Lithoautotrophy in the subsurface

Lithoautotrophy in the subsurface

Todd Stevens *

Paci¢c Northwest National Laboratory, P7-54, P.O. Box 999, Richland, WA 99352-0999, USA

Abstract

If microorganisms can carry out primary production within the Earth's crust, then the biosphere might not be totally

dependent on surface-based photosynthesis. Potential chemical energy from purely geochemical sources within the earth can

support growth of a number of known microorganisms, chiefly strict anaerobes, such as methanogens, homoacetogens, and

sulfate-reducers. (Chemo)lithoautotrophic microorganisms have been detected in sedimentary systems, but they have not been

shown to carry out primary production in situ, at least not without some dependence on surface-based photosynthesis.

Microbial communities within igneous rock formations might, of necessity, be based on in situ primary production. Evidence

has emerged for the presence of microorganism in basalt below the sea floor, but data on in situ activity are not yet in hand.

Microbial communities have been observed, within continental flood basalts and granitic plutons, which appear to be based on

in situ primary production by anaerobic bacteria. Geochemical measurements have confirmed that in situ activity is

lithoautotrophic. This evidence for subsurface lithoautotrophic microbial ecosystems, which are not dependent on surface

organisms, may have profound implications for life on the early Earth, and on other planets, including Mars.

Keywords: Subsurface; Chemolithoautotrophic; Primary production; Extraterrestrial ; Mars; Lithotrophy

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328

2. Types of subsurface ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328

3. Where to look? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329

4. The evidence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330

4.1. Field evidence: lithoautotrophy in sedimentary systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330

4.1.1. Karstic environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330

4.1.2. Oil bearing sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330

4.2. Field evidence: lithoautotrophy in igneous rock ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331

4.2.1. Sub-sea£oor ultrama¢c rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331

4.2.2. Continental basalt formations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332

4.2.3. Continental granite formations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333

5. Consequences and implications of SLMEs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333

5.1. Economic considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333

5.2. Evolution of the biosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333

5.3. Possible life on Mars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334

0168-6445 / 97 / $32.00 ß 1997 Federation of European Microbiological Societies. Published by Elsevier Science B.V.

PII S 0 1 6 8 - 6 4 4 5 ( 9 7 ) 0 0 0 1 5 - 6

FEMSRE 557 28-10-97

* Tel.: +1 (509) 373-0891; Fax: +1 (509) 376-9650; e-mail: [email protected]

FEMS Microbiology Reviews 20 (1997) 327^337

5.4. Life elsewhere in the solar system? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334

6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335

1. Introduction

The concept of bacteria living deep underground

now seems well-accepted in the scienti¢c world.

However, investigators are only beginning to under-

stand the functional roles of these organisms in the

Earth's biosphere. One question of current interest is

whether subsurface microorganisms carry out pri-

mary production underground, or whether all living

things are ultimately dependent on primary produc-

tion occurring at the Earth's surface. This article re-

views recent ¢ndings which bear direct and indirect

evidence for this question, and concludes by discus-

sing some of the wider implications of subsurface

primary production.

2. Types of subsurface ecosystems

Conceptually, at least, subsurface ecosystems

might be divided into `detrital systems' and `produc-

tive systems'. Most of what we know about subsur-

face microbiology, so far, has been learned in detrital

systems, although there is growing evidence for sev-

eral productive systems. In practice, the dividing line

between the two types may be somewhat blurred, but

properties of the two extremes (or `end-members' in

geological parlance) can be described, at least for

purposes of this discussion.

Detrital systems conform to the standard para-

digm of ecology, in which primary production is ac-

complished by photosynthetic organisms, at the

Earth's surface. All other organisms act as consum-

ers of the photosynthate, secondary consumers, or

recyclers of photosynthetic energy. This paradigm

is extended into the subsurface by burial of photo-

synthetically produced organic matter by sedimenta-

tion and other processes of the rock cycle. In a de-

trital subsurface ecosystem, microbial metabolism is

primarily heterotrophic, and is based upon consum-

ing buried organic matter. Such a system would seem

somewhat limited, since, with increasing time, the

organic matter would be depleted, and the remainder

would become more recalcitrant and less available,

due to compaction, cementation and decreased po-

rosity.

