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Chapter 21. Geomicrobiology of Selenium and Tellurium
21.3 TOXICITY OF SELENIUM AND TELLURIUM
Both selenium and tellurium are toxic when present in excess, but the minimum toxic doses vary
depending on the organism. As mentioned in Section 21.2, some plants accumulate selenium to the
extent of 1.5–2 g kg−1 dry weight of tissue (Stadtman, 1974). They usually grow in arid environments with unusually high concentrations of selenium in the soil. In the Kesterson National Wildlife
Refuge in California, where extensive selenium intoxication of wild animals has been observed,
selenate concentrations of 1.8–18 µM (0.14–1.4 ppm) have been reported, contrasted with normal
concentrations of ∼1.3–21.1 nM in the San Joaquin River, ∼0.4–1.3 nM in the Sacramento River,
and <0.2 nM in San Francisco Bay, all located in California (Zehr and Oremland, 1987). These
normal concentrations are below minimum inhibitory concentrations (MICs) of selenate [Se(VI)]
for three selenium-sensitive strains of bacteria from the same general area in California. Their
MICs were found to range from 0.78 to 1.56 mM for selenate and from 1.56 to 25 mM for selenite
[Se(IV)]. Selenium-resistant bacteria from the Kesterson National Wildlife Refuge exhibited MICs
of 50 to >200 mM selenate and selenite (Burton et al., 1987). By contrast, both selenium-resistant
and selenium-sensitive organisms from these same sites in California exhibited MICs for tellurate in
the range of 0.03–1 mM and for tellurite in the range of 0.03–4 mM (Burton et al., 1987).
Selenium and tellurium resistance appears to be regulated by different genes. In Escherichia
coli, tellurium resistance appears to be mediated by the arsenical ATPase efflux pump. The genetic
determinants for this pump reside on resistance plasmid R773 (Turner et al., 1992). Higher forms
of life appear to be relatively more sensitive to Se than bacteria, although they require Se as a
nutritional trace element. Biochemically, Se toxicity appears to be the result of superoxide or H2O2
production in excess of antioxidant production by a cell. A similar mechanism may be the basis for
Te toxicity (see references 31, 35, and 41 cited by Guzzo and Dubow, 2000).
21.4 BIOOXIDATION OF REDUCED FORMS OF SELENIUM
Some inorganic forms of selenium have been reported to be oxidizable by microorganisms.
Micrococcus selenicus isolated from mud (Breed et al., 1948), a rod-shaped bacterium isolated
from soil and thought to be autotrophic (Lipman and Waksman, 1923), and a purple bacterium
(Sapozhnikov, 1937) were observed to oxidize Se0 to SeO2–
4 . A strain of Bacillus megaterium from
topsoil in a river alluvium was found to oxidize Se0 to SeO2–
3 and traces of SeO 4 . Red selenium
was more readily attacked than gray selenium (Sarathchandra and Watkinson, 1981). Dowdle
and Oremland (1998) observed elemental selenium oxidation in soil slurries that was inhibited
by autoclaving the slurry or by addition of formalin, azide, 2,4-dinitrophenol, or the antibiotics
chloramphenicol + tetracycline or cycloheximide + nystatin. Se0 oxidation in the slurries was
enhanced by addition of sulfide, acetate, or glucose, suggesting that sulfur-oxidizing autotrophs and
heterotrophs were involved in the oxidation.
