Size Limits of Very Small Microorganisms

Proceedings of a Workshop

Steering Group for the Workshop on Size Limits of Very Small Microorganisms

Space Studies Board

Commission on Physical Sciences, Mathematics, and Applications

National Research Council

NATIONAL ACADEMY PRESS
Washington, D.C.



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Size Limits of Very Small Microorganisms Proceedings of a Workshop Steering Group for the Workshop on Size Limits of Very Small Microorganisms Space Studies Board Commission on Physical Sciences, Mathematics, and Applications National Research Council NATIONAL ACADEMY PRESS Washington, D.C.

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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. The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce Alberts is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. William A. Wulf is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Kenneth I. Shine is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce Alberts and Dr. William A. Wulf are chairman and vice chairman, respectively, of the National Research Council. Support for this project was provided by Contract NASW 96013 between the National Academy of Sciences and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the sponsor. International Standard Book Number 0-309-06634-4 Copyright 1999 by the National Academy of Sciences. All rights reserved. COVER: Design by Penny Margolskee. Copies of this report are available free of charge from: Space Studies Board National Research Council 2101 Constitution Avenue, NW Washington, DC 20418 Printed in the United States of America

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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

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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 *   Former member.

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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 1   D.S. McKay, et al. (1996). Science 273:924-930.

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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

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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

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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.

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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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CONTENTS Overview (Andrew Knoll, Steering Group Co-chair)   1 Panel 1   5     Discussion (Summarized by Christian de Duve, Panel Moderator, and Mary Jane Osborn, Steering Group Co-chair),   5     Metabolism and Physiology of Conventional Bacteria (Dan G. Fraenkel),   10     A Biophysical Chemist's Thoughts on Cell Size (Peter B. Moore),   16     Correlates of Smallest Sizes for Microorganisms (Monica Riley),   21     Mechanical Characteristics of Very Small Cells (David Boal),   26     Gene Transfer and Minimal Genome Size (Jeffrey G. Lawrence),   32 Panel 2   39     Discussion (Summarized by Kenneth Nealson, Panel Moderator),   39     Can Large dsDNA-Containing Viruses Provide Information about the Minimal Genome Size Required to Support Life? (James L. Van Etten),   43     Suggestions from Observations on Nanobacteria Isolated from Blood (E. Olavi Kajander, Mikael Björklund, and Neva Çiftçioglu),   50     Properties of Small Free-Living Aquatic Bacteria (D.K. Button and Betsy Robertson),   56     Bacteria, Their Smallest Representatives and Subcellular Structures, and the Purported Precambrian Fossil “Metallogenium” (James T. Staley),   62     Smallest Cell Sizes Within Hyperthermophilic Archaea ("Archaebacteria") (Karl O. Stetter),   68     The Influence of Environment and Metabolic Capacity on the Size of a Microorganism (Michael W.W. Adams),   74     Diminutive Cells in the Ocean—Unanswered Questions (Edward F. DeLong),   81

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Panel 3   85     Discussion (Summarized by Andrew Knoll, Panel Moderator),   85     Fossils and Pseudofossils: Lessons from the Hunt for Early Life on Earth (J. William Schopf),   88     Taphonomic Modes in Microbial Fossilization (Jack Farmer),   94     Investigation of Biomineralization at the Nanometer Scale by Using Electron Microscopy (John Bradley),   103 Panel 4   107     Discussion (Summarized by Leslie Orgel, Panel Moderator, and Laura Ost, Consultant),   107     Primitive Life: Origin, Size, and Signature (James P. Ferris),   111     Constraints on the Sizes of the Earliest Cells (Jack W. Szostak),   120     How Small Can a Microorganism Be? (Steven A. Benner),   126 Appendixes     A   Steering Group Biographies,   139 B   Request from NASA,   142 C   Workshop Agenda,   143 D   Workshop Participants,   147

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This report is dedicated to the memory of DR. JOSEPH L. ZELIBOR, JR. (1953-1998) a respected friend and colleague whose boundless energy and vision shaped this workshop.

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