FEMSRE 557 28-10-97

Table 1

Some redox couples known to support chemolithoautotrophy by microorganisms

Microbial process Electron donor Electron acceptor Photosynthesis dependent? Deep subsurface geochemical source?

Sul¢de oxidation S

23O2 X

Anaerobic sul¢de oxidation S

2NO

3

3

Sulfur oxidation S

oO2 X

Thiosulfate oxidation S2O23

3O2 X

Iron oxidation Fe

2�O2 X

Anaerobic iron oxidation Fe

2�NO

3

3

Manganese oxidation Mn

2�O2 X

Nitri¢cation NH

4�O2 X

Methane oxidation CH4 O2 X

Anaerobic methane oxidation CH4 SO23

4X

Hydrogen oxidation H2 O2 X

Denitri¢cation H2 NO3

3

Sulfur reduction H2 S

oX

Sulfate reduction H2 SO23

4X

Methanogenesis H2 CO2 X

Acetogenesis H2 CO2 X

T. Stevens / FEMS Microbiology Reviews 20 (1997) 327^337328

The alternative to a detrital system is a subsurface

ecosystem in which primary production occurs, using

in situ energy sources of geochemical origin. Rather

than being based on photosynthesis, subsurface pri-

mary production must be based on chemolithoauto-

trophy. That is, both the energy source and the elec-

tron sink must be inorganic chemicals, and inorganic

carbon is converted to organic carbon. Chemo-

lithoautotrophy is a well-known physiological trait

of diverse microorganisms, as shown in Table 1. In

surface ecosystems, autotrophic organisms are as-

sumed to function primarily as recyclers, conserving

energy that would otherwise be lost during decom-

position of organic matter. For instance, an auto-

trophic methanogen consumes hydrogen and carbon

dioxide, which were produced from fermentation of

organic acids, yielding methane, and conserving the

potential (photosynthetic) energy that would other-

wise be lost. In a productive subsurface system, the

same organism could function as a primary pro-

ducer, if its substrates had an abiotic geochemical

source. This has the potential to lead to ecosystems

in which life is based on geochemically derived or

`terrestrial energy' [1], rather than solar energy.

This potential for subsurface primary production

has been noted by several reviewers (e.g., [1^4]) but

only recently has evidence become available to show

that this phenomenon actually occurs.

As noted above, if (chemo)lithoautotrophy occurs

in the subsurface (we can dismiss (photo)lithoauto-

trophy) both an electron donor and an electron ac-

ceptor must be supplied from geochemical sources.

Table 1 lists a number of redox couples which are

known to support lithoautotrophy. Those processes

which require the most oxidized electron acceptors,

such as oxygen and nitrate, might occur in the sub-

surface, but would be limited to near-surface or

young groundwater environments. This is because

oxygen is a product of photosynthesis, and must be

transported from the surface. The remaining lithoau-

totrophic processes are more likely to take advantage

of in situ subsurface geochemical resources, and they

conspicuously feature hydrogen as an electron do-

nor.

Hydrogen is widespread in the deep subsurface,

and has a number of geochemical sources (e.g.,

[5]). The most relevant to this discussion are the re-

actions of igneous rocks, which are the main compo-

nents of the Earth. These reactions form a continu-

um from fast high-temperature reactions of magma

outgassing and magma-water interactions, to slow,

low-temperature reactions that take place during

weathering of igneous rocks. Igneous rocks are

formed by extrusion of melted material from the

Earth's mantle into the crust. Rocks with greater

content of silica are classi¢ed as felsic, while those

with greater content of magnesium and ferrous iron

are classi¢ed as ma¢c. Those with the highest mag-

nesium and iron content are called ultrama¢c (e.g.,

[6]). The greater the content of reduced metals, the

more potential redox energy a rock should contain.

