Fundamentals of Geobiology E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 1614

Veröffentlichungsjahr: 2012

Beschreibung

2012 PROSE Award, Earth Science: Honorable Mention For more than fifty years scientists have been concerned with the interrelationships of Earth and life. Over the past decade, however, geobiology, the name given to this interdisciplinary endeavour, has emerged as an exciting and rapidly expanding field, fuelled by advances in molecular phylogeny, a new microbial ecology made possible by the molecular revolution, increasingly sophisticated new techniques for imaging and determining chemical compositions of solids on nanometer scales, the development of non-traditional stable isotope analyses, Earth systems science and Earth system history, and accelerating exploration of other planets within and beyond our solar system. Geobiology has many faces: there is the microbial weathering of minerals, bacterial and skeletal biomineralization, the roles of autotrophic and heterotrophic metabolisms in elemental cycling, the redox history in the oceans and its relationship to evolution and the origin of life itself.. This book is the first to set out a coherent set of principles that underpin geobiology, and will act as a foundational text that will speed the dissemination of those principles. The chapters have been carefully chosen to provide intellectually rich but concise summaries of key topics, and each has been written by one or more of the leading scientists in that field.. Fundamentals of Geobiology is aimed at advanced undergraduates and graduates in the Earth and biological sciences, and to the growing number of scientists worldwide who have an interest in this burgeoning new discipline. Additional resources for this book can be found at: href="http://www.wiley.com/go/knoll/geobiology">http://www.wiley.com/go/knoll/geobiology.

