Abstracts

Hyperthermophiles in the history of life

Karl O. Stetter

Lehrstuhl fur Mikrobiologie, Universitat Regensburg, Universitatstrasse 31, D-93053 Regensburg, Germany

Prokaryotes requiring extremely high growth temperatures (optimum 80–110°C) have recently been isolated from water-containing terrestrial, subterranean and submarine high temperature environments. These hyperthermophiles consist of primary producers and consumers of organic matter, forming unique high temperature ecosystems. Surprisingly, within the 16S rRNA-based phylogenetic tree, hyperthermophiles occupy all the shortest and deepest branches closest to the root. Therefore, they appear to be the most primitive extant organisms. Most of them (the primary producers) are able to grow chemolithoautotrophically, using CO2 as sole carbon source and inorganic energy sources, suggesting a hyperthermophilic autotrophic common ancestor. They gain energy from various kinds of respiration. Molecular hydrogen and reduced sulfur compounds serve as electron donors while CO2, oxidized sulfur compounds, NO3- and O2 (only rarely) serve as electron acceptors. Growth demands of hyperthermophiles fit the scenario of a hot volcanism-dominated primitive Earth. Similar anaerobic chemolithoautotrophic hyperthermophiles, completely independent of a sun, could even exist on other planets provided that active volcanism and liquid water were present.

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©1997 The Ciba Foundation


Phylogenetic perspective on microbial life in hydrothermal ecosystems, past and present

Susan M. Barns, Charles F. Delwiche, Jeffrey D. Palmer, Scott C. Dawson, Karen L. Hershberger and Norman R. Pace

Department of Biology and Institute for Molecular and Cellular Biology, Indiana University, Bloomington, IN 47405, USA

Understanding hydrothermal ecosystems, both past and present, requires basic information on the types of organisms present. Traditional methods, which require cultivation of microorganisms, fail to detect many taxa. We have used phylogenetic analyses of small subunit rRNA sequences obtained from microorganisms of a hot spring in Yellowstone National Park to explore the archaeal (archaebacterial) diversity present. Analysis of these sequences reveals several novel groups of archaea, greatly expanding our conception of the diversity of high temperature microorganisms, and demonstrating that hydrothermal systems harbour a rich variety of life. Many of these groups diverged from the archaeal line of descent early during evolution, and an understanding of their common properties may assist in inference of the nature of the last common ancestor of all life. The data also show a specific relationship between low-temperature marine archaea and some hot spring archaea, consistent with a thermophilic origin of life. Future use of rRNA-sequence-based techniques in exploration of hydrothermal systems should greatly facilitate study of modern thermophiles and give us insight into the activities of extinct communities as well.

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©1997 The Ciba Foundation


Hydrothermal systems as environments for the emergence of life

Everett L. Shock

Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St Louis, MO 63130, USA

Analysis of the chemical disequilibrium provided by the mixing of hydrothermal fluids and seawater in present-day systems indicates that organic synthesis from CO2 or carbonic acid is thermodynamically favoured in the conditions in which hyperthermophilic microorganisms are known to live. These organisms lower the Gibbs free energy of the chemical mixture by synthesizing many of the components of their cells. Primary productivity is enormous in hydrothermal systems because it depends only on catalysis of thermodynamically favourable, exergonic reactions. It follows that hydrothermal systems may be the most favourable environments for life on the Earth. This fact makes hydrothermal systems logical candidates for the location of the emergence of life, a speculation that is supported by genetic evidence that modern hyperthermophilic organisms are closer to a common ancestor than any other forms of life. The presence of hydrothermal systems on the early Earth would correspond to the presence of liquid water. Evidence that hydrothermal systems existed early in the history of Mars raises the possibility that life may have emerged on Mars as well. Redox reactions between water and rock establish the potential for organic synthesis in and around hydrothermal systems. Therefore, the single most important parameter for modelling the geochemical emergence of life on the early Earth or Mars is the composition of the rock which hosts the hydrothermal system.

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©1997 The Ciba Foundation


Chemical and physical context for life in terrestrial hydrothermal systems: chemical reactors for the early development of life and hydrothermal ecosystems

Richard W. Henley

Etheridge Henley Williams, PO Box 250, Deakin West, ACT 2600, Australia

The diversity of terrestrial hot spring systems, resulting from the large scale coupled transfer of heat and mass in the Earth's crust, maximize opportunities for evolving ecosystems by the continuous supply of nutrients (P, N, C, S) together with the metals (e.g. K, Mg, Mo, Zn) essential to biogenesis. Cyclic, evaporative microenvironments are common, and potentially catalytic mineral surfaces are also continually created through rock alteration and mineral deposition in and around hot springs. These dynamical systems constitute highly interactive, open, chemical environments capable of establishing complex biochemical microreactors. Volcanic collapse settings on oceanic islands, provide a highly dynamic scenario for the initiation of life and development of diverse ecosystems at the earliest stages of development of the Earth's crust.

