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| Titel |
Methanotrophy and chemoautotrophy within the redox gradient of a large and deep tropical lake (Lake Kivu, East Africa) |
| VerfasserIn |
Cedric Morana, Alberto V. Borges, François Darchambeau, Fleur Roland, Laetitia Montante, Jean-Pierre Descy, Steven Bouillon |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250091049
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| Publikation (Nr.) |
 EGU/EGU2014-5316.pdf |
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| Zusammenfassung |
| Lake Kivu (East Africa) is a large (2370 km2) and deep (maximum depth of 485 m)
meromictic lake. Its vertical structure consists of an oxic and nutrient-poor mixed layer down
to 70 m maximum, and a permanently anoxic monimolimnion rich in dissolved gases
(methane and carbon dioxide) and inorganic nutrients. Seasonal variation of the vertical
position of the oxic-anoxic interface is driven by contrasting precipitation and wind
speed regimes between rainy (October-May) and dry (June-September) season,
the latter being characterized by a deepening of the oxic zone, and an increased
input of dissolved gases and inorganic nutrients. Our work aimed at quantifying
methanotrophic and chemoautotrophic production within the redox gradient of
Lake Kivu and identifying the micro-organisms involved in these processes using
phospholipid-derived fatty acid markers and their carbon stable isotope composition. Our
approach combined both natural stable isotope abundance analysis and 13C-labelling
(13C-DIC ; 13C-CH4) experiments. Sampling was carried out at two stations in Lake
Kivu during rainy (February 2012) and dry (September 2012) season conditions.
Methanotrophic bacterial production rates were highly variable (from 0.1 to 7.0
μmol C L-1 d-1), but maximum values were always observed at the oxic-anoxic
interface when the CH4:O2 ratio varied between 0.1 and 10, suggesting that the
majority of methane was oxidized aerobically. Furthermore, strong stable isotope
labelling of monounsaturated C16 fatty acids indicate that active methane oxidizers
were related to the group of type I aerobic methanotrophs (gammaproteobacteria).
Despite the dominance of aerobic methane oxidation, significant methanotrophic
bacterial production rates were found below the oxic-anoxic interface during the rainy
season, indicating that at least a fraction of the upcoming methane may be oxidized
anaerobically. This observation was further confirmed by the strong labelling at these
depths of the 10Me16:0 fatty acid, biomarker for sulphate-reducing bacteria, the
syntrophic partners of anaerobic methane-oxidizing archaea. The methanotrophic
bacterial growth efficiency (MBGE) was variable (2-50%), and inversely related to
methane concentration. Maximum chemoautotrophic bacterial production rates
were recorded well below the oxycline, in sulfidic waters. However, during the
rainy season, significant dark C fixation rates were measured near the oxic-anoxic
interface, in a nitracline where sulphide was absent, suggesting that another energy
source was involved. Incorporation of labelled carbon in the 16:1Ï9c ; 16:1Ï7c and
18:1Ï7 fatty acids suggest that the active chemoautotrophic organisms belong to
the phylum proteobacteria. Together, the vertically integrated methanotrophic and
chemoautotrophic production rates were 31 mmol m-2 d-1 and 41 mmol-2 d-1
during the rainy and dry season, respectively. These values are comparable to the net
phytoplanktonic production rates in Lake Kivu ranging between 12 and 160 mmol
m-2 d-1 (on average 52 mmol m-2 d-1). Our results indicate that methanotrophs
and chemoautotrophs contribute substantially to the carbon cycle in Lake Kivu. |
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