The slow oxidation of ultrama¢c rocks by ground-

waters is known as serpentinization (e.g., [7]).

H2O� �FeO�x�SiO2�y

� H2 � X�FeO3=2� � Y�SiO2�

Although serpentinization is often assumed to re-

quire elevated temperatures (several hundred de-

grees) and geological time scales, we have recently

demonstrated that microbially signi¢cant quantities

of hydrogen can be formed during room tempera-

ture reactions of igneous rocks over a few hours or

days [8]. Even some rather felsic rocks produced hy-

drogen, if strictly anaerobic conditions were main-

tained.

Of course, while productive subsurface ecosystems

would depend upon primary production by lithoau-

totrophs, we know of no reason why heterotrophic

organisms would not be present as well. Conceiv-

ably, complex subsurface ecosystems could be sup-

ported by in situ primary production. However, since

rates of metabolism are extremely low in the subsur-

face [9,10], metabolism will be primarily directed to-

wards maintenance, rather than growth (e.g., [11])

and geochemical signatures, such as isotope ratios,

might re£ect only the primary energy reaction.

3. Where to look?

The vast majority of subsurface microbiology

studies have been carried out on samples obtained

from sedimentary formations, and there are good

reasons for this. Most subsurface sediments con-

tained signi¢cant populations of microorganisms at

the time of deposition. Sediments often contain large

FEMSRE 557 28-10-97

T. Stevens / FEMS Microbiology Reviews 20 (1997) 327^337 329

concentrations of organic matter, so they can sup-

port microbial metabolism. The extensive pore space

in many sedimentary rocks provides ample habitat

volume for subsurface microorganisms. Because sedi-

ments are typically porous media, slow £uxes of

groundwater are able to distribute nutrients through

the formations. These factors make sedimentary for-

mations likely habitats for subsurface microorgan-

isms, and indeed, such organisms are often found

to thrive in them. Because of these same character-

istics, however, one might hypothesize that any ac-

tive microbial ecosystem in a sedimentary environ-

ment is likely to be a detrital ecosystem.

Igneous rocks, at ¢rst glance, might seem unlikely

to provide a habitat for subsurface microorganisms.

Because they are formed from molten magma, one

would expect that they contain little or no organic

matter. (However, there have been sometimes con-

troversial reports of small amounts of kerogenous

organic matter within igneous rocks [12,13].) Because

they are crystalline rocks, and usually not porous,

they provide little habitat volume. Any microorgan-

isms must be con¢ned to fractures, cooling joints,

and rubble zones (which form between successive

lava £ows), as must groundwater and associated nu-

trient £uxes. Nevertheless, although the habitat den-

sity may be lower than sedimentary rocks, when the

sheer volume of igneous rocks in the Earth's crust is

considered, these fracture networks can form a vast

total volume. As discussed above, reactions between

water and igneous rock in fracture networks can

result, under the correct conditions, in production

of energy gases, which might be used by lithoauto-

trophic microorganisms. Because of these character-

istics, one might hypothesize that any active micro-

bial ecosystem in an igneous rock habitat is likely to

be based on in situ primary production.

4. The evidence

4.1. Field evidence: lithoautotrophy in sedimentary

systems

We have hypothesized that active microbial eco-

systems in sedimentary formations will be detrital.

Yet there have been reports of lithoautotrophy in

sedimentary systems. Is the hypothesis incorrect?