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©1997 The Ciba Foundation


Stable light isotope biogeochemistry of hydrothermal systems

David J. Des Marais

Space Science Division, NASA Ames Research Center, Moffett Field, CA 94035-1000, USA

The stable isotopic composition of the elements O, H, S and C in minerals and other chemical species can indicate the existence, extent, conditions and the processes (including biological activity) of hydrothermal systems. Hydrothermal alteration of the 18O/16O and D/H values of minerals can be used to detect fossil systems and delineate their areal extent. Water–rock interactions create isotopic signatures which indicate fluid composition, temperature, water–rock ratios, etc. The 18O/16O values of silica and carbonate deposits tend to increase with declining temperature and thus help to map thermal gradients. Measurements of D/H values can help to decipher the origin(s) of hydrothermal fluids. The 34S/32S and 13C/12C values of fluids and minerals reflect the origin of the S and C as well as oxygen fugacities and key redox processes. For example, a wide range of 34S/32S values which are consistent with equilibration below 100°C between sulfide and sulfate can be attributed to sulfur metabolizing bacteria. Depending on its magnitude, the difference in the 13C/12C value of CO2 and carbonates versus organic carbon might be attributed either to equilibrium at hydrothermal temperatures or, if the difference exceeds 1% (10‰), to organic biosynthesis. Along the thermal gradients of thermal spring outflows, the 13C/12C value of carbonates and 13C-depleted microbial organic carbon increases, principally due to the outgassing of relatively 13C-depleted CO2.

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©1997 The Ciba Foundation


High temperature ecosystems and their chemical interactions with their environment

Allan Pentecost

Division of Life Sciences, King's College London, Campden Hill Road, London W8 7AH, UK

Phototrophic thermal ecosystems consist of microbial mats whose composition is largely determined by water temperature, dissolved oxygen, sulfide and pH. Mats exposed to sunlight consist of an upper zone of phototrophic bacteria and cyanobacteria and an undermat of heterotrophic bacteria. There is little or no net accumulation of reduced carbon and a quasi-equilibrium is established between the synthesis and oxidation of reduced carbon. The flux of carbon and other metabolites induces chemical change in the interstitial water which may assist the deposition of hydrothermal minerals. Uptake of carbon dioxide by phototrophs is favourable to calcium carbonate (travertine) deposition. Thermal systems also contain a range of chemolithotrophs and sulfate reducers potentially capable of depositing carbonate. Microbial acid production by sulfate reducers may have the ability to precipitate silica from alkaline thermal waters but has not yet been demonstrated in vivo. Deposition of thermal ochre is also possible via bacterial oxidation of reduced iron and manganese. It appears that bacteria play a minor role in the deposition of hydrothermal minerals through chemical interaction. However, they may play a more important physical role by providing a large surface area suitable for mineral nucleation. If hydrothermal deposits occur on Mars, the distribution of travertine is likely to be restricted if there is a lack of pre-existing sedimentary carbonate. Less biologically interactive deposits of silica and ochre may predominate.

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©1997 The Ciba Foundation


Ancient hydrothermal ecosystems on Earth: a new palaeobiological frontier

M. R. Walter

School of Earth Sciences, Macquarie University, NSW 2109, Australia

Thermal springs are common in the oceans and on land. Early in the history of the Earth they would have been even more abundant, because of a higher heat flow. A thermophilic lifestyle has been proposed for the common ancestor of extant life, and hydrothermal ecosystems can be expected to have existed on Earth since life arose. Though there has been a great deal of recent research on this topic by biologists, palaeobiologists have done little to explore ancient high temperature environments. Exploration geologists and miners have long known the importance of hydrothermal systems, as they are sources for much of our gold, silver, copper, lead and zinc. Such systems are particularly abundant in Archaean and Proterozoic successions. Despite the rarity of systematic searches of these by palaeobiologists, already 12 fossiliferous Phanerozoic deposits are known. Five are 'black smoker' type submarine deposits that formed in the deep ocean and preserve a vent fauna like that in the modern oceans; the oldest is Devonian. Three are from shallow marine deposits of Carboniferous age. As well as 'worm tubes', several of these contain morphological or isotopic evidence of microbial life. The oldest well established fossiliferous submarine thermal spring deposit is Cambro-Ordovician; microorganisms of at least three or four types are preserved in this. One example each of Carboniferous and Jurassic sub-lacustrine fossiliferous thermal springs are known. There are two convincing examples of fossiliferous subaerial hydrothermal deposits. Both are Devonian. Several known Proterozoic and Archaean deposits are likely to preserve a substantial palaeobiological record, and all the indications are that there must be numerous deposits suitable for study. Already it is demonstrable that in ancient thermal spring deposits there is a record of microbial communities preserved as stromatolites, microfossils, isotope distribution patterns and hydrocarbon biomarkers.

