Science - USA (2021-11-12)

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INSIGHTS | PERSPECTIVES

atures and higher pressures in
deeper parts of Earth’s crust lead
to side chain degradation of aro-
matic compounds and demethyl-
ation of the methoxyl groups ( 4 ,
5 ). The loss of methoxyl groups is
thus a key stage in the transfor-
mation of wood to coal and likely
an important source of methane
in coal beds. However, whether
the mechanism of this process
is purely a chemical reaction by
thermal decomposition (thermo-
genic methane) or an enzyme-
catalyzed biodegradation by
microbes (microbial methane)
has been vigorously debated for
many years.
Recent evidence supports mi-
crobial methanogenesis in coal-bearing ma-
rine sediments at depths down to 2.5 km ( 6 ).
Additionally, specific methanogens belong-
ing to the genus Methermicoccus can utilize
coal methoxyl groups to produce methane
( 7 ). However, the extent to which methano-
gens transform methoxyl groups to methane
in the deep subsurface is still unknown.
Progress in understanding the key role
that plant methoxyl groups play in form-
ing C1 volatiles came about by exploiting
the Zeisel reaction (to determine the num-
ber of methoxyl groups) and isotope ratio
mass spectrometry. This enabled measure-
ment of the ratio of the naturally occur-
ring stable isotopes of carbon (the amount
of^13 C relative to^12 C, expressed as d^13 C val-
ues) of the methoxyl groups. The d^13 C val-
ues of these groups can be utilized not only
to gain deeper insights into the processes
that control the formation of C1 volatiles in
the environment but also to trace the ori-
gin of several atmospheric C1 gases ( 8 ). In
this context, it is important to note that,
traditionally, thermogenic methane is con-
sidered to contain more^13 C than microbial
methane, which therefore has more nega-
tive d^13 C values ( 9 ).
Lloyd et al. measured both the abundance
and isotopic fingerprint of methoxyl groups
from organic matter collected from several
localities around the globe that represent
the major maturity stages, from wood to
bituminous coal. They suggest that micro-
organisms directly metabolize coal struc-
tures that fuel methane formation in deep
sediments (see the figure). These results are
of considerable interest for understanding
methane generation from organic carbon of
terrestrial deep and surface sediments and
the cycling of carbon in the past, present,
and future.
The reported carbon isotopic patterns of
coal methoxyl groups support the idea that
microbial-mediated processes rather than

abiotic reactions drive methane formation

from plant methoxyl groups on geological

time scales of tens of millions of years. It is

also important to point out that the results

considerably expand the measured range

of carbon isotopic signatures for methoxyl

groups on Earth, as Lloyd et al. reported

very positive d^13 C values [up to +27 per mil

(‰)], which are usually only observed for

organic compounds from extraterrestrial

environments, such as meteorites. Thus,

when considering the source substrate,

these isotopic constraints might elegantly

explain why CBM often shows an enrich-

ment in^13 C that nevertheless could still be

of microbial origin. These results will also

alter thinking about the traditional as-

signment of isotopic patterns of methane

sources ( 9 ) because they offer a straightfor-

ward explanation for why microbial meth-

ane from coal is often enriched in^13 C rela-

tive to other microbial methane sources.

Currently, CBM makes up a substantial

portion of the world’s natural gas resources

and may become a transition fuel when

moving from a carbon-based to a sustain-

able green economy. It may displace coal

in electricity generation, thereby

reducing  carbon dioxide emis-

sions and greatly decreasing emis-

sions of particulate matter and

other pollutants, such as sulfur.

However, CBM production wells

often have limited life spans, and

thus there are efforts to enhance

microbial methanogenesis in coal

beds ( 10 , 11 ). The abundance of

methoxyl groups in the lignite and

coal samples reported by Lloyd

et al. not only show that these

groups affect the yield of CBM,

they limit the potential produc-

tion of methane from coal by the

addition of microbes when almost

all methoxyl groups have already

been removed during matura-

tion processes. A major known downside of

mining activities is that the release of CBM

currently accounts for 5 to 10% of the global

atmospheric methane budget and thus con-

tributes to global climate change ( 12 ).

The potent role of methane as a green-

house gas is also of interest when the find-

ings of Lloyd et al. are considered for future

carbon cycling and global climate change,

given the potential thawing of large per-

mafrost areas that are considered to be

one of the tipping elements of the Earth

system ( 13 ). The methoxyl moieties in the

vast amounts of organic matter that have

accumulated over thousands of years and

have been preserved for up to 650,000

years in permafrost ( 14 ) might be rapidly

transformed to methane by methanogens

once warmer climates allow their growth

and multiplication. If not consumed by

degrading microbes, the formed meth-

ane might eventually end up in the atmo-

sphere, potentially providing a positive

feedback on climate change. j

REFERENCES AND NOTES

1. S. Zeisel, Monatsh. Chem. 6 , 989 (1885).


2. M. K. Lloyd et al., Science 374 , 894 (2021).


3. W. Boerjan, J. Ralph, M. Baucher, Annu. Rev. Plant Biol.
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4. P. G. Hatcher, D. J. Clifford, Org. Geochem. 27 , 251 (1997).


5. H. Lee, X. Feng, M. Mastalerz, S. J. Feakins, Org.
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6. F. Inagaki et al., Science 349 , 420 (2015).


7. D. Mayumi et al., Science 354 , 222 (2016).


8. F. Keppler, R. M. Kalin, D. B. Harper, W. C. McRoberts, J. T.
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9. M. J. Whiticar, in Hydrocarbons, Oils and Lipids: Diversity,
   Origin, Chemistry and Fate, H. Wilkes, Ed. (Springer,
   2020), pp. 669–746.


10. D. Ritter et al., Int. J. Coal Geol. 146 , 28 (2015).


11. D. Strąpoć et al., Annu. Rev. Earth Planet. Sci. 39 , 617
    (2011).


12. M. Saunois et al., Earth Syst. Sci. Data 12 , 1561 (2020).


13. W. Steffen et al., Proc. Natl. Acad. Sci. U.S.A. 115 , 8252
    (2018).


14. J. B. Murton et al., Quat. Res. 1 0. 1 0 1 7/
    qua.2021.27(2021).
    10.1126/science.abm6027

Litter

Humic

material

Peat

Lignite

Coal

Ve g e t a t i o n

Microbes

Methanogens

Weight percent

Stable carbon

isotope ratio

Time

0 5 10 /

“The reported carbon isotopic

patterns of coal methoxyl

groups support the idea that

microbial-mediated processes

rather than abiotic reactions

drive methane formation from

plant methoxyl groups on

geological time scales of tens

of millions of years.”

The fate of methoxyl groups: From plants to coal

Microbial conversion of methoxyl groups plays an important role in generating

methane on geological time scales.

822 12 NOVEMBER 2021 • VOL 374 ISSUE 6569