FEMS Microbiology Letters 185 (2000) 9^16
www.fems-microbiology.org
MiniReview
Exploration of deep intraterrestrial microbial life:
current perspectives
K. Pedersen *
Department of Cell and Molecular Biology, Microbiology Section, Go«teborg University, P.O. Box 462, SE-405 30 Go«teborg, Sweden
Received 28 September 1999; received in revised form 26 January 2000; accepted 26 January 2000
Abstract
Intraterrestrial life has been found at depths of several thousand metres in deep sub-sea floor sediments and in the basement crust beneath
the sediments. It has also been found at up to 2800-m depth in continental sedimentary rocks, 5300-m depth in igneous rock aquifers and in
fluid inclusions in ancient salt deposits from salt mines. The biomass of these intraterrestrial organisms may be equal to the total weight of
all marine and terrestrial plants. The intraterrestrial microbes generally seem to be active at very low but significant rates and several
investigations indicate chemolithoautotrophs to form a chemosynthetic base. Hydrogen, methane and carbon dioxide gases are
continuously generated in the interior of our planet and probably constitute sustainable sources of carbon and energy for deep
intraterrestrial biosphere ecosystems. Several prospective research areas are foreseen to focus on the importance of microbial communities
for metabolic processes such as anaerobic utilisation of hydrocarbons and anaerobic methane oxidation. ß 2000 Federation of European
Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.
Keywords : Autotrophy; Biosphere ; Groundwater; Rock; Subsurface
1. Introduction
1.1. How deep is deep ?
Jules Verne's famous book, Journey to the Centre of the
Earth, reports how Professor Lidenbrock and his nephew,
Axel, found advanced life in natural caves and tunnels
deep below Europe. At the time it was written the story
was genuine science ¢ction. Now, 135 years later, it is
obvious that Jules Verne in a sense was correct because
deep under the terrestrial ground as well as deep under the
sea £oor, life is very abundant, albeit in cavities smaller
than imagined by Verne. Research directed towards exploring intraterrestrial microbial life is a rapidly growing
¢eld re£ected by an increasing number of meetings and
workshops in the research ¢eld and also by numerous
intriguing articles in popular science journals [1^4]. The
purpose of this mini-review is to give an overview of microbial life in various deep intraterrestrial habitats that are
at present under exploration and to identify central questions regarding our understanding of microbial processes
deep inside the earth, in the dark realm of deep sediments,
rocks and minerals.
The superdeep well, SG-3, 12 262 m deep, in the Pechenga-Zapolyarny area of Kola Peninsula, Russia, is currently
the deepest drilled hole in the world [5]. One of many
important reasons for drilling this borehole was to see
how deep a borehole could be drilled. The prospect of
drilling a superdeep borehole becomes increasingly di¤cult
with increasing depth and success depends on the geological formation drilled, the quality of equipment used and
the skills of the drilling personnel. In addition, the drill
string may get stuck in a layer with bad borehole stability
and this is always a major uncertainty in deep drilling
projects.
Other superdeep boreholes include several Cretaceous^
Tertiary boundary (KTB) boreholes drilled by the German
Continental Deep Drilling Programme into the crystalline
rock of the Bavarian Black Forest (Schwarzwald) basement in Central Europe [5]. The deepest of the six wells
drilled was 9100 m and it reached an in situ temperature
of 265³C at that depth. One of these KTB wells was
searched for hyperthermophiles at a depth of 4000 m.
Culturable microorganisms could not be demonstrated,
possibly because the temperature of the sampled £uids,
118³C, was too high for life [6]. So far, the highest cultur-
* Tel. : +46 (31) 773 25 78; Fax: +46 (31) 773 25 78;
E-mail : [email protected]
0378-1097 / 00 / $20.00 ß 2000 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 1 0 9 7 ( 0 0 ) 0 0 0 6 1 - 6
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ing temperature for hyperthermophiles has not exceeded
113³C [7].
Another very deep borehole was drilled in Gravenberg,
Sweden, in the search for deep earth gases [8]. It reached
6800 m and thermophilic bacteria were successfully enriched and isolated from a depth of 5278 m where the
temperature was 65^75³C [9].