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©1997 The Ciba Foundation


The Rhynie cherts: an early Devonian ecosystem preserved by hydrothermal activity

Nigel H. Trewin

Department of Geology and Petroleum Geology, Meston Building, Kings College, University of Aberdeen, Aberdeen AB9 2UE, UK

The Rhynie cherts contain a remarkable early Devonian terrestrial to freshwater biota preserved in siliceous sinter by the action of a precious-metal-bearing hot spring system. Arthropods, vascular and non-vascular plants, algae, fungi and cyanobacteria are present. Preservation ranges from perfect 3D cellular permineralization to compacted coalified films, and can be related to both silicification processes and stages of biological and physical degradation of the plants at the time of silicification. Plants occasionally have original subaerial vertical axes preserved in growth position, and rhizomes bearing rhizoids. The plant litter of the substrate is also partly silicified. Silicification of organic material took place in hot spring pools, by surface flooding of areas with growing plants, and by permeation of the substrate. Sinters recognized include botryoidal geyserite typical of vent margins, and laminated sinter comparable with that of modern sinter terraces. Massive, vuggy, brecciated and nodular sinter textures are also present. At the microscopic level, textures associated with filamentous elements of the biota, and with the preservation of plants, closely match those present in modern sinters. Oxygen isotope and organic geochemical data from the Rhynie cherts indicate a temperature of 90–120°C. This is apparently greater than the temperature at which elements of the biota were preserved and represents subsequent shallow burial in the hot spring system. The range of temperature and chemistry present at the surface provided high local environmental gradients. Current work attempts to identify thermophilic elements of the biota and document environmental zonation of biota relative to hot spring vents.

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©1997 The Ciba Foundation


Fossilization processes in siliceous thermal springs: trends in preservation along thermal gradients

S. L. Cady and J. D. Farmer

NASA Ames Research Center, MS 239-4, Moffett Field, CA 94035-1000, USA

To enhance our ability to extract palaeobiological and palaeoenvironmental information from ancient thermal spring deposits, we have carried out studies of the processes responsible for the development and preservation of stromatolites in modern subaerial thermal spring systems in Yellowstone National Park (USA). We investigated specimens collected from silica-depositing thermal springs along the thermal gradient using petrographic techniques and scanning electron microscopy. Although it is known that thermophilic cyanobacteria control the morphogenesis of thermal spring stromatolites below 73°C, we have also found that biofilms which contain filamentous thermophiles contribute to the microstructural development of subaerial geyserites that occur along the inner rims of thermal spring pools and geyser effluents. Biofilms intermittently colonize the surfaces of subaerial geyserites and provide a favoured substrate for opaline silica precipitation. We have also found that the preservation of biotically produced microfabrics of thermal spring sinters reflects dynamic balances between rates of population growth, decomposition of organic matter, silica deposition and early diagenesis. Major trends in preservation of thermophilic organisms along the thermal gradient are defined by differences in the mode of fossilization, including replacement, encrustation and permineralization.

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©1997 The Ciba Foundation


Lipid biomarkers for bacterial ecosystems: studies of cultured organisms, hydrothermal environments and ancient sediments

Roger E. Summons, Linda L. Jahnke and Bernd R.T. Simoneit

Australian Geological Survey Organisation, PO Box 378, Canberra, ACT 2601, Australia, Planetary Biology Branch, NASA Ames Research Center, Moffett Field, CA 94035, USA and College of Oceanic & Atmospheric Sciences, Oregon State University, Corvallis, OR 97331-5503, USA