Leseprobe

Contents

Contributors

1 What is Geobiology?

1.1 Introduction

1.2 Life interacting with the Earth

1.3 Pattern and process in geobiology

1.4 New horizons in geobiology

2 The Global Carbon Cycle: Biological Processes

2.1 Introduction

2.2 A brief primer on redox reactions

2.3 Carbon as a substrate for biological reactions

2.4 The evolution of photosynthesis

2.5 The evolution of oxygenic phototrophs

2.6 Net primary production

2.7 What limits NPP on land and in the ocean?

2.8 Is NPP in balance with respiration?

2.9 Conclusions and extensions

3 The Global Carbon Cycle: Geological Processes

3.1 Introduction

3.2 Organic carbon cycling

3.3 Carbonate cycling

3.4 Mantle degassing

3.5 Metamorphism

3.6 Silicate weathering

3.7 Feedbacks

3.8 Balancing the geological carbon cycle

3.9 Evolution of the geological carbon cycle through Earth’s history: proxies and models

3.10 The geological C cycle through time

3.11 Limitations and perspectives

4 The Global Nitrogen Cycle

4.1 Introduction

4.2 Geological nitrogen cycle

4.3 Components of the global nitrogen cycle

4.4 Nitrogen redox chemistry

4.5 Biological reactions of the nitrogen cycle

4.6 Atmospheric nitrogen chemistry

4.7 Summary and areas for future research

5 The Global Sulfur Cycle

5.1 Introduction

5.2 The global sulfur cycle from two perspectives

5.3 The evolution of S metabolisms

5.4 The interaction of S with other biogeochemical cycles

5.5 The evolution of the S cycle

5.6 Closing remarks

6 The Global Iron Cycle

6.1 Overview

6.2 The inorganic geochemistry of iron: redox and reservoirs

6.3 Iron in modern biology and biogeochemical cycles

6.4 Iron through time

6.5 Summary

7 The Global Oxygen Cycle

7.1 Introduction

7.2 The chemistry and biochemistry of oxygen

7.3 The concept of redox balance

7.4 The modern O2 cycle

7.5 Cycling of O2 and H2 on the early Earth

7.6 Synthesis: speculations about the timing and cause of the rise of atmospheric O2

8 Bacterial Biomineralization

8.1 Introduction

8.2 Mineral nucleation and growth

8.3 How bacteria facilitate biomineralization

8.4 Iron oxyhydroxides

8.5 Calcium carbonates

9 Mineral–Organic–Microbe Interfacial Chemistry

9.1 Introduction

9.2 The mineral surface (and mineral-bio interface) and techniques for its study

9.3 Mineral-organic-microbe interfacial processes: some key examples

10 Eukaryotic Skeletal Formation

10.1 Introduction

10.2 Mineralization by unicellular organisms

10.3 Mineralization by multicellular organisms

10.4 A brief history of skeletons

10.5 Summary

11 Plants and Animals as Geobiological Agents

11.1 Introduction

11.2 Land plants as geobiological agents

11.3 Animals as geobiological agents

11.4 Conclusions

12 A Geobiological View of Weathering and Erosion

12.1 Introduction

12.2 Effects of biota on weathering

12.3 Effects of organic molecules on weathering

12.4 Organomarkers in weathering solutions

12.5 Elemental profiles in regolith

12.6 Time evolution of profile development

12.7 Investigating chemical, physical, and biological weathering with simple models

12.8 Conclusions

13 Molecular Biology’s Contributions to Geobiology

13.1 Introduction

13.2 Molecular approaches used in geobiology

13.3 Case study: anaerobic oxidation of methane

13.4 Challenges and opportunities for the next generation

14 Stable Isotope Geobiology

14.1 Introduction

14.2 Isotopic notation and the biogeochemical elements

14.3 Tracking fractionation in a system

14.4 Applications

14.5 Using isotopes to ask a geobiological question in deep time

14.6 Conclusions

15 Biomarkers: Informative Molecules for Studies in Geobiology

15.1 Introduction

15.2 Origins of biomarkers

15.3 Diagenesis

15.4 Isotopic compositions

15.5 Stereochemical considerations

15.6 Lipid biosynthetic pathways

15.7 Classification of lipids

15.8 Lipids diagnostic of Archaea

15.9 Lipids diagnostic of Bacteria

15.10 Lipids of Eukarya

15.11 Preservable cores

15.12 Outlook

16 The Fossil Record of Microbial Life

16.1 Introduction

16.2 The nature of Earth’s early microbial record

16.3 Paleobiological inferences from microfossil morphology

16.4 Inferences from microfossil chemistry and ultrastructure (new technologies)

16.5 Inferences from microbialites

16.6 A brief history, with questions

16.7 Conclusions

17 Geochemical Origins of Life

17.1 Introduction

17.2 Emergence as a unifying concept in origins research

17.3 The emergence of biomolecules

17.4 The emergence of macromolecules

17.5 The emergence of self-replicating systems

17.6 The emergence of natural selection

17.7 Three scenarios for the origins of life

18 Mineralogical Co-evolution of the Geosphere and Biosphere

18.1 Introduction

18.2 Prebiotic mineral evolution I – evidence from meteorites