Acidithiobacillus ferrooxidans has been shown to oxidize copper selenide (CuSe) to cupric copper
(Cu2+) and elemental selenium (Se0) (Torma and Habashi, 1972). The reaction may be written as
CuSe + 2H+ + 0.5O2 → Cu2+ + Se0 + H2O
21.5 BIOREDUCTION OF OXIDIZED SELENIUM COMPOUNDS
Various inorganic selenium compounds have been found to be reduced anaerobically by some
microorganisms. Crude cell extract of Micrococcus lactilyticus (also known as Veillonella lactilyticus) has been shown to reduce selenite but not selenate to Se0, and Se0 to HSe−. The reductant was
hydrogen (H2 ) (Woolfolk and Whiteley, 1962). Cell extracts from strains of Desulfovibrio desulfuricans and Clostridium pasteurianum were also found to reduce selenite with hydrogen. The enzyme
hydrogenase mediated electron transfer from hydrogen in these reactions (Woolfolk and Whiteley,
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Geomicrobiology of Selenium and Tellurium
1962). A variety of other bacteria, actinomycetes, and fungi have been shown to reduce selenate and
selenite to Se0 (Bautista and Alexander, 1972; Lortie et al., 1992; Stolz and Oremland, 1999; Tomei
et al., 1992; Zalokar, 1953). The bacteria include Pseudomonas stutzeri, Wolinella succinogenes,
and Micrococcus sp. Acidithiobacillus ferrooxidans is able to reduce the red form of Se0 to H2Se
anaerobically, albeit in small amounts (Bacon and Ingledew, 1989).
A relatively recently discovered bacterium, Thauera selenatis, can grow anaerobically with selenate or nitrate as terminal electron acceptor (Macy et al., 1993; Rech and Macy, 1992). In the
absence of nitrate, it reduces selenate to selenite (DeMoll-Decker and Macy, 1993). The reductases
for selenate and nitrate in this organism are distinct enzymes with different pH optima. Thus in
contrast to the response of a selenate-reducing enrichment culture (Steinberg et al., 1992), nitrate
does not inhibit selenate reduction by T. selenatis. Indeed, when present together, both selenate and
nitrate are reduced simultaneously, with selenate reduced to elemental selenium (DeMoll-Decker
and Macy, 1993). The selenate reductase in this organism, which catalyzes the reduction of selenate
to selenite, is found in the periplasm, whereas its nitrate reductase, which catalyzes the reduction of
nitrate to nitrite, is found in its cytoplasmic membrane (Rech and Macy, 1992), Selenate reductase
is a metalloprotein containing Mo, Fe, acid-labile sulfur, and a cytochrome b subunit (Schroeder
et al., 1997). Nitrite reductase is found in the periplasm of T. selenatis and plays a role in selenite
reduction, besides catalyzing nitrite reduction (DeMoll-Decker and Macy, 1993). This explains why
T. selenatis produces elemental selenium in the presence of nitrate, but selenite in its absence.
Selenite does not support growth of T. selenatis (DeMoll-Decker and Macy, 1993).
A selenite reductase enzyme has been obtained from the fungus Candida albicans (Falcone
and Nickerson, 1963; Nickerson and Falcone, 1963). It reduces selenite to Se0. A characterization
of the enzyme has shown that it requires a quinone, a thiol compound (e.g., glutathione), a pyridine
nucleotide (NADP), and an electron donor (e.g., glucose 6-phosphate) for activity. Electron transfer
between NADP and quinone is probably mediated by flavin mononucleotide in this system. It is
possible that this enzyme is part of an assimilatory SeO2–
4 and SeO 3 reductase system. How this
enzyme compares with that in T. selenatis remains to be established.
Sulfurospirillum barnesii (formerly Geospirillum barnesii, also called strain SES-3) (Oremland
et al., 1994; Stolz et al., 1999) is another bacterium that can reduce selenate to elemental selenium.
Cells of this organism grew with lactate as carbon and energy source and selenate as terminal electron acceptor, which was reduced to selenite. As with Thauera selenatis, resting cells of Ssp. barnesii but not growing cells were able to reduce selenite to Se0 (Oremland et al., 1994). One important
difference between Ssp. barnesii and T. selenatis is that Ssp. barnesii is able to use a much wider
range of reducible anions as terminal electron acceptors than T. selenatis (Stolz and Oremland,
1999). Ssp. barnesii can reduce selenate and nitrate simultaneously whether pregrown on selenate
or nitrate, consistent with the observation that selenate reductase is constitutive in this organism
(Oremland et al., 1999).