This paper forms part of our long-term goal of using molecular structure and carbon isotopic signals preserved as hydrocarbons in ancient sediments to improve understanding of the early evolution of Earth's surface environment. Here, we combine bacterial biochemistry with the organic geochemistry of contemporary and ancient hydrothermal ecosystems to construct models for the nature, behaviour and preservation potential of primitive microbial communities. We use a combined molecular and isotopic approach to characterize lipids produced by cultured bacteria and test a variety of culture conditions which affect their biosynthesis. This information is then compared with lipid mixtures isolated from contemporary hot springs and evaluated for the kinds of chemical change which would accompany burial and incorporation into the sedimentary record. In this study we have shown that temperature of growth does not appear to alter isotopic fractionation within the lipid classes produced by a methanotropic bacterium. We also found that cultured cyanobacteria biosynthesize diagnostic methylalkanes and dimethylalkanes with the latter only made when growing under low pCO2. In an examination of a microbial mat sample from Octopus Spring, Yellowstone, we could readily identify chemical structures with 13C contents which were diagnostic for the phototrophic organisms such as cyanobacteria and Chloroflexus. We could not, however, confirm that a methane cycle was operating in this particular sample.

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©1997 The Ciba Foundation


The limits of palaeontological knowledge: finding the gold among the dross

Andrew H. Knoll and Malcom R. Walter

Botanical Museum, Harvard University, 26 Oxford Street, Cambridge MA 02138, USA and School of Earth Sciences, Macquarie University, NSW 2109 and Rix & Walter Pty Ltd, 265 Murramarang Road, Bawley Point, NSW 2539, Australia

Palaeontological interpretation rests on two interwoven sets of comparisons with the modern world. Palaeobiological interpretation relies on the placement of fossils within a phylogenetic and functional framework based primarily on the comparative biology of living organisms. Analogy to currently observable chemical, physical, and taphonomic processes enables palaeoenvironmental inferences to be drawn from geological data. In older rocks, comparisons with the modern Earth can become tenuous, limiting palaeontological interpretation. The problem reaches its apogee in Archaean successions, yet pursuit of multiple lines of evidence establishes that complex microbial communities, fuelled by autotrophy and, likely, photoautotrophy, existed 3500 million years ago. Although Archaean palaeontology has to date focused on silicified coastal sediments, improved understanding of Earth's earliest biosphere may depend on the development of alternative environmental and taphonomic analogies. Spring precipitates and hydrothermal metal deposits are promising candidates. Terrestrial organisms may be of limited value in interpreting such fossils as may be found on Mars, although some points of comparison could prove general. Given limited opportunities for exploration, proper choice of environmental analogy is critical. Spring precipitates constitute excellent deposits for addressing questions of biology on another planet.

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©1997 The Ciba Foundation


The role of remote sensing in finding hydrothermal mineral deposits on earth

Jonathan F. Huntington

CSIRO Exploration and Mining, Mineral Mapping Technologies Group, PO Box 136, North Ryde, NSW 2113, Australia

The identification of surface mineralogical composition using hyperspectral sensors is now the major remote sensing opportunity for exploration geologists seeking refined vectors to potential ore-bearing hydrothermal systems. This involves no less than remote, visible and infrared spectroscopy of the molecular composition of geological materials from remote platforms using a large number of calibrated spectral bands. From field-portable systems to those flying in high-flying aircraft on the edges of space, it is now possible to define a long list of minerals and their weathering products detectable at these wavelengths. These include hydroxyl-bearing minerals such as hydrothermal clays, sulfates, ammonium-bearing minerals, phyllosilicates, iron oxides, carbonates, and a wide range of silicates. Indeed, even the chemical composition of micas and chlorites has been mapped remotely using subtle wavelength shifts in their diagnostic reflectance spectra, indicating varying degrees of Na, K, Al, Mg and Fe substitution. Spatial zones, relative abundances and assemblages of these minerals allow geologists to reconstruct the mineralogical, chemical and sometimes thermal disposition of ancient hydrothermal systems in their search for optimal drilling targets. Such minerals not only result directly from the hydrothermal processes involved but may also 'expose' older host rocks caught up in the process and brought to the surface. New microwave radar systems are also shedding new light on landscape processes, textures and structure and occasionally penetrating dry surface layers to reveal buried structures.

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©1997 The Ciba Foundation


Exploration strategies for hydrothermal deposits

Robert A. Horn

Inco Limited, 2060 Flavelle Boulevard, Mississauga, Ontario, Canada L5K 1Z9

With unlimited money the most certain strategy for finding most hydrothermal metal deposits would be by drilling to 5000m at 50m spacing. However, the cost would far outweigh the benefit of the discoveries. Geological knowledge and exploration techniques may be used to obtain the greatest benefit for minimum cost, and to concentrate human and material resources in the most economic way in areas with the highest probability of discovery. This paper reviews the economic theory of exploration based on expected value, and the application of geological concepts and exploration techniques to exploration for hydrothermal deposits. Exploration techniques for hydrothermal systems on Mars would include geochemistry and particularly passive geophysical methods.