The borehole windows into superdeep environments are
still very few and none have been drilled with microbiology as its major motive. Boreholes drilled with the purpose of exploring microbial life are rarely deeper than
1000 m [10,11]. However, depth is not the only limiting
factor for survival of deep life. Rather, it is temperature
that probably sets the ultimate limit for how deep life can
penetrate into our planet. A temperature that is too high
for life is reached at very di¡erent depths, from the sea
£oor surface at marine hot springs to 10.000 m down or
deeper in massive sedimentary rock formations. The absolute majority of boreholes explored for microbial life does
not reach such depths because the drilling cost for a borehole increases with depth which, together with increasing
technical challenges, limits the number of superdeep boreholes drilled thus far to very few. The exploration of intraterrestrial life is consequently, in parallel to the exploration of possible life on other planets, strongly dependent
on sophisticated technological equipment and skills. The
exploration of the (super)deep intraterrestrial biosphere
has merely begun.
1.2. Global microbial biomass distribution at a glance
The total number of intraterrestrial microorganisms
varies notably, depending on the site studied. Values in
the range of 103 to 108 per ml groundwater or g sediment
are commonly reported [10,12]. Although the dry weight
of 108 cells in 1 g of sediment is very small, viz. in the
range of 1^10 Wg, the weight of microorganisms over many
square kilometres of ocean £oor and continental shelf
sediments, rock aquifers and so on may reach an impressive total. An attempt to estimate the total carbon in terrestrial and intraterrestrial environments was recently
made [13]. Calculations indicate that the total amount of
carbon in intraterrestrial organisms may equal that of all
terrestrial and marine plants (Table 1). Although subject
to a great deal of uncertainty, the estimates therefore suggest that the biomass of intraterrestrial life is very large. A
wealth of microbial life may exist deep inside the earth,
with many unknown species and microorganisms with
unique physiological and biochemical features and is
awaiting exploration.
1.3. Exploration in whose interests?
The interests in investigations of intraterrestrial life are
represented by a very diverse array of general social, professional and industrial motives. Some of the current reasons for such investigations are listed below.
1. Microbial activity in oil wells may have both negative
(e.g. through corrosion and well souring) and positive
(e.g. through surfactant production) e¡ects on oil extraction. The oil industry, therefore, shows an interest
in deep oil reservoir microbiology [14^16].
2. The contamination of groundwater from surface and
underground disposal sites, accidental spills, leakage
and other human activities has triggered a widespread
interest in the possibilities of restoring contaminated
underground sites with the help of autochthonous
and/or allochthonous microorganisms [17].
3. Disposal of radioactive wastes and heavy metals in
deep geological formations requires in-depth knowledge
about the host rock environment, including possible
e¡ects of microbes on future repositories [18].
4. There are enormous reservoirs of energy in the methane
gas hydrates that are found globally in sub-ocean £oor
sediments, possibly twice the amount of energy contained in known oil and gas reservoirs [19]. Most of
this methane is believed to have been produced by
methanogens living deep below the deposits [20].
5. An increasing number of scientists argue for an underground origin of life, possibly in the vicinity of hydrothermal systems [21]. If life did originate subterraneously, then it must have been present underground for
as long as there has been life on our planet. A diverse
and extended underground life on, or within, our planet
suggests that life on other planets should be searched
for underground rather than on the surface.
6. Last but not least, the increasing knowledge about intraterrestrial life may signi¢cantly expand our knowledge of microbial diversity and, especially, of the metabolic capabilities of living organisms (see, for example,
[22]).
Table 1
Global carbon content in plant and prokaryotic biomass
Ecosystem
Continental
Oceanic
Total
Carbon content, 1012 kg of C
plant
soil and aquatic prokaryotes
intraterrestrial prokaryotes
total
560
1.8
561.8
26
2.2
28.2
22^215
303
325^518
608^801
307
915^1108
Source: [13]
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Each of these interest areas focuses on more or less
speci¢c intraterrestrial environments and some of the currently studied ones are discussed below.
2. Environments for intraterrestrial life
11
ultimately be a driving force for active life of the microbes
dwelling in both deep rock aquifers and sediments. The
intraterrestrial microbes can, however, not be more active
than the support of energy over time allows and this may
be a very slow, di¡usion-limited process.
