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| Titel |
The stable isotopic signature of biologically produced molecular hydrogen (H2) |
| VerfasserIn |
S. Walter, S. Laukenmann, A. J. M. Stams, M. K. Vollmer, G. Gleixner, T. Röckmann |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 9, no. 10 ; Nr. 9, no. 10 (2012-10-24), S.4115-4123 |
| Datensatznummer |
250007342
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| Publikation (Nr.) |
 copernicus.org/bg-9-4115-2012.pdf |
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| Zusammenfassung |
Biologically produced molecular hydrogen (H2) is characterised by a
very strong depletion in deuterium. Although the biological source to the
atmosphere is small compared to photochemical or combustion sources, it
makes an important contribution to the global isotope budget of H2.
Large uncertainties exist in the quantification of the individual production
and degradation processes that contribute to the atmospheric budget, and
isotope measurements are a tool to distinguish the contributions from the
different sources. Measurements of δ D from the various H2
sources are scarce and for biologically produced H2 only very few
measurements exist.
Here the first systematic study of the isotopic composition of biologically
produced H2 is presented. In a first set of experiments, we investigated
δ D of H2 produced in a biogas plant, covering different
treatments of biogas production. In a second set of experiments, we
investigated pure cultures of several H2 producing microorganisms such
as bacteria or green algae. A Keeling plot analysis provides a robust overall
source signature of δ D = −712‰ (±13‰) for the
samples from the biogas reactor (at 38 °C, δ DH2O=
+73.4‰), with a fractionation constant ϵH2-H2O
of −689‰ (±20‰) between H2 and the water. The five
experiments using pure culture samples from different microorganisms give a
mean source signature of δ D = −728‰ (±28‰), and a
fractionation constant ϵH2-H2O of −711‰
(±34‰) between H2 and the water. The results confirm the
massive deuterium depletion of biologically produced H2 as was predicted
by the calculation of the thermodynamic fractionation factors for hydrogen
exchange between H2 and water vapour. Systematic errors in the isotope
scale are difficult to assess in the absence of international standards for
δ D of H2.
As expected for a thermodynamic equilibrium, the fractionation factor is
temperature dependent, but largely independent of the substrates used and the
H2 production conditions. The equilibrium fractionation coefficient is
positively correlated with temperature and we measured a rate of change of
2.3‰ / °C between 45 °C and 60 °C, which is
in general agreement with the theoretical prediction of
1.4‰ / °C.
Our best experimental estimate for ϵH2-H2O at a
temperature of 20 °C is −731‰ (±20‰) for
biologically produced H2. This value is close to the predicted value of
−722‰, and we suggest using these values in future global H2
isotope budget calculations and models with adjusting to regional
temperatures for calculating δ D values. |
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