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©1997 The Ciba Foundation


Water on early Mars

Michael H. Carr

US Geological Survey, MS-975, 345 Middlefield Road, Menlo Park, CA 94025, USA

Large flood channels, valley networks and a variety of features attributed to the action of ground ice indicate that Mars emerged from heavy bombardment 3.8Ga ago, with an inventory of water at the surface equivalent to at least a few hundred metres spread over the whole planet, as compared with 3km for the Earth. The mantle of Mars is much drier than that of the Earth, possibly as a result of global melting at the end of accretion and the lack of plate tectonics to subsequently reintroduce water into the interior. The surface water resided primarily in a porous, kilometres-thick megaregolith created by the high impact rates. Under today's climatic conditions groundwater is trapped below a thick permafrost zone. At the end of heavy bombardment any permafrost zone would have been much thinner because of the high heat flows, but climatic conditions may have been very different then, as suggested by erosion rates 1000 times higher than subsequent rates. Water trapped below the permafrost periodically erupted onto the surface to form large flood channels and lakes. Given abundant water at the surface and sustained volcanism, hydrothermal activity must have frequently occurred but we have yet to make the appropriate observations to detect the results of such activity.

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©1997 The Ciba Foundation


Hydrothermal systems on Mars: an assessment of present evidence

Jack D. Farmer

NASA Ames Research Center, MS 239-4, Moffett Field, CA 94035-1000, USA

Hydrothermal processes have been suggested to explain a number of observations for Mars, including D/H ratios of water extracted from Martian meteorites, as a means for removing CO2 from the Martian atmosphere and sequestering it in the crust as carbonates, and as a possible origin for iron oxide-rich spectral units on the floors of some rifted basins (chasmata). There are numerous examples of Martian channels formed by discharges of subsurface water near potential magmatic heat sources, and hydrothermal processes have also been proposed as a mechanism for aquifer recharge needed to sustain long term erosion of sapping channels. The following geological settings have been identified as targets for ancient hydrothermal systems on Mars: channels located along the margins of impact crater melt sheets and on the slopes of ancient volcanoes; chaotic and fretted terranes where shallow subsurface heat sources are thought to have interacted with ground ice; and the floors of calderas and rifted basins (e.g. chasmata). On Earth, such geological environments are often a locus for hydrothermal mineralization. But we presently lack the mineralogical information needed for a definitive evaluation of hypotheses. A preferred tool for identifying minerals by remote sensing methods on Earth is high spatial resolution, hyperspectral, near-infrared spectroscopy, a technique that has been extensively developed by mineral explorationists. Future efforts to explore Mars for ancient hydrothermal systems would benefit from the application of methods developed by the mining industry to look for similar deposits on Earth. But Earth-based exploration models must be adapted to account for the large differences in the climatic and geological history of Mars. For example, it is likely that the early surface environment of Mars was cool, perhaps consistently below freezing, with the shallow portions of hydrothermal systems being dominated by magma–cryosphere interactions. Given the smaller gravitational field, declining atmospheric pressure, and widespread, permeable megaregolith on Mars, volatile outgassing and magmatic cooling would have been more effective than on Earth. Thus, hydrothermal systems are likely to have had much lower average surface temperatures than comparable geological settings on Earth. The likely predominance of basaltic crust on Mars suggests that hydrothermal fluids and associated deposits should be enriched in Fe, Mg, Si and Ca, with surficial deposits being dominated by lower temperature, mixed iron oxide and carbonate mineralogies.

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©1997 The Ciba Foundation


The transfer of viable microorganisms between planets

P. C. W. Davies

Department of Physics and Mathematical Physics, The University of Adelaide, Adelaide, SA 5001, Australia

There is increasing acceptance that catastrophic cosmic impacts have played an important role in shaping the history of terrestrial life. Large asteroid and cometary impacts are also capable of displacing substantial quantities of planetary surface material into space. The discovery of Martian rocks on Earth suggests that viable microorganisms within such ejecta could be exchanged between planets. If this conjecture is correct, it will have profound implications for the origin and evolution of life in the solar system.

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©1997 The Ciba Foundation