2.2. The sub-sea £oor sediment and basement rock
2.1. General considerations
The realm of sediments, rocks and minerals o¡ers environments for life that are very di¡erent from terrestrial
and aquatic habitats. Water is common, but there is a
very large solid surface-area-to-water-volume ratio and
generally there is relatively little space for water and life
per volume subsurface. The intraterrestrial microbes dwell
in the pores of consolidated sediments and coarser, unconsolidated material, in fractures in hard rock and in £uid
inclusions (see Section 2.4). A microscopic cell is ideally
suited to these environments. Almost all very deep environments are anaerobic, with the exception of places
where radioactivity may cause radiolysis of water, producing hydrogen and oxygen. With an increase in depth, the
amount of dissolved solids in the groundwater tends to
increase and so does the temperature, but the gradients
of these increases vary, depending on the geological formation. The temperature trend implies that at all places on
the planet, there will be a maximum depth below which
life as we know it is impossible, but the maximum depth
for life is highly variable. The upper known maximum
temperature for microbial growth, 113³C, is reached at
the ocean £oor at hydrothermal vents, but not before
5000^10 000-m depth or more in shield rocks, mountains
and deep sediments.
There is a set of questions that are commonly addressed
relating to the microbiology of most intraterrestrial environments. These are largely equivalent to the questions
posed in studies of terrestrial and aquatic ecosystems.
Numbers of individual cells and the members of various
physiological groups, as well as the diversity of microbial
populations are frequently analysed. Once the presence,
abundance and guild structure of microorganisms have
been demonstrated, species diversity may be addressed,
but a key question is the metabolic state of the community, that is, are the intraterrestrial microbes metabolically active at any level or are they resting in dormant
states ?
Microbes are experts on utilising any energy that becomes thermodynamically available for biochemical reactions in the environment. Such energy can be extracted
from various sources, depending on the type of geological
formation. Recharging groundwater may carry a slow £ow
of organic carbon from the ground surface down into hard
rock aquifers. In sedimentary rock layers, utilisable organic carbon may have remained since their formation in an
ancient sea. Finally, a £ow of reduced gases such as hydrogen and methane from the mantle of our planet could
The Ocean Drilling Program (ODP) is an international
partnership of scientists and research institutions organised to explore the evolution and structure of the earth
(http://www.oceandrilling.org/). With its drill ship,
JOIDES Resolution, the ODP can drill cores (long cylinders of sediment and rock) in water depths of up to 8.2
km. Since January 1985, the ODP has recovered more
than 160 000 m of cores. Results obtained thus far show
microbial life to be abundant both in the deep sub-sea
£oor sediments and in the basement crust under the sediments [23,24]. As a result, exploration of the deep sub-sea
£oor biosphere is now a high-priority research objective of
the ODP.
The basement of the sea £oor is born at spreading ocean
ridges that commonly stretch over long distances on the
bottom of the sea. Lava and mantle rock move up at the
site of these ridges and thus reach the ocean £oor. At the
ridges, hydrothermal vents are the obvious indicators of
an ongoing convective circulation of saltwater through
¢ssures and cracks in the newborn hot basement. Some
calculations indicate that all oceanic water passes through
the basement rocks every 30 million years. Large parts of
the new ocean £oor consist of pillow lavas with amorphic
volcanic glass on the pillow surface. Microscopic and culturing investigations strongly suggest that microorganisms
are involved in the weathering of this volcanic glass
[23,25]. Channels, cavities and holes in the micrometer
range penetrate the glass and the tips of the tunnels commonly stain with nucleic acid speci¢c stains, suggesting
the presence of microorganisms. The circulation of saltwater through the basement and the weathering activities of microorganisms in the £ow paths therefore may
have a signi¢cant impact on the composition of saltwater
[26].
Due to plate tectonic forces the new ocean basin that
forms at spreading ocean ridges migrates towards subduction zones where it is engulfed under continental shelves
and returned to the interior of our planet. This tour takes
at most 170 million years. During that time, more and
more sediments build up on top of the hard basement
rock and eventually layers are formed that can be thousands of metres thick. During the sedimentation process,
microorganisms are mixed with the sediment and they
appear to be important factors in the diagenesis of the
sediments [24]. The continued presence of active bacterial
populations in deep sediments that are over 10 million
years old may relate to acetate generation from organic
matter during burial. This was indicated by the fact that
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K. Pedersen / FEMS Microbiology Letters 185 (2000) 9^16
at two sites in the Atlantic Ocean, pore-water acetate
concentration has been found to increase at depths
below about 150 m [24]. The increase appears to be associated with a signi¢cant increase in bacterial acetogenic
activity.
The acetate produced in the sediments is available for
acetoclastic methanogenesis and this may be a possible
mechanism for formation of the enormous reservoirs of
methane in gas hydrates found globally in sub-sea £oor
sediments [19]. Methane formation by bacterial methanogenesis based on the burial of and acetogenic decomposition of organic carbon in marine sediments has been demonstrated to roughly explain the content of gas hydrates in
the marine sediments investigated [20]. However, for the
local formation of massive concentrations of gas hydrates,
some accumulation processes are required. Accumulation
of gas hydrates near the base of the gas hydrate stability
zone (which depends on pressure and temperature) is possible if methane migrates from depths below the hydrate
deposits. The enormous reservoirs of energy in the form of
methane gas hydrates in sub-sea £oor sediments seem to
have been produced by methanogens living deep underneath the deposits.