18.3 Prebiotic mineral evolution II – crust and mantle reworking

18.4 The anoxic Archean biosphere

18.5 The Great Oxidation Event

18.6 A billion years of stasis

18.7 The snowball Earth

18.8 The rise of skeletal mineralization

18.9 Summary

19 Geobiology of the Archean Eon

19.1 Introduction

19.2 Carbon cycle

19.3 Sulfur cycle

19.4 Iron cycle

19.5 Oxygen cycle

19.6 Nitrogen cycle

19.7 Phosphorus cycle

19.8 Bioaccretion of sediment

19.9 Bioalteration

19.10 Conclusions

20 Geobiology of the Proterozoic Eon

20.1 Introduction

20.2 The Great Oxidation Event

20.3 The early Proterozoic: Era geobiology in the wake of the GOE

20.4 The mid-Proterozoic: a last gasp of iron formations, deep ocean anoxia, the ‘boring’ billion, and a mid-life crisis

20.5 The history of Proterozoic life: biomarker records

20.6 The history of Proterozoic life: mid-Proterozoic fossil record

20.7 The late Proterozoic: a supercontinent, oxygen, ice, and the emergence of animals

20.8 Summary

21 Geobiology of the Phanerozoic

21.1 The beginning of the Phanerozoic Eon

21.2 Cambrian mass extinctions

21.3 The terminal Ordovician mass extinction

21.4 The impact of early land plants

21.5 Silurian biotic crises

21.6 Devonian mass extinctions

21.7 Major changes of the global ecosystem in Carboniferous time

21.8 Low-elevation glaciation near the equator

21.9 Drying of climates

21.10 A double mass extinction in the Permian

21.11 The absence of recovery in the early Triassic

21.12 The terminal Triassic crisis

21.13 The rise of atmospheric oxygen since early in Triassic time

21.14 The Toarcian anoxic event

21.15 Phytoplankton, planktonic foraminifera, and the carbon cycle

21.16 Diatoms and the silica cycle

21.17 Cretaceous climates

21.18 The sudden Paleocene-Eocene climatic shift

21.19 The cause of the Eocene-Oligocene climatic shift

21.20 The re-expansion of reefs during Oligocene time

21.21 Drier climates and cascading evolutionary radiations on the land

22 Geobiology of the Anthropocene

22.1 Introduction

22.2 The Anthropocene

22.3 When did the Anthropocene begin?

22.4 Geobiology and human population

22.5 Human appropriation of the Earth

22.6 The carbon cycle and climate of the Anthropocene

22.7 The future of geobiology

Plates

Index

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Library of Congress Cataloging-in-Publication Data

Fundamentals of geobiology / edited by Andrew H. Knoll, Donald E. Canfield & Kurt O. Konhauser.p. cm.Includes index.

ISBN 978-1-4051-8752-7 (pbk.) – ISBN 978-1-1182-8081-2 (hardcover)

1. Geobiology. I. Knoll, Andrew H. II. Canfield, Donald E. III. Konhauser, Kurt.QH343.4.F86 2012577–dc23

2011046005

A catalogue record for this book is available from the British Library.

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books.

COMPANION WEBSITEThis book has a companion website:

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Contributors

GIOVANNI ALOISI UMR CNRS 7159LOCEAN, UniversitéPierre et Marie Curie, Paris, France

ARIEL D. ANBAR School of Earth and Space Exploration and Department of Chemistry and Biochemistry, Arizona State University, Tempe AZ 85287, USA

DAVID J. BEERLING Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK

ROGER BUICK Department of Earth & Space Sciences and Astrobiology Program, University of Washington, Seattle WA 98195, USA

NICHOLAS J. BUTTERFIELD Department of Earth Sciences, University of Cambridge, Cambridge, CB2 2EQ, UK

SUSAN L. BRANTLEY Center for Environmental Kinetics Analysis, Earth and Environmental Systems Institute, Pennsylvania State University, University Park PA 16802, USA

DONALD E. CANFIELD Institute of Biology Nordic Center for Earth Evolution, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark

PATRICIA M. DOVE Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg VA 24061, USA

PAUL G. FALKOWSKI Department of Earth and Planetary Sciences and Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick NJ 08901, USA

JAMES FARQUHAR Department of Geology and ESSIC, University of Maryland, College Park MD 20742, USA

W.W. FISCHER Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

LAURA M. HAMM Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg VA 24061, USA

ELISABETH M. HAUSRATH Department of Geosciences,University of Nevada, Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV 89154, USA

ROBERT M. HAZEN Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, DC 20015, USA

D.T. JOHNSTON Department of Earth and Planetary Sciences, Harvard University, Cambridge MA 02138, USA

ANDREAS KAPPLER Geomicrobiology, Center for Applied Geosciences, University of Tübingen, Sigwartstrasse 10, 72076, Tübingen, Germany

JAMES F. KASTING Department of Geosciences, Pennsylvania State University, University Park, PA 16802, USA