Two newly discovered selenate reducers, both gram-positive bacteria, are Bacillus arsenicoselenatis and B. selenitireducens (Switzer Blum et al., 1998). The first forms spores but the second
does not. Both were isolated from anoxic muds from Mono Lake, California, which is alkaline,
hypersaline, and arsenic-rich. B. arsenicoselenatis reduces selenate to selenite whereas B. selenitireducens reduces selenite to elemental selenium as forms of anaerobic respiration. In coculture,
the two strains together can reduce selenate to elemental selenium. Both strains can reduce arsenate
as well as selenate (Switzer Blum et al., 1998). Sulfurospirillum barnesii and B. arsenicoselenatis
can reduce selenate and nitrate simultaneously, but unlike the selenate reductase in Ssp. barnesii,
that in B. arsenicoselenatis is not constitutive because it does not appear in nitrate-grown cells
(Oremland et al., 1999). Therefore, in order for B. arsenicoselenatis to reduce selenate and nitrate
simultaneously, it has to be grown in the presence of a mixture of the two electron acceptors.
A moderately halophilic selenate reducer was isolated from Dead Sea (Israel) sediment. It
reduced selenate to selenite and elemental selenium. It is a gram-negative organism and has been
named Selenihalanaerobacter shriftii (Switzer Blum et al., 2001). When it respires on glycerol or
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glucose, it forms acetate and CO2. Nitrate and trimethylamine N-oxide could serve as alternative
electron acceptors, but reduced forms of sulfur, nitrite, arsenate, fumarate, or dimethylsulfoxide
All previously mentioned selenate- and selenite-reducing bacteria belong to the domain Bacteria.
Recently, a hyperthermophilic member of the domain Archaea capable of respiring organotrophically on selenate was isolated from a hot spring near Naples, Italy (Huber et al., 2000) (see also
Chapter 14). Its name is Pyrobaculum arsenaticum. It reduces selenate to elemental selenium.
Previously isolated P. aerophilum (Völkl et al., 1993) was found capable of respiring organotrophically on selenate and selenite and autotrophically on selenate with H2 as electron donor (Huber
et al., 2000). Elemental selenium was the reduction product.
In most studies of bacterial reduction of selenate and selenite, elemental selenium (red form),
when formed, is usually found to be a major, if not the only, product. This is noteworthy because
sulfate and sulfite cannot be directly reduced to S0 but are reduced to H2S without intermediate formation of S0. Yet selenium and sulfur are members of the same chemical family. The implication is
that enzymatic mechanisms of reduction for oxidized forms of these two elements are different. To
date, none of the true selenate respirers have been found capable of sulfate respiration, which could
be related to the significantly higher energy yield in selenate respiration (∆G′, −15.53 kcal mol−1 e−1)
than in sulfate respiration (∆G′, 0.10 kcal mol−1 e−1) (Newman et al., 1998). It must be noted, however, that Desulfovibrio desulfuricans subsp. aestuarii has been found to reduce nanomolar but not
millimolar quantities of selenate to selenite (Zehr and Oremland, 1987). Sulfate inhibited reduction of selenate, suggesting but not proving that the mechanism of sulfate and selenate reduction
in this case may be a common one. As Zehr and Oremland (1987) pointed out, when sulfate is
being reduced to H2S in the absence of selenate, some of the H2S formed may subsequently reduce
biogenically formed selenite chemically to Se0. They found that in nature, the sulfate reducer can
reduce selenate only if the ambient sulfate concentration is <4 mM. Hockin and Gadd (2003) found
that in mixed biofilms, Desulfomicrobium norvegicum could reduce selenite that diffused into the
biofilm with H2S it produced anaerobically by reduction of sulfate, resulting in the formation of S0
and Se0. This reaction was abiotic and can be formulated as follows:
3HS− + SeO2–
4 + 5H → 3S + Se + 4H2O
The sulfur and selenium precipitated within the biofilm as nanometer-sized selenium–sulfur granules. By contrast, Sulfurospirillum barnesii, Bacillus selenitireducens, and Selenihalanaerobacter
shriftii can form nanospheres consisting exclusively of Se0 when reducing selenite enzymatically
(Oremland et al., 2004).