A recently discovered global warming catastrophe occurred 55 million years ago as a result of rapid melting
of methane gas hydrates [27]. That event demonstrates the
delicate balance of our global climate and should be taken
as a serious warning. We cannot ignore the possibility that
a continuation of the newly observed rapid melting of the
arctic ice [28] may trigger a similar, but modern such catastrophe. A future Antarctic sea without its ice cover
would in summer time take up 80% solar energy instead
of re£ecting 80%, as done presently with warming of the
oceans as an obvious result. The past event of global
warming is a good example of how microbes may have
an enormous in£uence on global processes, albeit indirectly, via methane formation in deep sub-ocean £oor environments.
Successful enrichments of thermophiles have been performed from samples from sub-sea £oor and continental
oil reservoirs [14^16,29], indicating that microorganisms
survive in deep hot oil reservoirs. However, the need for
quality assurance for good microbiology sampling, with
protocols for contamination control, has not been emphasised in the oil industry. It has still not been conclusively shown that these isolates were indigenous to the
deep oil wells. This is surprising, since it is well known
to oil drillers that microorganisms, especially sulfate-reducing bacteria, may cause expensive problems related to
corrosion and souring of wells. Solving the question of the
origin of microbes in deep oil wells would de¢nitely reduce
uncertainties about how these problems can best be attacked. The costs for proper contamination control would
be extremely small in comparison with the costs that can
be saved by reducing microbial corrosion and oil well
souring.
2.3. Continental sedimentary rocks
A signi¢cant e¡ort was devoted to a good contamination control during the US Department of Energy's investigative programme on the microbiology of subsurface
sediments [10]. For instance, the need for rapid initiation
of analyses after sediment sample acquisition was demonstrated for measurements of in situ microbial properties
[30]. Subsequently, the experience gained was used in studies of the environmental conditions under which microorganisms exist in deep continental hydrocarbon reservoirs. The samples included sidewall cores collected from
a natural gas-bearing formation 2800 m below the surface
in Taylorsville Basin, Virginia, in the USA [31]. Microbiological data, and data from chemical and microbial tracers
and controls indicated that the interiors of some sidewall
cores contained microorganisms indigenous to the deep
hot rock formation. The cultured microorganisms were
composed primarily of saline-tolerant, thermophilic, fermenting, Fe(III)-reducing and sulfate-reducing bacteria
with physiological capabilities compatible with the in situ
temperature (76³C) and pressure (32 MPa).
Large phylogenetically and culturally diverse communities of Archaea and Bacteria in North American deep
subsurface continental sediments have been demonstrated
[32], which is in line with reported biodiversity from other
investigated underground sites in Africa and Scandinavia
[33,34]. The possible in situ activity of microorganisms in
the subsurface sedimentary rocks studied was also addressed. As in other oligotrophic habitats, activity occurred, but at very slow rates. Also, there was usually a
signi¢cant correlation between the type of geological strata, for instance in the pore size and organic content, and
activity [35].
The research of continental sedimentary rocks shows
that living microbes are present and active down to the
deepest levels studied (about 3000 m). They will probably
be present even deeper, as deep as the temperature allows,
as discussed in Section 1.1. The results have important
implications for long-term maintenance of anoxic conditions and the impact of anaerobic biochemical processes
on groundwater chemistry. They also imply that natural
attenuation of contaminated groundwater by indigenous
microbes can occur at depth. Further work is needed to
unravel details about these signi¢cant processes.
2.4. Ancient salt deposits
Salt crystals are formed when saline water evaporates.
During this process, small parts of the brine become
trapped in the salt and end up as £uid inclusions. Any
halophilic microorganisms present in the £uid will be
trapped as well. Large, ancient deep deposits of salt exist
all over our planet. For instance, there are extensive salt
deposits deep under Germany, Denmark and the North
Sea, formed 250 million years ago from shallow seas which
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K. Pedersen / FEMS Microbiology Letters 185 (2000) 9^16
evaporated when the climate in Europe was much warmer
because of an equatorial position of the European continent caused by plate tectonics. Similar formations are
found elsewhere on the planet. When ancient £uid inclusions from such deposits are opened and the £uid is transferred to culture media for halophiles, growth occurs and
the growing organisms exhibit a large species diversity
[36]. It seems very probable that these growing halophiles
were captured 250 million years ago during the formation
of the salt deposits. This ¢nding invokes many intriguing
questions, regarding how long microorganisms can survive
and speci¢cally, how halophiles could stay alive for 250
million years in their small, saline prisons. These observations do indeed imply that some microorganisms may have
eternal life, at least in relation to the brief lifetime of most
plants and animals.