BRIAN KENDALL School of Earth and Space Exploration, Arizona State University, Tempe AZ 85287, USA

ANDREW H. KNOLL Department of Organismic and Evolutionary Biology, Harvard University, Cambridge MA 02138, USA

KURT O. KONHAUSER Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada

MARINA LEBEDEVA Center for Environmental Kinetics Analysis, Earth and Environmental Systems Institute, Pennsylvania State University, University Park PA 16802, USA

JONATHAN R. LLOYD Williamson Research Centre for Molecular Environmental Science and School of Earth, Atmospheric and Environmental Science, University of Manchester, Manchester M13 9PL, UK

SARA A. LINCOLN Massachusetts Institute of Technology, Department of Earth and Planetary Sciences, 77 Massachusetts Ave., Cambridge MA 02139, USA

GORDON LOVE Department of Earth Sciences, University of California, Riverside CA 92521, USA

TIMOTHY W. LYONS Department of Earth Sciences, University of California, Riverside CA 92521, USA

DIANNE K. NEWMAN Howard Hughes Medical Institute, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA

VICTORIA J. ORPHAN Division of Geological and Planetary Sciences,California Institute of Technology, Pasadena, CA 91125, USA

DOMINIC PAPINEAU Department of Earth and Environmental Sciences, Boston College, 140 Commonwealth Avenue, Chestnut Hill, MA 02467, USA

CHRISTOPHER T. REINHARD Department of Earth Sciences,University of California, Riverside CA 92521, USA

ANNA-LOUISE REYSENBACH Portland State University, Portland, OR 97207, USA

ROBERT RIDING Department of Earth & Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA

DANIEL P. SCHRAG Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA

STEVEN M. STANLEY Department of Geology and Geophysics, University of Hawaii, 1680 East-West Road, Honolulu HI 96822, USA

ROGER E. SUMMONS Massachusetts Institute of Technology, Department of Earth and Planetary Sciences, 77 Massachusetts Ave., Cambridge MA 02139, USA

DAVID J. VUAGHAN, Williamson Research Centre for Molecular Environmental Science and School of Earth, Atmospheric and Environmental Science, University of Manchester, Manchester M13 9PL, UK

ADAM F. WALLACE Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley CA 94720, USA

KLAUS WALLMANN Leibniz Institute for Marine Sciences (IFM-GEOMAR), Wischhofstrasse, 1-3; 24148, Kiel, Germany

DONGBO WANG Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg VA 24061, USA

BESS WARD Department of Geosciences, Princeton University, Princeton NJ 08540 USA

SHUHAI XIAO Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg VA 24061, USA

1

WHAT IS GEOBIOLOGY?

Andrew H. Knoll1, Donald E. Canfield2, and Kurt O. Konhauser3

1 Department of Organismic and Evolutionary Biology, Harvard University, Cambridge MA 02138, USA2 Nordic Center for Earth Evolution, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark3 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3 Canada

1.1 Introduction

Geobiology is a scientific discipline in which the principles and tools of biology are applied to studies of the Earth. In concept, geobiology parallels geophysics and geochemistry, two longer established disciplines within the Earth sciences. Beginning in the 1940s, and accelerating through the remainder of the twentieth century, scientists brought the tools of physics and chemistry to bear on studies of the Earth, transforming geology from a descriptive science to a quantitative field grounded in analysis, experiment and modeling. The geophysical and geochemical revolutions both reflected and drove a strong disciplinary emphasis on plate tectonics and planetary differentiation, not least because, for the first time, they made the Earth’s interior accessible to research.