Whereas selenate and selenite reduction by the previously described organisms resulted in extracellular deposition of Se0, intracellular deposition of Se0 has been observed with some other organisms. Chromatium vinosum can deposit Se0 intracellularly as a result of an interaction of H2Se,
which is produced by Desulfovibrio desulfuricans in selenate reduction in coculture with Chr.
vinosum. The Se0 is stored in the form of globules in the Chr. vinosum cells (Nelson et al., 1996).
Rhodobacter spheroides deposited red Se0 in or on its cells when it reduced selenate and selenite
(Van Fleet-Stadler et al., 2000). Ralstonia metallidurans CH34 can reduce selenite to red Se0, which
it stores in its cytoplasm and occasionally in its periplasm (Roux et al., 2001).
OTHER PRODUCTS OF SELENATE AND SELENITE REDUCTION
In Escherichia coli, a significant portion of selenite reduced during glucose metabolism is deposited
as Se0 on its cell membrane but not in its cytoplasm (Gerrard et al., 1974), and another portion is
incorporated as selenide in organic compounds such as selenomethionine (Ahluwalia et al., 1968).
Some soil microbes reduce selenate or selenite to dimethylselenide [(CH3)2Se] at elevated selenium
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Geomicrobiology of Selenium and Tellurium
concentrations (Kovalskii et al., 1968; Fleming and Alexander, 1972; Alexander, 1977; Doran and
Alexander, 1977). Other volatile selenium compounds may also be formed, their relative quantities
depending on reaction conditions (Reamer and Zoller, 1980). The compounds include dimethyl
diselenide [(CH3)2Se2] and dimethyl selenone [(CH3)2SeO2].
Some fungi have been found to be effective in forming methylated selenium compounds (Barkes and
Fleming, 1974). Alternaria alternata isolated from seleniferous water from a sample series collected
from evaporation ponds at the Kesterson Reservoir, Lost Hills, and Peck Ranch in California formed
dimethylselenide more rapidly from selenate and selenite than from selenium sulfide (SeS2) or various organic Se compounds. Methionine, a known biochemical methyl donor, and methylcobalamin, a
known methyl carrier in biochemical transmethylation, stimulated dimethylselenide formation by the
fungus (Thompson-Eagle et al., 1989). Crude cell extracts and a supernatant fraction from the fungus
Pichia guillermondi after centrifugation at 144,000g reduced selenite but not selenate (Bautista and
Alexander, 1972). In a mechanism proposed by Reamer and Zoller (1980), all methylated forms of
selenium arise by methylation of selenite and subsequent reductions and, where needed, by additional
methylation of the methylated products. Dimethylselenone is viewed as a precursor of dimethylselenide,
whereas methylselenide [(CH3)SeH] and (CH3)SeOH are viewed as precursors of dimethyldiselenide.
The archaeon Methanococcus voltae has been found to be able to use dimethylselenide [(CH3)2Se]
as a source of Se required for its growth. Demethylation in this instance involved a corrinoid protein
and two methyltransferases (Niess and Klein, 2004).
SELENIUM REDUCTION IN THE ENVIRONMENT
Bacterial reduction of selenate and selenite has been detected in situ, especially in environments
with significant soluble selenium. In the Kesterson National Wildlife Refuge in California, Maiers
et al. (1988) reported that 4% of water samples, 92% of sediment samples, and 100% of the soil
samples they collected exhibited microbial selenium reduction. Of 100 mg selenate per liter, up
to 75% was reduced to red Se0, the rest to selenite. In the interstitial water of core samples from a
wastewater evaporation pond in Fresno, California, selenate removal was stimulated by H2 and the
addition of acetate, and inhibited by O2, NO 3– , MnO2, CrO2–
4 , and WO 4 , but not by the addition of
SO 4 , MoO 4 , or FeOOH (Oremland et al., 1989). At other sites in California and also in Nevada,
Steinberg and Oremland (1990) found measurable selenate-reducing activity in surficial sediment
samples from bodies of freshwater to waters with salinities of 250 g L−1 but not 320 g L−1. Nitrate,
nitrite, molybdate, and tungstate added separately to samples from the agricultural drains were
inhibitory to different extents. Sulfate partially inhibited the reduction of selenate in a sample
from a freshwater site but not in one from a site with water having a salinity of 60 g L−1. These
differences are likely reflections of differences in the type of selenate reducers in the different
samples and therefore of differences in mechanisms of selenate reduction. Additional studies in the
agricultural drainage region of western Nevada revealed a selenate turnover rate of 0.04–1.8 h−1
at ambient Se oxyanion concentrations (13–455 nM). Rates of removal of selenium oxyanions
ranged from 14 to 155 µmol m−2 per day (Oremland et al., 1991). The formation of elemental Se has
a potential for selenium immobilization in soil and sediment under anaerobic conditions. Owing
to the possibility of Se0 reoxidation under aerobic conditions, remobilization may occur. However,
such reoxidation has been found to be a slow process compared to microbial selenate reduction
(Dowdle and Oremland, 1998).
