Publication Date:
2022-05-25
Description:
Author Posting. © The Author(s), 2018. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 237 (2018): 339-356, doi:10.1016/j.gca.2018.06.029.
Description:
The abundance of methane isotopologues with two rare isotopes (e.g., 13CH3D) has been
proposed as a tool to estimate the temperature at which methane is formed or thermally
equilibrated. It has been shown, however, that microbial methane from surface environments and
from laboratory cultures is characterized by low 13CH3D abundance, corresponding to
anomalously high apparent 13CH3D equilibrium temperatures. We carried out a series of batch
culture experiments to investigate the origin of the non-equilibrium signals in microbial methane
by exploring a range of metabolic pathways, growth temperatures, and hydrogen isotope
compositions of the media. We found that thermophilic methanogens
(Methanocaldococcus jannaschii, Methanothermococcus thermolithotrophicus, and
Methanocaldococcus bathoardescens) grown on H2+CO2 at temperatures between 60 and 80°C
produced methane with Δ13CH3D values (defined as the deviation from stochastic abundance) of
0.5 to 2.5‰, corresponding to apparent 13CH3D equilibrium temperatures of 200 to 600°C.
Mesophilic methanogens (Methanosarcina barkeri and Methanosarcina mazei) grown on
H2+CO2, acetate, or methanol produced methane with consistently low Δ13CH3D values, down to
-5.2‰. Closed system effects can explain part of the non-equilibrium signals for methane from
thermophilic methanogens. Experiments with M. barkeri using D-spiked water or D-labeled
acetate (CD3COO-) indicate that 1.6 to 1.9 out of four H atoms in methane originate from water,
but Δ13CH3D values of product methane only weakly correlate with the D/H ratio of medium water.
Our experimental results demonstrate that low Δ13CH3D values are not specific to the metabolic
pathways of methanogenesis, suggesting that they could be produced during enzymatic reactions
common in the three methanogenic pathways, such as the reduction of methyl-coenzyme M.
Nonetheless C-H bonds inherited from precursor methyl groups may also carry part of non-equilibrium signals.
Description:
Grants from the National Science Foundation (EAR-1250394 to S.O.), N. Braunsdorf and D. Smit
of Shell PTI/EG (to S.O.), the Deep Carbon Observatory (to S.O., M.K., K.-U.H., D.S.G.), the
Gottfried Wilhelm Leibniz Program of the Deutsche Forschungsgemeinschaft (HI 616-14-1 to K.-
U.H.), and the Heisenberg Program (KO3651-3-1 to M.K.) of the Deutsche
Forschungsgemeinschaft supported this study. D.S.G. was also supported by a National Science
Foundation Graduate Research Fellowship, the Neil and Anna Rasmussen Foundation Fund, the
Grayce B. Kerr Fellowship, and a Shell-MIT Energy Initiative Graduate Fellowship. D.T.W. was
supported by a National Defense Science and Engineering Graduate Fellowship. L.C.S. was
supported by a NASA Earth and Space Science Fellowship (grant NNX11AP78H).
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
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