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10.5061/dryad.jq2bvq89m.
ACKNOWLEDGMENTS
We thank N. Tomioka and an anonymous reviewer for their
helpful comments.Funding:This work was supported by awards
NSF-EAR-1838330, -EAR-1942042, and -EAR-1322082; the NSF
Cooperative Agreement No. DMR-1644779; and the State of Florida.
Use of the Advanced Photon Source and the Advanced Light Source
were supported by the US Department of Energy, Basic Energy
Sciences, contracts DE-AC02-06CH11357 and DE-AC02-05CH11231,
respectively.Author contributions:O.T., S.H., S.Y., and M.H.
participated in design, interpretation, data collection, and analysis of
the reported results and in drafting and revising the manuscript.
W.L., S.N.G.C., H.A.B., J.T., and G.R.R. participated in data collection
and revising the manuscript.Competing interests:The authors
have no competing interests.Data and materials availability:
Additional chemical and crystallographic information about
davemaoite is provided in the supplementary materials. Raw data
are deposited at Dryad ( 23 ). Crystallographic and chemical
information on type davemaoite is deposited in the InorganicCrystal Structure Database. The type material is deposited with
the NHMLA under accession number 74541.SUPPLEMENTARY MATERIALS
science.org/doi/10.1126/science.abl8568
Materials and Methods
Figs. S1 to S4
Tables S1 and S2
References ( 24 – 40 )
Data S1
9 August 2021; accepted 21 September 2021
10.1126/science.abl8568FOSSIL CARBON
Methoxyl stable isotopic constraints on the origins
and limits of coal-bed methane
M. K. Lloyd1,2*, E. Trembath-Reichert1,3, K. S. Dawson1,4, S. J. Feakins^5 , M. Mastalerz^6 , V. J. Orphan^1 ,
A. L. Sessions^1 , J. M. Eiler^1
Microbial coal-bed methane is an important economic resource and source of a potent greenhouse gas,
but controls on its formation are poorly understood. To test whether the microbial degradability of
coal limits microbial methane, we monitored methoxyl group demethylation—a reaction that feeds
methanogenesis—in a global sample suite ranging in maturity from wood to bituminous coal. Carbon
isotopic compositions of residual methoxyl groups were inconsistent with a thermal reaction, instead
implying a substrate-limited biologic process. This suggests that deep biosphere communities
participated in transforming plant matter to coal on geologic time scales and that methoxyl abundance
influences coal-bed methane yield. Carbon isotopic enrichments resulting from microbial methylotrophy
also explain an enigmatic offset in the carbon-13 content of microbial methane from coals and conventional
hydrocarbon deposits.
C
oal is currently the world’s dominant
electricity source, but its combustion
carries great environmental costs ( 1 ).
Coal-bed methane (CBM) produces less
CO 2 per unit of electricity and far less
particulate pollution, but its viability as a
transition fuel is debated: CBM production
wells often have limited life spans, and efforts
to enhance microbial methanogenesis in coal
beds have seen inconsistent success ( 2 ). CBM
emissions also account for ~10% of global
atmosphericinputsofthispotentgreenhouse
gas ( 3 ), but substantial uncertainties regarding
present and future emissions remain ( 4 ). The
outcomes of these efforts depend on under-
standing what factors limit CBM generation.
Most of the world’s coal resources are of low
rank (e.g., lignite or subbituminous), where
CBM is formed by subsurface microbial com-
munities metabolizing degraded components
of the coal structure ( 5 , 6 ). One substrate of
particular interest is the methoxyl group
(R–O–CH 3 ). Methoxyl groups initially make
up about one in eight carbon atoms in lignin,
and their loss (termed O-demethylation) is a
key reaction in the chemical transformation
of woody plant tissue to coal (coalification)
that is inferred to source the first peak of
methane production in humic coals ( 7 – 9 ).