A recent study of selenate-respiring bacteria detected in culture enrichments with sediments from
water bodies in Chennai, India, and New Jersey, United States, revealed the presence of a diverse
group of bacteria classifiable with Gammaproteobacteria, Deltaproteobacteria, Deferribacteres,
and Chrysiogenetes (Narasingarao and Häggblom, 2007).
Methylation of selenium in aquatic environments has also been observed (e.g., Chau et al., 1976;
Frankenberger and Karlson, 1992, 1995). This activity has a potential for Se removal from polluted
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soils and waters. Ranjard et al. (2002) demonstrated transmethylation of different forms of selenium
with bacterial thiopurine methyltransferase in a strain of Escherichia coli, DH10B, acting on selenate, selenite, (methyl)selenocysteine, and selenomethionine.
Ecologically, anaerobic reduction of selenate and selenite to selenium (Se0) represents a respiratory, energy-conserving process in some microorganisms and serves to detoxify the immediate
environment for all organisms as long as anaerobic conditions are maintained. Selenium volatilization serves as a permanent detoxification process in water, soils, and sediments, and can occur
aerobically, although in at least one instance it was more effective anaerobically (Frankenberger and
The existence of a selenium cycle in nature was suggested by Shrift (1964). However, some of the
details of this cycle are still obscure. The ultimate source of selenium must be igneous rocks, but
whether microbes play a role in mobilizing the selenium from selenium-containing minerals is
unknown. Similarly, little is known about the role that microbes play in mobilizing selenium in soil
and sediment. Such an activity, when it occurs, is of great importance in understanding and controlling selenium pollution, as has occurred, for instance, in the Kesterson National Wildlife Refuge in
California. The source of selenium in that case appears to be the drainage of irrigation water applied
to farmland in the San Joaquin Valley. The irrigation water leached the selenium from the soil.
This drainage has been collecting in the wildlife refuge. Different processes in selenium cycling in
wetlands include redox reactions involving selenium, methylation and volatilization of selenium,
organic and inorganic complexation of selenium, precipitation and dissolution of Se-containing
minerals, and sorption and desorption of ionic species of selenium (Masscheleyn and Patrick, 1993).
The known biochemical steps of a selenium cycle are shown in Figure 21.1.
21.7 BIOOXIDATION OF REDUCED FORMS OF TELLURIUM
Microbial oxidation of reduced forms of tellurium has so far not been reported. This may mean
that this process does not occur in nature, but it is more likely that so far it has not been sought by
investigators. Its geomicrobial importance is likely to be limited because the natural occurrence of
tellurium is much rarer than that of selenium (see Section 21.1).
b, c, ?
FIGURE 21.1 The selenium cycle. (a) Escherichia coli, (b) bacteria, (c) actinomycetes, (d) fungi, (e)
Micrococcus lactilyticus, (f) Acidithiobacillus ferrooxidans. (See also Doran JW, Alexander M, Appl Environ
Microbiol, 33, 31–37, 1977.)