4.1.1. Karstic environments

A number of investigators have reported evidence

for lithoautotrophy in caves located in karstic ter-

rains. Karst is a chemical sediment, composed

mainly of limestone, which is readily dissolved by

acids in groundwater. This results in extensive cav-

erns and underground streams (e.g., [6,14]). These

caves and streams conduct oxygen, from the atmos-

phere, into the subsurface. In several places, anaero-

bic water, containing reduced gases has been found

to enter cave systems from some deeper source. This

results in a chemical environment that is analogous

to that at geothermal hot springs. In several caves,

sul¢de- and sulfur-oxidizing bacteria have been

found to form mats, in which lithoautotrophic car-

bon ¢xation occurs [15^17]. In at least one case, in-

vertebrate fauna have been found to graze on these

mats, and the isotopic composition of their biomass

showed that the animals depend entirely on bacterial

lithoautotrophic carbon ¢xation [18]. However, the

source of the reduced gases, which provide energy

for these systems, has not been determined. To

understand whether this is true subsurface primary

production, perhaps the scale of the system should

be rede¢ned to include this unknown energy source.

If the reduced sul¢de is a product of microbial sul-

fate-reduction, coupled to fermentation of organic

matter in a deeper aquifer, then the lithoautotrophic

mats in the karst are recycling and conserving photo-

synthetic energy, as their relatives do in surface sedi-

ments. If the source of reduced gases is volcanic

activity, then the system would be analogous to

deep-sea hydrothermal vents. However, since O2,

produced at the surface by photosynthetic organ-

isms, is required as an electron acceptor, these karst

systems (as well as the deep-sea vent organisms) are

ultimately dependent on photosynthetic processes,

and do not constitute an independent subsurface

ecosystem.

4.1.2. Oil bearing sediments

Lithoautotrophic microorganisms have been de-

tected in several very deep sedimentary formations.

Thermophilic hydrogen-oxidizing bacteria have been

detected in £uids produced from formations over

3 km deep [19^22]. These include hyperthermophilic

sul¢dogenic archaea, methanogenic archaea, and

thermophilic sul¢dogenic and metal-reducing bacte-

FEMSRE 557 28-10-97


ria. The possibility that these organisms are an arti-

fact of well drilling or sea-water injection (which is

used to enhance oil recovery) cannot be completely

ruled out [23,24]. However, some lithoautotrophs

have been recovered from wells which were not af-

fected by water injection. The physiology of micro-

organisms isolated from these environments is often

consistent with the conditions at depth, in terms of

optimal salinity and temperature, which is elevated

due to geothermal gradients.

Core samples may be less likely to contain con-

taminating organisms than borehole £uids, and

chemical tracers can be used to detect contamina-

tion. Lithoautotrophs were also isolated from core

samples taken from 2800 m deep shale-sandstone

formations, and which tracer methods indicated

were uncontaminated. In these samples, lithoauto-

trophs were found to outnumber heterotrophs, at

least as counted by laboratory growth-based assays

[25]. These organisms included hydrogen-oxidizing

metal-reducing and sul¢dogenic bacteria [26].

Unfortunately, no in situ measurements of meta-

bolic activity are available for deep sedimentary for-

mations, so the ecological role of the recovered mi-

croorganisms, which are often facultatively

heterotrophic, can only be inferred. It may be that

the apparent high incidence of lithoautotrophs in

deep sediments is an artifact of laboratory methods

used to recover microorganisms. Alternatively, it

may be that hydrogen is an important electron donor

in these formations. Produced from organic matter

by fermentation or thermal cracking, hydrogen

would be transported through the low-porosity envi-

ronments of the deep subsurface more readily than

many other nutrients. Because these hydrocarbon-

bearing sediments are rich in fossil organic matter,

we can assume that they are ultimately detrital eco-

systems.

In summary, it is evident that lithoautotrophy oc-

curs in sedimentary subsurface habitats. However, as

mentioned above, it can be seen already that there is

not always a clear-cut distinction between detrital

and productive systems. The reader will have noticed

that this discussion belabors the point of independ-

ence from surface photosynthesis. What di¡erence

does this make? Is it really a useful distinction?

We will return to this point in the ¢nal section be-

low.

4.2. Field evidence: lithoautotrophy in igneous rock

ecosystems

Relatively few investigations have studied micro-

organisms in subsurface igneous rock formations.

