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| Titel |
Aerobic methanotrophs drive the formation of a seasonal anoxic benthic nepheloid layer in monomictic Lake Lugano |
| VerfasserIn |
Jan Blees, Helge Niemann, Christine B. Wenk, Jacob Zopfi, Carsten J. Schubert, Joël S. Jenzer, Mauro L. Veronesi, Moritz F. Lehmann |
| 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 |
250098728
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| Publikation (Nr.) |
 EGU/EGU2014-14430.pdf |
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| Zusammenfassung |
| In the southern basin of Lake Lugano, thermal stratification of the water column during
summer and autumn leads to a lack of exchange between surface and deep water
masses, and consequently to seasonal bottom water anoxia, associated with high
methane concentrations. With the onset of bottom water anoxia, a dense layer of high
particulate matter concentration – a so-called benthic nepheloid layer (BNL) –
develops in the bottom waters. A sharp redox gradient marks the upper boundary of
the BNL. At its maximum, the BNL extends 15 – 30 m from the sediment into
the water column. We investigated the identity of the BNL and key environmental
factors controlling its formation in the framework of a seasonal study. Compound
specific C-isotope measurements and Fluorescence In Situ Hybridisation (FISH) of
suspended particulate organic matter, radioactive tracer based measurements of
methane oxidation, as well as investigation of geochemical water column parameters
were performed in spring and autumn. Our analyses revealed that the microbial
biomass within the BNL is dominated by methanotrophic bacteria. Aerobic methane
oxidation (MOx) was restricted to a narrow zone at the top of the BNL, reaching
maximum rates of up to 1.8 μM/day. The rates of MOx activity effectively consumed
most (>99%) of the uprising methane, leading to the formation of a sharp CH4
concentration gradient and a strongly suppressed kinetic isotope effect (É = -2.8o). CH4
oxidation was limited by the diffusive supply of O2 from the upper hypolimnion,
implying that methanotrophy is the primary driver of the seasonal expansion of the
anoxic bottom water volume, and explaining the vertical migration of the BNL in
response to its own O2 consumption. The bulk organic matter extracted from the
BNL was strongly depleted in 13C (δ13C < -60o), providing evidence for the
incorporation of CH4-derived carbon into the biomass, suggesting that the BNL was
composed of MOx-communities. This was further evidenced by four dominant fatty acid
biomarkers (C16:1Ï5, C16:1Ï6, C16:1Ï7, and C16:1Ï8), which were strongly C-isotopically
depleted, with δ13C values between -62o (Ï6) and -80o (Ï7). The fingerprint of
isotopically depleted FAs indicates a dominance of Type I MOx bacteria in the
BNL, which we could confirm with FISH using specific probes. Isotope mixing
considerations suggest that 77 – 96 % of fatty acid carbon in the BNL is CH4-derived. FISH
revealed that up to 30% of microbial cells in the BNL are methanotrophic. The cell
size of methanotrophs was significantly larger than of other microbial cells, and an
independent approach to quantify the contribution of methanotroph-carbon to the BNL
biomass, based on methanotrophic cell size, confirmed our C-isotope-based estimate. |
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