25th International Meeting on Organic Geochemistry IMOG 2011

P-404
Amino sugars as biomarkers for transformation of organic
nitrogen compounds in two Swiss lakes
Dörte Carstens, Krista E. Köllner, Gijs Nobbe, Helmut Bürgmann, Bernhard Wehrli,
Carsten J. Schubert
Swiss Federal Institute for Aquatic Science and Technology (Eawag), Surface Waters Research and
Management, Kastanienbaum, Switzerland (corresponding author:doerte.carstens@eawag.ch)
Studies on the fate of organic nitrogen
compounds in aquatic systems mostly concentrate on
marine environments. As amino sugars (AS) are
nitrogen and carbon containing molecules they are
important contributors to the nitrogen and carbon
cycle. Microorganisms mediate substantially
biogeochemical cycles as they represent both
contributors and degraders of organic matter. In order
to study the fate of organic nitrogen compounds and
the role of bacteria to organic matter transformation in
lacustrine systems Lake Brienz (oligotrophic,
holomictic) and Lake Zug (eutrophic, meromictic)
were chosen as study sites.
Water and particulate matter (> 0.7 μm) were
sampled in ten depths distributed over the entire
water columns in fall 2009. Plankton as source of
autochthonous material was collected in the
epilimnion. Additionally, sediment cores were
recovered. Concentrations of single AS in the
particulate fraction of the water samples, the plankton
and the first centimeters of the sediments were
measured by gas chromatography (GC). AS
concentrations of the unfiltered water samples were
determined by high performance liquid
chromatography. GlcN:GalN ratios were used to
identify the AS sources, as chitin-rich plankton has
GlcN:GalN ratios >14 and heterotrophic bacterial
communities ratios smaller than three [1]. Additionally
the Chlorin Index (CI) was applied as a pigment
based degradation parameter for organic matter
freshness. Concentrations of pheophytin and longchain
fatty acids in the sediments were measured by
GC. With the pheophytin and long-chain fatty acids
concentrations the input of terrestrial material to the
sediments was estimated.
The results showed decreasing carbon
normalized AS yields with increasing water depths
indicating enhanced degradation of AS compared to
the bulk organic matter in both lakes. Within the
sediments the AS yields were fluctuating with an
increasing trend for Lake Brienz and a decreasing
trend for Lake Zug with depth. The values of the CI
displayed degradation of particulate matter settling
towards the bottom of the lake and within the
sediments. The GlcN:GalN ratios showed a
replacement of the AS composition of the plankton by
the AS signature of heterotrophic microorganisms
along the water column. Even smaller values for the
ratio were found in the sediment samples indicating
ongoing degradation of the buried organic matter by
bacteria. Fluctuating AS yields within the sediments
could be explained by different fractions of
allochthonous material in the samples. Higher AS
yields were found in sediment layers with major input
of terrigenous organic matter. These findings indicate
that AS from autochthonous sources are more
degradable than those from allochthonous sources,
which are accumulating in the lake sediments.
Reference
[1] Benner, R. and Kaiser, K. (2003). Abundance of
amino sugars and peptidoglycan in marine particulate
and dissolved organic matter. Limnology and
Oceanography, 48(1):118–128.
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