According to the hypothesis above, however, active

microbial ecosystems in igneous rock might be ex-

pected to be based on in situ primary production.

4.2.1. Sub-sea£oor ultrama¢c rocks

The most reactive igneous rocks are ophiolite

suites, which are ultrama¢c rocks produced from

magma at mid-ocean spreading centers. This is also

the location of the well-known chemolithoauto-

trophic (but oxygen-dependent) communities on the

sea-£oor. There has been considerable speculation

that a `deep hot biosphere' might exist within the

basalt, in the hydrothermal convection cell of the

hot springs (e.g., [27]). Such a hypothetical system

would, of necessity, be based on anaerobic chemo-

lithoautotrophy. This would require life at temper-

atures above 200³C, which most investigators cur-

rently believe to be unlikely [1]. Thermophilic

anaerobic lithoautotrophs have been detected on

the walls of `black smoker' chimneys, but not in

the very hot £uids emitted from the vents. There is

currently no geochemical or isotopic evidence for

active lithoautotrophy below the hot springs. Lilley

et al. [28] reported anomalous isotope compositions

for methane emitted from an unsedimented hydro-

thermal system which were consistent with microbial

lithoautotrophy, but attributed the gases to pyrolysis

of organic matter buried below the basalt.

Though conditions below mid-ocean spreading

centers may lie outside the feasible range for living

organisms, the rocks produced there are steadily

transported outward, and temperatures decrease

concomitantly. Zones of active serpentinization

have been observed in cooler ophiolites, and it seems

feasible that this could support microbial commu-

nities below the sea £oor. Plumes of dissolved meth-

ane have been detected in the ocean above these

serpentinization zones [29,30], as might be expected

from a bacterial system. The origin of this methane

has been explained by abiotic geochemical models

[31] though it may be useful to reexamine these sites

as our understanding of microbial interactions with

igneous rocks improves.

FEMSRE 557 28-10-97


Unfortunately, while it is di¤cult to obtain sam-

ples from the subsurface that are useful for micro-

biology, it is even more di¤cult to do so at the

bottom of the ocean! Nevertheless, several research-

ers have recently reported encouraging results from

studies of basalt cores obtained by the Ocean Drill-

ing Program [32]. In these cores, basaltic glass was

altered along natural fractures, through which sea

water entered and reacted with the rocks, over long

periods of time. At the interface between unaltered

glass and the alteration materials, where contami-

nants could not have penetrated during sampling,

evidence was found which suggested in situ bacterial

activity. The evidence included bacterium-shaped

etched channels in the glass, putative bacteria which

were stained by DNA-speci¢c £uorescent dyes [32],

detection of DNA [33], and enrichment of potassium

in the altered glass, which may be an indicator of

biomass [34]. No evidence for lithoautotrophy in

these samples has been obtained so far, but one

might predict aerobic iron-oxidation near the sea

£oor, and anaerobic hydrogen-oxidation in deeper

strata.

In summary, no ¢rm evidence is yet available for

lithoautotrophic microbial ecosystems below the sea

£oor. However, the few observations that are cur-

rently available appear to provide encouraging cir-

cumstantial support. Ophiolite suites with associated

serpentinization zones are also found on the conti-

nents (e.g., [35]), though they have not been exam-

ined for the presence of microorganisms, it may

prove fruitful to do so.

4.2.2. Continental basalt formations

Ma¢c rocks contain less reduced iron than ultra-

ma¢c rocks, and are correspondingly less reactive.

Nevertheless, they can react with water, under ap-

propriate conditions, to produce microbially avail-

able hydrogen. We have recently reported evidence

for a subsurface lithoautotrophic microbial ecosys-

tem (SLME) within the Columbia River Basalt

group (CRB) [8]. The CRB is a series of continental

£ood basalts which form a layered structure up to

3 km deep and covering over 300 000 km2in western

North America. Con¢ned aquifers between the ba-

salt £ows contain anaerobic, reducing water, indicat-

ing extensive water-rock interactions, as well as

abundant dissolved hydrogen, methane, sul¢de and

bacterial cells. Dissolved inorganic carbon (DIC) is

progressively depleted with increasing methane con-

centration, suggesting that lithoautotrophic metha-

nogens consume bicarbonate to produce methane.