While geochemistry and geophysics occupied centre stage in the Earth sciences, another multidisciplinary transformation was taking shape nearer to the field’s periphery. Paleontology had long brought a measure of biological thought to geology, in no small part because fossils provide a basis for correlating sedimentary rocks. But while it was obvious that life had evolved on the Earth, it was less clear to most Earth scientists that life had actually shaped, and been shaped, by Earth’s environmental history. For example, in Tempo and Mode in Evolution, paleontology’s key contribution to the Neodarwinian synthesis in evolutionary biology, G.G. Simpson (1944) devoted less than a page to questions of environmental interactions. As early as 1926, however, the Russian scientist Vladimir Vernadsky had published The Biosphere, setting forth the argument that life has shaped our planet’s surface environment throughout geologic time. Vernadsky also championed the idea of a noosphere, a planet transformed by activities of human beings. A few years later, the Dutch microbiologist Lourens Baas-Becking (1934) coined the term geobiology to describe the interactions between organisms and environment at the chemical level. Whereas most paleontologists stressed morphology and systematics, Vernadsky and Baas-Becking focused on metabolism – and in the long run that made all the difference.

Geobiological thinking moved to centre stage in the 1970s with articulation of the Gaia Hypothesis by James Lovelock (1979). Much like Vernadsky before him, Lovelock argued that life, air, water and rocks interact in complex ways within an integrated Earth system. More controversially, he posited that organisms regulate the Earth system for their own benefit. While this latter view, sometimes called ‘strong Gaia,’ has found little favor with biologists or Earth scientists, most now accept the more general view that Earth surface environments cannot be understood without input from the life sciences. The seeds of these ideas may have been planted earlier, but it was Lovelock who really captured the attention of a broad scientific community.

As the twentieth century entered its final decade, interest in geobiology grew, driven by an increasing emphasis within the Earth sciences on understanding our planetary surface, and supported by accelerating research on the microbial control of elemental cycling, the ecological diversity of microbial life under even the most harsh environmental conditions (commonly referred to as extremeophiles), the use of microbes to ameliorate pollution (bioremediation) or recover valuable metals from mine waste (biorecovery), Earth’s ancient microbial history, and efforts to understand human influences on the Earth surface system. And, in the twenty-first century, universities are increasingly supporting research and education in geobiology, international journals (e.g., Geobiology, Biogeosciences) have prospered, textbooks have been published (e.g., Schlesinger, 1997; Canfield et al., 2005; Konhauser, 2007; Ehrlich and Newman, 2009), and conferences occur regularly. Without question, geobiology has come of age.

1.2 Life interacting with the Earth

Geobiology is predicated on the observation that biological processes interact with physical processes at and near the Earth’s surface. Take, for example, carbon, the defining element of life. Within the biosphere – the sum of all environments that support life on Earth – carbon exists in a number of forms and in several key reservoirs. It is present as CO2 in the atmosphere; as CO2, and CO32− dissolved in fresh and marine waters; as carbonate minerals in soils, sediments and rocks; and as a huge variety of organic molecules in organisms, in sediments and soils, and dissolved in lakes and oceans. Physical processes move carbon from one reservoir to another; for example, volcanoes add CO2 to the atmosphere and chemical weathering removes it. Biological processes do as well. In two notable examples, photosynthesis reduces CO2 to sugar, and respiration oxidizes organic molecules to CO2. Since the industrial revolution, humans have oxidized sedimentary organic matter (by burning fossil fuels) at rates much higher than those characteristic of earlier epochs, making us important participants in the Earth’s carbon cycle. Given the centrality of the carbon cycle to both ecology and climate, its biological and geological components are explored in two early chapters of this book (Chapters 2 and 3) and revisited in the context of human activities in Chapter 22.

Other biologically important elements also cycle through the biosphere. Sulfur, nitrogen, and iron (Chapters 4–6) all link the physical and biological Earth, interacting with each other and, importantly, with the carbon cycle. And oxygen, key to environments that support large animals, including humans, is regulated by a complex and incompletely understood set of processes that, again, have both biological and physical components (Chapter 7).