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Geomicrobiology of Selenium and Tellurium
21.8 BIOREDUCTION OF OXIDIZED FORMS OF TELLURIUM
Microbial reduction of tellurates and tellurites to elemental tellurium (Te0) and dimethyltelluride
[(CH3)2Te] has been reported (Woolfolk and Whiteley, 1962; Silverman and Ehrlich, 1964; Nagai,
1965; Bautista and Alexander, 1972; Trutkoet al., 2000; Klonowska et al., 2005; Csotonyi et al.,
2006; Baesman et al., 2007). Trutko et al. (2000) presented evidence that in some gram-negative
bacteria the respiratory chain was involved in tellurite reduction. The tellurite was reduced to tellurium crystallites, which appeared in the periplasmic space or on the outer or inner surface of the
plasma membrane. The makeup of the respiratory chain differed to some extent among the different
bacterial cultures tested. Klonowska et al. (2005) found that although Shewanella oneidensis was
able to reduce selenite and tellurite anaerobically, the electron transport pathway to the two electron
acceptors diverged upstream from tetracytochrome c, CymA.
Baesman et al. (2007) found that Bacillus selenitireducens and Sulfurospirillum barnesii
produced Te0 in the form of nanocrystals when respiring on tellurate or tellurite. With B. selenitireducens, Te 0 was deposited in the form of nanorods on the surface of the cells, which subsequently formed clusters, called shards, and rosettes. Nanorods also appeared in the bulk phase.
Some crystals in the form of nanorods that aggregated into shard-like nanocrystals also formed
inside the cells. Ssp. barnesii deposited Te 0 as irregularly shaped nanospheres (∼20 nm diameter) frequently attached to the cell surface, which coalesced into larger clusters (500–1000 nm
diameter). Nanospheres were also observed inside the cells. A question arises whether the difference in morphologies of the Te 0 nanocrystals is somehow related to a difference in the gramstaining properties of these two organisms, B. selenitireducens being gram-positive whereas
Ssp. barnesii being gram-negative. This difference in gram reactivity may reflect a difference in
organization of their respective electron transport systems, as is probably the case, for instance,
in Mn(II)-oxidation and Mn(IV)-reduction by gram-positive and gram-negative bacteria
(see Sections 17.5 and 17.6).
The fungus Penicillium sp. has been found to produce (CH3)2Te from several inorganic tellurium
compounds, provided only that reducible selenium compounds were also present (Fleming and
Alexander, 1972). The amount of dialkyltelluride formed was related to the relative concentrations
of Se and Te in the medium. Microbial reduction of oxidized forms of tellurium may represent
detoxification reactions rather than a form of respiration, but this needs further investigation.
Selenium, although a very toxic element, is nutritionally required by some bacteria, plants, and animals. Microorganisms have been described that can oxidize reduced selenium compounds. At least
one, Acidithiobacillus ferrooxidans, can use selenide in the form of CuSe as a sole source of energy,
oxidizing the compound to elemental selenium (Se0) and Cu2+. Oxidized forms of inorganic selenium compounds can be reduced by microorganisms, including members of the domains Bacteria
and Archaea, and Fungi. Selenate and selenite may be reduced to one or more of the following:
Se0, H2Se, dimethylselenide [(CH3)2Te], dimethyl diselenide [(CH3)2Se2], and dimethyl selenone
[(CH3)2SeO2]. The reductions are enzymatic and in some bacteria represent a form of respiration.
The microbial interactions with various forms of selenium contribute to a selenium cycle in nature.
Microbial selenate and selenite reduction to elemental selenium in soil and sediment is a form of
selenium immobilization that is potentially reversible. Microbial selenate and selenite reduction
to volatile forms of selenium in soil, sediment, and water columns of bodies of water is a form of
selenium removal that is permanent.