The stable isotope composition of the DIC is consis-

tent with this model. These observations suggest that

microbial lithoautotrophic metabolism occurs in situ

in the CRB. If the system were heterotrophy-based,

DIC would increase, rather than decrease with in-

creasing methane.

The ferrous silicate minerals in basalt react with

water, under appropriate conditions, to generate hy-

drogen gas [36]. This reaction is inhibited by molec-

ular oxygen and promoted by low pH, high temper-

ature, and greater reacting surface area. Since

meteoritic water is initially aerobic, if H2 production

is to occur, groundwaters must be isolated from

communication with the atmosphere, and remain in

contact with subsurface rocks long enough for all the

O2 to be removed. These properties should allow

some general predictions about the sorts of aquifers

where SLMEs might be found. The rock matrix

should contain signi¢cant quantities of ferrous-sili-

cate minerals. Depth below surface should be ad-

equate to preclude di¡usion of oxygen from the at-

mosphere. Groundwater £ow rates should be low, or

£ow paths long, to ensure su¤cient water rock inter-

action. The two currently proposed SLMEs are in

deep con¢ned aquifers with groundwater ages of sev-

eral tens of thousands of years. Future research

might allow more quantitative description of

SLME habitat. For instance, volcanic ash deposits

might form a high surface-area aquifer with rela-

tively fast reaction rates, compared to bulk igneous

rock. However, no subsurface microbiology investi-

gations have yet examined such formations.

A number of important questions remain unan-

swered in these systems. What happens when the

fresh rock surfaces become oxidized? What are the

exact mineral reactions that occur, and what are the

limiting factors? Do microorganisms actively mine

basalt, or passively consume products of abiotic re-

actions? What sorts of competitive or syntrophic in-

teractions occur between the di¡erent types of micro-

organisms observed?

It should be noted that, while primary production

occurs in sul¢dogenic basalt microcosms (Landau,

N. and Stevens, T., unpublished), the stable isotope

FEMSRE 557 28-10-97


signatures measured in sul¢dogenic CRB ground-

waters do not conform to the lithoautotrophic pat-

tern, though no known source of organic carbon is

present. These aquifers warrant further study, espe-

cially since they may be analogous to sub-sea£oor

basalts, which would also contain elevated sulfate,

due to the presence of seawater.

Microbial communities from the CRB can be

grown in laboratory microcosms containing basalt

as the sole electron donor, as can some well-charac-

terized pure cultures of bacteria [36]. These micro-

cosms provide an opportunity to carry out manipu-

lative experiments to test hypotheses about SLMEs.

4.2.3. Continental granite formations

Granites are felsic igneous rocks with rather low

iron content, yet they can contain enough ferrous

silicate to react with water to form hydrogen, though

less than ma¢c or ultrama¢c rocks [8]. It seems pos-

sible that SLMEs could exist in large granitic plu-

tons. In fact, natural gas deposits with anomalous

isotopic and hydrocarbon composition, apparently

of microbial origin, have been observed within the

Canadian and Fenno-Scandian shields [37,38]. Mi-

croorganisms within granite formations in Sweden

have been studied extensively [39,40]. Recent results

from this formation reveal that anaerobic lithoauto-

trophs are abundant in deep granitic groundwaters,

and geochemical measurements indicate that these

organisms are active in situ [56]. Accumulating evi-

dence indicates that a lithoautotrophy based ecosys-

tem may exist within this deep granite groundwater

system.

To summarize, accumulated evidence seems to in-

dicate that SLMEs have the potential to exist in

aquifers within most kinds of large igneous rock

bodies. Indeed, the strongest ¢eld evidence, so far,

comes from formations composed of some of the less

reactive igneous rocks. This suggests that prospects

are good for, and studies are certainly warranted of,

similar systems in ultrama¢c subsurface environ-

ments.