Despite its importance, little is known of the
organisms that catalyze coal O-demethylation
in the environment or whether this initial step
is biogenic at all ( 6 ). Some have suggested that
direct microbial modification of plant matter
in nature ends during peat formation ( 5 ), and
therefore abiotic mechanisms [e.g., temper-
ature, acid hydrolysis ( 10 )] catalyze subse-
quent O-demethylations. However, laboratory
enrichment cultures of coal-hosted anaerobes
directly O-demethylate humic materials beyond
the peat stage ( 11 , 12 ), and scientific drilling
continues to extend the depths and temper-
atures at which hydrocarbon-degrading life
can persist ( 13 , 14 ). Therefore, the extent to
which deep-biosphere microbes catalyze early
coalification reactions such as O-demethylation
( 6 )—or merely consume the by-products ( 10 )—
is an open question central to understanding
the viability of CBM as a transition fuel and
its contribution to atmospheric methane. Ifthe ambient temperature, activity, or com-
position of indigenous microbial assemb-
lages limits methanogenesis in low-rank coals,
strategic amendments of microbes or nutrients
could enhance CBM generation or emission.
Alternatively, if coal’s microbial degradability
(hereafter, bioavailability) limits CBM forma-
tion, only thermal maturation on geologic time
scales would measurably increase the abun-
dance of this methane source.
Stable isotopes can distinguish between
reaction mechanisms that impart different
isotopic fractionations ( 15 ). However, bulk
isotopic analyses are insufficient for macro-
molecular organics like lignin because the
carbon sites that participate in coalification
reactions are diluted by those that do not. To
constrain how coals are O-demethylated, we
analyzed the site-specific concentration and
carbon isotopic composition of methoxyl
groups in a global suite of natural humic
materials representing the major coalifica-
tion stages from pristine wood to bituminous
coal (materials and methods, section S1).
Methoxyl abundances (reported as the frac-
tion of all lignin carbon that is methoxyl
carbon) andd^13 C values [reported in per mil
versus Vienna Pee Dee belemnite (VPDB):
d^13 Cmethoxyl=(^13 Rmethoxyl/^13 RVPDB)−1] in wood
range from 0.13 to 0.19 and−23 to−32 per
mil (‰), respectively (Fig. 1). Lignites from
Belchatow, Poland (vitrinite reflectance of
0.27 to 0.28% Ro), have 25 to 40% of the initial
methoxyl groups remaining and d^13 Cmethoxyl
values of−19 to− 13 ‰. Lignites and sub-
bituminous coals retrieved offshore of the
Shimokita peninsula (0.37 to 0.43% Ro) have
0.1 to 2% of the methoxyl groups in lignin
andd^13 Cmethoxylvalues of +6 to +27‰. Sub-
bituminous coals from the Powder River Basin
are marginally more mature (0.45 to 0.47% Ro),
have lower methoxyl contents (0.05 to 0.08%
remaining), andd^13 Cmethoxylvalues of +4 to
+13‰. Bituminous coals from the San Juan
and Michigan Basins (0.5 to 0.8% Ro) contain
no methoxyl groups above detection limits.
Bulk organic carbond^13 C are all between− 27
and− 22 ‰(table S1).
A reduction in methoxyl concentration and
a marked rise ind^13 Cmethoxylvalues accom-
pany coalification. Our normalization scheme894 12 NOVEMBER 2021•VOL 374 ISSUE 6569 science.orgSCIENCE
(^1) Division of Geological and Planetary Sciences, California
Institute of Technology, Pasadena, CA 91125, USA.
(^2) Department of Geosciences, The Pennsylvania State
University, University Park, PA 16802, USA.^3 School of Earth
and Space Exploration, Arizona State University, Tempe, AZ
85287, USA.^4 Department of Environmental Sciences,
Rutgers, The State University of New Jersey, New Brunswick,
NJ 08901, USA.^5 Department of Earth Sciences, University of
Southern California, Los Angeles, CA 90089, USA.^6 Indiana
Geological and Water Survey, Indiana University,
Bloomington, IN 47405, USA.
*Corresponding author. Email: [email protected]
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