Unlike physical processes, life evolves, and so the array of biological processes in play within the biosphere has changed through time. The state of the environment supporting biological communities has changed as well. Indeed, given the close relationship between environment and population distributions on the present day Earth, it is reasonable to hypothesize that evolving life has significantly influenced the chemical environment through time and, conversely, that environmental change has influenced the course of evolution.

While metabolism encompasses many of the biological cogs in the biosphere, other processes also play important roles. For example, many organisms precipitate minerals, either indirectly by altering local chemical environments (Chapter 8), or directly by building mineralized skeletons (Chapter 10). Today, skeletons dominate the deposition of carbonate and silica on the seafloor, although this was not true before the evolution of shells, spicules and tests. More subtly, organisms interact with clays and other minerals in a series of surface interactions that are only now beginning to be understood (Chapter 9). While much of geobiology focuses on chemical processes, organisms influence the Earth through physical activities as well – think of microbial communities that can stabilize sand beds (Chapter 16) or worms that irrigate sediments as they burrow (Chapter 11). The example of burrowing reminds us that while microorganisms garner much geobiological attention, plants and animals also act as geobiological agents, and have done so for more than 500 million years (Chapter 11).

In short, Earth surface processes once considered to be largely physical in nature – for example weathering and erosion – are now known to have key biological components (Chapter 12). Life plays a critical role in the Earth system.

1.3 Pattern and process in geobiology

Geobiologists, then, study how organisms influence the physical Earth and vice versa, and how biological and physical processes have interacted through our planet’s long history. Much of this research focuses on illuminating process: field and experimental studies of how organisms participate in the Earth system, and what consequences these activities have for local to global environmental state. Geobiological research can be fundamental – that is, aimed at achieving a basic understanding of the Earth system and its evolution – or it can be applied. In the case of the latter, microbial populations have been deployed and even engineered to perform tasks that range from concentrating gold dispersed in the talus piles of mines, and removing arsenic from the water supply of Los Angeles, to respiring vast amounts of the petroleum that gushed into the Gulf of Mexico in 2010. Building on earlier chapters, Chapters 13–16 focus on techniques that are prominent in modern geobiological research.

Elucidating the changing role of life through Earth history, sometimes called historical geobiology, begins with a basic understanding of geobiological processes, but from there takes on a distinctly geological slant. We would like to interpret the geologic record in terms of active processes and chemical states, but rocks preserve only pattern. Thus, the geobiological interpretation of ancient sedimentary rocks requires that we understand how biological processes and aspects of the ambient environmental state are reflected in the geologically preservable patterns they create. For example, we can use the sulfur isotopic composition of minerals in billion-year-old shales to constrain the biological workings of the ancient sulfur cycle and sulfate abundance in ancient seawater, but can do so only in light of present day observations and experiments that show how biological and physical processes result in particular isotopic patterns.

Of course, there are at least two features that complicate this linkage of geobiological process to geologic pattern. For one, populations evolve, so biological processes observable today may not been active during the deposition of ancient sedimentary rocks. For this reason, historical geobiology has among its goals the establishment of evolutionary pattern in Earth history. The second complication is that many environmental states on the ancient Earth have no modern counterpart. Most obviously, modern surface environments are permeated with oxygen in ways unlikely to have existed during the first two billion years of our planet’s development. Other differences exist, as well. Therefore, the present-day Earth system is far removed from the earliest systems where life evolved and then spread out across the planet; it represents a long accumulation of biological, physical and chemical changes through Earth history. Following a chapter on the origin of life (Chapter 17), perhaps the ultimate example of the intimate relationship between biological and physical processes, we present three chapters that outline Earth’s geobiological history (Chapters 19–21). Oxygen, biological evolution and chemical change dominate these discussions, but there are other aspects to the story. For example, Chapter 18 discusses how the diversity of minerals found on Earth has expanded through time as the biosphere has changed, providing a twenty-first century account of an intriguing subject suggested long ago by Vernadsky.