5. Consequences and implications of SLMEs

Are SLMEs merely a scienti¢c novelty, or do they

have broader signi¢cance that makes them worthy of

attention? And what di¡erence could it make, if

SLMEs could truly function independently of the

surface biosphere? The following discussions are

highly speculative, but illustrate why it may be im-

portant to study these subsurface phenomena, and

perhaps outline some research questions for the fu-

ture.

5.1. Economic considerations

Certainly, microorganisms may have impacts on

economically important activities that take place in

their habitat. This has motivated most SLME inves-

tigations, to date.

Current evidence suggests that SLMEs may con-

tribute to natural gas formation, in certain settings.

If this should be con¢rmed, it could lead to a better

understanding of this important resource, and possi-

bly of certain other mineral resources.

Several countries have planned high-level nuclear

waste repositories in geological formations which

may contain SLMEs. Microbial activity in these set-

tings may have important long-term controls on cor-

rosion of containment vessels, and migration of ra-

dionuclides.

A variety of groundwater pollutants are present in

igneous rock formations around the world. It is pos-

sible that active SLMEs could degrade or transform

these contaminants, or a¡ect their mobility. For in-

stance, many of the anaerobic microorganisms

known to occur in SLMEs can carry out reductive

dehalogenation of solvents, or reductive transforma-

tion of heavy metals. It remains to be seen whether

the in situ rates of metabolism in a SLME are great

enough to interdict contaminant plumes. If so,

SLMEs could form a stable long-term barrier to

waste migration.

5.2. Evolution of the biosphere

If SLMEs can truly function without input from

the surface, it may mean that the depth of the bio-

sphere is limited only by increasing geothermal tem-

perature. This could have several important implica-

tions for the history of early life on Earth [41].

SLMEs could be a model for how ecosystems func-

tioned before the evolution of photosynthesis. It is

interesting that the energy currency of reduced iron

FEMSRE 557 28-10-97


and sulfur compounds that appear to support

SLMEs is reminiscent of the abiotic chemistry in-

voked in the origin-of-life theories that are col-

lectively known as `iron-sulfur hypotheses' (e.g.,

[42]).

The ability to survive in the subsurface, without

input from the surface, could also confer on organ-

isms the ability to survive cosmological events which

sterilize the surface of the planet. Many such events

are believed to have occurred during the early his-

tory of the Earth [43,44], yet the geological record

suggests that relatively advanced microorganisms

may have been present soon afterwards [45^47]. In-

terestingly, the oldest known biogenic signatures, in

3.8 Ga carbonates, seem to indicate lithoautotrophic

methanogenesis [47]. Perhaps the earliest continu-

ously habitable biosphere was in the deep subsur-

face, sandwiched between geological heat from be-

low, and impact heat from above [41].

5.3. Possible life on Mars

The surface of Mars is uninhabitable by, and

probably lethal to, any known organism (e.g., [48]).

The temperature and pressure are too low for water

to exist as liquid at the surface, though water is

abundant on the planet. Yet geological outwash fea-

tures, observed from orbit, suggest that large quan-

tities of water, possibly as liquid, exist in the subsur-

face [49]. Mechanisms have been suggested which

could allow a hydrologic cycle to exist in the subsur-

face (e.g., [50]). Thus, it seems possible that there

could be conditions in the deep subsurface of Mars

where chemolithoautotrophic microorganisms could

exist [3]. SLMEs in igneous rock formations on

Earth are the only known examples of functioning

ecosystems that could survive on Mars today, if they

existed there. Water, basalt, and inorganic carbon

should be abundant, in the Martian subsurface,

though the depths at which pressures and temper-

atures reach permissible levels for life may be more

than 2 km [51].