2.5. Aquifers in igneous terrestrial rocks
Igneous rocks are the predominant solid constituents of
the earth, formed through cooling of molten or partly
molten material at or beneath the earth's surface. These
rocks are penetrated by humans for a variety of purposes
such as mining for metals, the extraction of groundwater
and the building of tunnels and vaults for communication,
transport, defence, storage, deposition and waste disposal.
The concept of underground storage of hazardous wastes
is commonly applied to materials that are impossible to
transform to a non-hazardous form. One group of such
materials is the toxic metals, especially heavy metals and
radionuclides. The disposal concepts vary from country to
country, but igneous rocks are commonly in target for
countries with access to these types of geological structures. Deposition of long-lived hazardous wastes requires
extremely good knowledge about the igneous rock environment to be used as a host. Particularly the nuclear
waste industry invests large sums in research on the safe
underground disposal of all types of radioactive wastes
[37].
Early studies in the Swedish long-term nuclear waste
disposal research programme on subterranean microbiology revealed previously unknown microbial ecosystems in
igneous rock aquifers at depths exceeding 1000 m [38].
This discovery triggered a thorough exploration of the
subterranean biosphere in the aquifers of the Fennoscandian (Baltic) Shield [39]. Similarly, the Canadian radioactive waste disposal programme has stimulated investigations of microorganisms in deep igneous rock aquifers of
the Canadian Shield [40].
Other investigations examined the potential risk of radionuclide migration caused by microorganisms able to
survive in the deep groundwater systems [41]. It soon became apparent that microbial communities exist in most, if
not all, deep aquifers [38]. Attention was then shifted to
assay the activity potential of these microorganisms using
radiotracer methods [42^45]. The results suggested re-
13
markable metabolic and species diversity, which led to
DNA extraction and 16/18S rDNA cloning and sequencing for assessment of subterranean microbial diversity
[34,46,47]. This work has also revealed several hitherto
unknown microbial species adapted to life in igneous
rock aquifers [48^50].
The repeated observations of autotrophic, hydrogen-dependent microorganisms in the deep aquifers imply that
hydrogen may be an important electron and energy source
and carbon dioxide an important carbon source in deep
subsurface ecosystems. Hydrogen, methane and carbon
dioxide have been found in WM concentrations at all sites
that have been tested for these gases [51]. Methane is a
major product of autotrophic methanogens, which have
î spo« Hard Rock Labobeen shown to be present at the A
ratory in Sweden, situated 400 km south of Stockholm
[51]. Therefore, a model has been proposed of a hydrogen-driven biosphere in deep igneous rock aquifers in the
Fennoscandian Shield [39]. A similar model has been suggested for deep basalt aquifers [52]. The organisms at the
base of these ecosystems are assumed to be autotrophic
acetogens capable of reacting hydrogen with carbon dioxide to produce acetate, autotrophic methanogens that produce methane from hydrogen and carbon dioxide, and
acetoclastic methanogens that produce methane from the
acetate product of the autotrophic acetogens (Fig. 1). All
components needed for the life cycle in Fig. 1 have been
found in deep igneous rock aquifers and the microbial
activities expected have been demonstrated [51]. Consequently, the model is supported by the qualitative data
obtained so far. Ideally, the next step will be to obtain
quantitative data, which would require very sensitive experimental conditions, because of the very slow metabolic
rates that are expected under non-disturbed conditions.
The central question in such an experimental endeavour
is whether or not in situ hydrogen-driven microbial chemolithoautotrophic activities at depth are in balance with
estimated renewal rates of hydrogen. An indisputable positive answer to this question is crucial for the unequivocal
con¢rmation of a deep hydrogen-driven biosphere.
