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Titel |
The preferential growth of branched GDGT source microorganisms under aerobic conditions in peat revealed by stable isotope probing experiments |
VerfasserIn |
Arnaud Huguet, Travis B. Meador, Fatima Laggoun-Defarge, Martin Könneke, Sylvie Derenne, Kai-Uwe Hinrichs |
Konferenz |
EGU General Assembly 2016
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Medientyp |
Artikel
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Sprache |
en
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 18 (2016) |
Datensatznummer |
250127454
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Publikation (Nr.) |
EGU/EGU2016-7334.pdf |
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Zusammenfassung |
Branched glycerol dialkyl glycerol tetraether (brGDGTs) membrane lipids are widely
distributed in aquatic and terrestrial environments and are being increasingly used as
temperature proxies. Nevertheless, little is known regarding the microorganisms that
produce these lipids, which are found in especially high abundance in the anaerobic
horizons of peat bogs. We initiated stable isotope probing incubations of peat samples
from a Sphagnum-dominated peatland (Jura Mountains, France) to measure the
incorporation of (D)-D2O and 13C-labeled dissolved inorganic carbon (DIC) into
brGDGTs, and thus gauge the activity, growth, and turnover times of their source
organisms. Peat samples were collected from two adjacent sites with contrasting
humidity levels (hereafter called “fen” and “bog” sites). For each site, samples from
the surficial aerobic layer (acrotelm) and deeper anaerobic layer (catotelm) were
collected and were incubated under both anaerobic and aerobic conditions for the
acrotelm samples and only anaerobic conditions for the catotelm. The incubations were
performed at 12 ˚ C, consistent with the mean summer air temperature at the sampling
site.
After two months of incubation, there was no incorporation of 13C label in brGDGTs
for samples incubated under either aerobic or anaerobic conditions, showing that
brGDGT-producing bacteria are heterotrophic microorganisms, as previously observed in
organo-mineral soils (Weijers et al., 2011). Similarly, little to no deuterium incorporation was
observed for brGDGTs isolated from anaerobically-incubated deep samples. In contrast, in
the aerobic incubations of acrotelm samples from bog and fen, the weighted average δD of
brGDGT core lipids (CLs) increased by up to 3332‰ and 933‰ after two months,
respectively, indicating that fresh brGDGT CLs were biosynthesized at the peat surface. D
incorporation into brGDGT CLs converted to production rates ranging from 30-106
ng cm−3y−1 in the aerobic acrotelm from bog and fen, whereas corresponding
rates in the anaerobic acrotelm incubations were more than an order of magnitude
slower (< 3 ng cm−3 y−1). Production rates of bacterial fatty acids approached or
exceeded 1 μg cm−3 y−1 in both aerobic and anaerobic incubations, and were
therefore much higher than those of brGDGTs. This suggests that the brGDGT
producers are a minor constituent of the microbial community in Sphagnum-dominated
peatlands or brGDGTs are a small component of the microbial cell membrane in
comparison to fatty acids, despite the typically high brGDGT concentrations observed in
peat.
In conclusion, our results reveal that brGDGT source microorganisms preferentially grow
under oxic to sub-oxic conditions, likely as facultative anaerobes. We show for the first time
that these microorganisms are especially active at the peat surface, in contrast to
the deeper layers, implying that the high abundance of brGDGTs observed in the
catotlem should result from the accumulation of the brGDGTs actively produced in the
acrotelm.
Reference
Weijers, J.W.H., Wiesenberg, G.L.B., Bol, R., Hopmans, E.C., Pancost, R.D., 2010.
Biogeosciences 7, 2959–2973. |
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