Earlier in the history of the solar system, around

the time of the origin of life, conditions on Earth and

Mars were more similar. If life arose on one planet,

it is not unreasonable to suspect that it arose on the

other as well. As the surface of Mars became unin-

habitable, any life that was there must have perished,

or retreated into the subsurface [52]. If life exists

there today, it is most likely in the deep subsurface.

Several recently recognized meteorites of Martian

origin are known on the Earth, and have been the

subject of much recent study. These are subsurface

igneous rocks ejected from Mars by cosmic impacts.

Some investigators have even suggested that one may

contain evidence of ancient microbial life [53].

Whether or not these hypotheses are eventually

proved, Earthly SLMEs are an exact analog of the

proposed biological system, and are the positive con-

trol by which to evaluate putative Martian life.

Though the connection remains speculative, an

understanding of SLMEs and the geochemical signa-

tures they leave behind in rocks may be highly sig-

ni¢cant to Martian exploration. During the upcom-

ing decade, several spacecraft are planned to land

and conduct investigations of Mars. Though planned

experiments do not focus on life detection per se, a

number of geological and chemical observations

could shed light on the possibility that life ever ex-

isted there. A better understanding of microorgan-

ism-rock interactions could provide possible targets

for some of these remote investigations.

5.4. Life elsewhere in the solar system?

Are there possible abodes for life beyond Mars?

Liquid water is assumed to be the primary require-

ment for life [52], and probably does not exist on

Mercury, Venus, or the gas giant planets. However,

though we know relatively little about them, there

are several other planets where liquid water, and

hence life, may be possible.

The Galilean moons of Jupiter ^ Io, Europa, Cal-

listo, and Ganymede ^ appear to be tectonically ac-

tive and may have enough interior heat to posses

liquid water. The surfaces of the latter three planets

are composed largely of water ice. Europa, in partic-

ular, is hypothesized to contain a subsurface ocean

of liquid water [54]. These planets have rocky cores

that could supply reduced minerals to act as electron

donors for microorganisms. The Galileo spacecraft is

currently exploring the Jovian system, and accumu-

lating new information about these worlds [55].

Much more must be learned, but it remains feasible

that SLME type life could survive within one or

more of these planets.

FEMSRE 557 28-10-97


Titan, the largest moon of Saturn, is often men-

tioned as a possible location for extraterrestrial life,

or at least interesting pre-biotic chemistry [54]. Little

is known of this planet, which is hidden beneath an

atmosphere containing methane and ammonia. Up-

coming exploration of the Saturn system by the Cas-

sini spacecraft will include `Huygens', an autono-

mous probe that will be dispatched to Titan to

learn more about this remote, enigmatic planet.

While it is far too early to reach conclusions about

any of the above hypotheses, it is reasonable to begin

formulating methods for testing them. The emerging

understanding of the ecology of the terrestrial sub-

surface may provide direction in the search for life in

distant space and time.

6. Conclusion

SLMEs have frequently been invoked in the liter-

ature, but only recently has evidence been found that

they exist in nature. The potential for SLMEs in the

Earth's crust is widespread, since presumably suit-

able igneous bodies are ubiquitous. Field evidence

for SLMEs is restricted to only a few locations, how-

ever, mostly because of the di¤culty and expense of

observing the deep subsurface. Continuing and fu-

ture studies, including ¢eld measurements and study

of laboratory microcosms, should contribute to the

emerging understanding of the ecological functions

and biogeochemical activities of SLMEs. Such new

information could lead to techniques for more

simply detecting the past or present activity of

SLMEs in the Earth's crust, or elsewhere in the solar

system.

Acknowledgments

I thank many colleagues for stimulating discus-

sions on this topic, including P. Long, C. McKay,

J. McKinley and K. Zahnle. Preparation of this re-

view was supported by the Subsurface Science Pro-

gram, O¤ce of Health and Environmental Research,

U.S. Department of Energy (DOE). Paci¢c North-

west Laboratory is operated for DOE by Battelle

Memorial Institute under Contract DE-AC06-

76RLO 1830.

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Lithoautotrophy in the subsurface

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