Finally, there is the question of us. Either directly or indirectly, humans appropriate nearly half of the total primary production on Earth’s land surface. We fix as much nitrogen as bacteria do, and shuttle phosphate from rocks to the oceans at unprecedented rates. As Vernadsky predicted in his early discussion of the noosphere, humans have become extraordinarily important agents of geobiological change. In areas that range from climate change to eutrophication, from ocean acidification to Earth’s declining supplies of fossil fuels and phosphate fertilizer, the human footprint on the biosphere is large and growing. Our societal future depends in part on understanding the geobiological influences of humans and in governing the technological processes that have come to play such important roles in the modern Earth system (Chapter 22).

1.4 New horizons in geobiology

It is difficult, if not impossible, to predict the future, and while it would be fun to attempt a forecast of the status of geobiology in say 20 years, we will avoid this. Rather, we highlight that under all circumstances, geobiology will increasingly look to the heavens. Astrobiology can be thought of as the application of geobiological principles to the study of planets and moons beyond the Earth. At the moment, claims about life in the universe largely constitute under-constrained statistical extrapolations from our terrestrial experience: some hold that life is abundant throughout the universe, but intelligent life is rare (Ward and Brownlee, 2000), while others suggest that life is rare, but intelligence more or less inevitable wherever life occurs (Conway Morris 2004). Clearly, the way forward lies in exploration. Both remote sensing and lander operations have made remarkable strides during the past decade (e.g., Squyres and Knoll, 2006), so we can be confident that on planets and moons within our solar system, direct observation of potentially geobiological patterns will sharply constrain arguments about life in our planetary neighborhood. And arguments about life in nearby solar systems will be framed in terms of geobiological models of planetary atmospheres glimpsed by Kepler and its technological descendents (Kasting, 2010).

This book, then, is a status report. It contains detailed but accessible summaries of key issues of geobiology, hopefully capturing the state and breadth of this emerging discipline. We have tried to be inclusive in our choice of topics covered within this volume. We recognize, however, that the borders defining geobiology are fluid, and we have likely missed or underrepresented some relevant geobiological topics. We apologize in advance for this. We also hope and trust that in the future, geobiology will expand in both depth and breadth well beyond what is offered here. Our crystal ball is cloudy, but we can be certain that a similar book written twenty years from now will differ fundamentally from this one.

References

Baas-Becking LGM (1934) Geobiologie of inleiding tot de milieukunde Diligentia Wetensch, Serie 18/19. van Stockum’s Gravenhange, The Hague.

Canfield DE, Kristensen E, Thamdrup B (2005) Aquatic Geomicrobiology. Elsevier, Amsterdam.

Conway Morris S (2004) Life’s Solution: Inevitable Humans in a Lonely Universe. Cambridge University Press, Cambridge.

Ehrlich HL, Newman DK (2009) Geomicrobiology, 5th edn. Marcel Dekker, New York.

Kasting J (2010) How to Find a Habitable Planet. Princeton University Press, Princeton, NJ.

Konhauser K (2007) Introduction to Geomicrobiology. Blackwell Publishing, Malden, MA.

Lovelock J (1979) Gaia: A New Look at Life on Earth. Oxford University Press, Oxford.

Schlesinger WH (1997) Biogeochemistry: An Analysis of Global Change. Academic Press, San Diego, CA.

Simpson GG (1944) Tempo and Mode in Evolution. Columbia University Press, New York.

Squyres S, Knoll AH, eds (2006) Sedimentary Geology at Meridiani Planum, Mars. Elsevier Science, Amsterdam. [Also published as Earth and Planetary Science Letters240(1).]

Vernadsky VL (1926) The Biosphere. English translation by D.B. Langmuir, Copernicus, New York, 1998.

Ward P, Brownlee D (2000) Rare Earth: Why Complex Life Is Uncommon in the Universe. Springer-Verlag, Berlin.