Tellurium occurs in such low concentrations in nature that it does not seem geomicrobially
important. Nevertheless, microbial reduction of tellurate and tellurite to elemental tellurium (Te0)
and dimethyltelluride [(CH3)2Te] has been observed. Microbial oxidation of tellurides has so far not
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of Fossil Fuels
Although much of the organic carbon in the biosphere is continually recycled, a very significant
amount has become trapped in special sedimentary formations, where it is inaccessible to mineralization by microbes until it becomes reexposed to water through natural causes or human intervention. Microbial mineralization of such reexposed organic carbon also depends on the access
to suitable terminal electron acceptors, that is, oxygen in air in the case of aerobes, and inorganic
electron acceptors in the case of facultative or anaerobic microbes. The trapped organic carbon
exists in various forms. The degree of its chemically reduced state is related to the length of time
it has been trapped and any secondary changes that it has undergone during this time. Some of
this trapped carbon has value as a fuel, a source of energy for industrial and other human activity,
and is exploited for this purpose. Because of the great age of this material, it is known as fossil
fuel. The remainder of the trapped carbon is chiefly kerogen and bitumen, some of which can be
converted to fuel by human intervention. Fossil fuels include methane gas, natural gas (which is
largely methane), petroleum, oil shale, coal, and peat. They are generally considered to have had a
microbial origin (Ourisson et al., 1984).
NATURAL ABUNDANCE OF FOSSIL FUELS
A major portion of the total carbon at the Earth’s surface is in the form of carbonate (Figure 22.1). It
represents a major sink for carbon. The other sink is the trapped organic carbon that is not directly
accessible for microbial mineralization. The carbonate carbon is not an absolute sink unless it is
deeply buried because it is in a steady-state relationship with dissolved carbonate/bicarbonate and
atmospheric CO2, which in turn are in a steady-state relationship with organic carbon in living and
dead biomass. The passage of carbon from one compartment into another is under biological control
(Figure 22.1; Fenchel and Blackburn, 1979).
Methane at atmospheric pressure and ambient temperature is a colorless, odorless, and flammable
gas. Because its autoignition temperature is 650°C, it does not catch fire spontaneously. It is sparingly soluble in water (3.5 mL per 100 mL of water) but readily soluble in organic solvents, including
liquid hydrocarbons. It may have an abiotic or biogenic origin. Biogenic accumulations of methane
may occur in nature when it is formed in consolidated sediment from which it cannot readily escape.
In some deep-ocean sediments under conditions of high pressure and low temperature, methane
accumulations are found in the form of methane hydrates (Kvenvolden, 1988; Haq, 1999). A special,
mixed community of certain members of the Archaea and Bacteria living very close to methane
hydrate has been detected in the forearc basin of the Nankai Trough off the east coast of Japan by
Reed et al. (2002) and in solid gas hydrates in the Gulf of Mexico by Mills et al., 2005.
Bioformation of methane comes about when organic matter in the sediment is undergoing anaerobic microbial breakdown in the absence of significant quantities of alternative terminal electron
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6.4 × 1017 g C
3.8 × 1019 g C
8.3 × 1017 g C
1.5 × 1018 g C
in sediments and
Limestone and other
3.5 × 1018 g C
1.8 × 1022 g C
natural gas, coal,
kerogen, and bitumen
2.5 × 1022 g C
FIGURE 22.1 Microbial and physical processes contributing to carbon transfer among different compartments in the biosphere. a, Microbial assimilation; b, burial; d, decomposition; e, excretion; m, microbial mineralization; r, respiration; s, sedimentation. (Quantitative estimates from Fenchel T, Blackburn TH, Bacteria
and Mineral Cycling, Academic Press, London, U.K., 1979; Bowen HJM, Environmental Chemistry of the
Elements, Academic Press, London, U.K., 1979.)
acceptors such as nitrate, Fe(III), Mn(IV), or sulfate. If gas pressure due to methane builds up sufficiently in anaerobic lake or coastal sediment, it may escape in the form of large gas bubbles that
break at the water surface to release their methane into the atmosphere (Martens, 1976; Zeikus,
1977). In marshes, escaping methane may be ignited (by biogenic phosphene?) to burn as the socalled will-o’-the-wisps.
Many of the methane accumulations on Earth are of biogenic origin. Methane may occur in association with peat, coal, and oil deposits, or independent of them. That which occurs in association
with coal and oil was probably microbially generated in the early stages of their formation, although
some may have been formed abiotically in later diagenetic phases. Methane associated with coal
deposits can be the cause of serious mine explosions when accidentally ignited. Such methane is
called coal damp by coal miners.
Biogenic methane formation is a unique biochemical process that appears to have arisen very
early in the evolution of life. Indeed, the methanogenesis that results from the microbial reduction
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