffrey Lawrence, University of Pittsburgh,
Marsha I. Lester, University of Pennsylvania, and
Gary J. Olsen, University of Illinois at Urbana-Champaign.
Although the individuals listed above have provided many constructive comments and suggestions, responsibility for the final content of this report rests with the the individual contributing authors, the workshop steering group, and the NRC.
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