2

THE GLOBAL CARBON CYCLE: BIOLOGICAL PROCESSES

Paul G. Falkowski

Department of Earth and Planetary Sciences and Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ 08901, USA

2.1 Introduction

Carbon is the fourth most abundant element in our solar system and its chemistry forms the basis of all life on Earth. It is used both as the fundamental building block for all structural biological molecules and as an energy carrier. However, the vast majority of carbon on the surface of this planet is covalently bound to oxygen or its hydrated equivalents, forming mineral carbonates in the lithosphere, soluble ions in the ocean, and gaseous carbon dioxide in the atmosphere. These oxidized (inorganic) forms of carbon are moved on time scales of centuries to millions of years between the lithosphere, ocean and atmosphere via tectonically driven acid-based reactions. Because these reservoirs are so vast (Table 2.1) they dominate the carbon cycle on geological time scales, but because the reactions are so slow, they are also difficult to measure directly within a human lifetime.

The ‘geological’ or ‘slow’ carbon cycle is critical for maintaining Earth as a habitable planet (Chapter 2), but entry of these oxidized forms of carbon into living matter requires the addition of hydrogen atoms. By definition, the addition of hydrogen atoms to a molecule is a chemical reduction reaction. Indeed, the addition or removal of hydrogen atoms to and from carbon atoms (i.e., ‘redox’ reactions), is the core chemistry of life. The processes which drive these core reactions also form a second, concurrently operating global carbon cycle which is biologically catalysed and operates millions of times faster than the geological carbon cycle (Falkowski, 2001). In this chapter, we consider the ‘biological’, or ‘fast’ carbon cycle, focusing on how it works, how it evolved, and how it is coupled to the redox chemistry of a few other elements, especially nitrogen, oxygen, sulfur, and some selected transition metals.

2.2 A brief primer on redox reactions

When carbon is directly, covalently linked to hydrogen atoms, the resulting (reduced) molecules are called organic. Like acid–base reactions, all reduction reactions must be coupled to a reverse reaction in another molecule or atom; that is the reduction of carbon is coupled to the oxidation of another element or molecule. Under Earth’s surface conditions, the addition of hydrogen atoms to carbon requires the addition of energy, while the oxidation of carbon-hydrogen (C–H) bonds yields energy. Indeed the oxidation of C–H bonds forms the basis of energy production for all life on Earth.

Although biologically mediated redox reactions (see Box 2.1) occur rapidly, the products are often kinetically inert. Hence, while it is relatively easy to measure the rate at which a plant converts carbon dioxide into sugars, the product, sugar, is stable. It can be purchased from a local grocery store and kept in a jar in sunlight. It does not spontaneously catch fire or explode. Yet when you eat it, your body extracts the energy from the C–H bonds, and oxidizes the sugar to CO2 and H2O.

2.3 Carbon as a substrate for biological reactions

Approximately 75 to 80% of the carbon on Earth is found in an oxidized, inorganic form either as the gas carbon dioxide (CO2) or its hydrated or ionic equivalents, namely bicarbonate () and carbonate () (see ). These inorganic forms of carbon are interconvertible, depending largely on pH and pressure, and the three forms partition into the lithosphere, ocean and atmosphere (see Chapter 3). Virtually all inorganic carbon in the oceans is in the form of with an average concentration of about 2.5 mM. This carbon is removed in association with calcium and magnesium as carbonate minerals. Although the precipitation of carbonates is thermodynamically favourable in the contemporary ocean, it is kinetically hindered, and virtually all carbonates are formed by organisms. The biological precipitation of carbonates is a result of redox reactions, but rather of acid-base reactions; hence, although virtually all carbonates are biologically derived, they remain as oxidized, inorganic carbon. The mineral phases of inorganic carbon are inaccessible to further biological reactions. The total reservoir of inorganic carbon in the ocean is approximately 50 times that of the atmosphere. Indeed, the ocean controls the concentration of CO in the atmosphere on time scales of decades to millennia.