2.6. Caves
Caves may provide the link between surface and subsurface environments. The cave environment varies tremendously. One very large cave which may serve as an example is the Lechuguilla Cave in Carlsbad Caverns National
Park in New Mexico, USA. It is a deep, extensive, gypsum- and sulfur-bearing hypogenic cave, at least 90% of
which lies more than 300 m beneath the entrance. This
cave is therefore a good example of an intraterrestrial
cave environment. Allochthonous organic input to the
cave is limited due to the depth, but bacterial and fungal
colonisation is relatively extensive [53]. Various points of
evidence suggest that autotrophic bacteria are present in
the ceiling-bound residues and could act as primary pro-
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K. Pedersen / FEMS Microbiology Letters 185 (2000) 9^16
Fig. 1. The deep hydrogen-driven biosphere hypothesis, illustrated by its
carbon cycle. At relevant temperature and water availability conditions,
intraterrestrial microorganisms are capable of performing a life cycle
that is independent of sun-driven ecosystems. Hydrogen and carbon dioxide from the deep crust of the earth are used as energy and carbon
sources.
ducers in a unique subterranean microbial food chain.
Because other major sources of organic matter have not
been detected, it is suggested that these bacteria provide
requisite organic matter to the known heterotrophic bacteria and fungi in the residues. The cave-wide bacterial and
fungal distribution, the large volumes of corrosion residues and the presence of ancient bacterial ¢laments in
unusual calcite speleothems (biothems) attest to the apparent longevity of microbial occupation in this cave. More
research should be invested in this and other caves to
reveal the ecology and the biogeochemistry of cave ecosystems.
3. What's next ?
The search for the maximum depths for life in various
intraterrestrial environments will probably always be one
major desire in the exploration of deep life. Knowing more
about these limits to life within our planet will enable
more precise calculations of the total amount of intraterrestrial organisms. It is possible that the maximum in situ
temperature for life overrides the 113³C shown at laboratory conditions. Deep, high-pressure environments are required to keep water liquid above the boiling point
(100³C) at atmospheric pressure. Therefore, these deep
crustal environments are relevant targets for the search
of the maximum temperature for life.
A paper on anaerobic degradation and methane production from long chain alkanes by a consortium of microorganisms was recently published [54]. A team of seven
di¡erent microorganisms were observed to crack the hydrocarbon, to produce methane and carbon dioxide and
also to expel hydrogen sul¢de. This is an encouraging
result for those who believe that any catabolic reaction
that can be linked to oxygen as the electron acceptor,
may also occur in the absence of oxygen, albeit slower
and as a result of collaboration between species. The experiment showing anaerobic degradation of alkanes continued for almost two years before conclusive results were
obtained. One implication deduced from these results is
that intraterrestrial microbes play in a very di¡erent league
from those in well-fed, aerobic, pure cultures. Patience, a
great deal of time and advanced culturing methods will be
required for successful exploration of the metabolic activities of intraterrestrial microbial communities.
Most deep environments contain methane in varying
concentrations. Methane oxidation in aerobic environments is well documented and very widespread on the
planet. As yet, there exists no microorganism, or community of microorganisms, in culture that oxidises methane
under anaerobic conditions, although much indirect evidence for the occurrence of this process in nature has been
published (see, e.g. [55]). If a metabolic process for anaerobic methane oxidation can be demonstrated, then a tremendous source of energy and carbon will become available for models of how intraterrestrial life is fuelled.
The theory of a deep biosphere driven by hydrogen
generated in deep geological strata (Fig. 1) requires more
research. There are at least six possible processes whereby
crustal hydrogen is generated [56]:
1. The reaction between dissolved gases in the carbon^
hydrogen^oxygen^sulfur system in magmas, especially
in those with basaltic a¤nities.
2. The decomposition of methane to carbon (graphite)
and hydrogen at temperatures above 600³C.
3. The reaction between CO2 , H2 O and CH4 at elevated
temperatures in vapours.
4. Radiolysis of water by radioactive isotopes of uranium
and thorium and their daughter isotopes, and potassium.
5. Cataclasis of silicates under stress in the presence of
water.
6. Hydrolysis by ferrous minerals in ma¢c and ultrama¢c
rocks.
It is important to explore the scale of these processes
and the rates at which the produced hydrogen is becoming
available for deep microbial ecosystems.
The idea, in conclusion, that life originated on the surface of our planet, where it was strongly dependent on a
hypothetical primordial soup, has recently come up
against strong competition. Today there are several suggestions that life originated in the form of a thermophilic
lithotroph [57] and that the birthplace was intraterrestrial,
perhaps a hydrothermal vent area [58]. Consequently it
should be obvious that the search for extraterrestrial life
should concentrate on samples from under the surface of
other planets [59]. However, that is another topic and far
too speculative for this mini-review.
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