25th International Meeting on Organic Geochemistry IMOG 2011

P-237
Nature of C29 sterols in the high-molecular weight dissolved
organic matter (HMW-DOM) from a freshwater lake
Kazuo Fukushima 1 , Koji Takahashi 1 , Yasuko Yoshiyama 1 , Yoshito Chikaraishi 2
1 Shinshu University, Matsumoto, Japan, 2 JAMSTEC-IFREE, Yokosuka, Japan (corresponding
author:kfukush@shinshu-u.ac.jp)
In our previous works, HMW-DOM (High-Molecular
Weight Dissolved Organic matter) were quantitatively
separated from some freshwater and blackish water
lakes in Japan, by using a Tangential-flow
ultrafiltration apparatus (TFF) [1,2]. The gross
chemical nature of the HMW-DOM as revealed by
bulk carbon (� 13 C) and nitrogen (� 15 N) isotope
composition, and fatty acid composition in the HMW-
DOM suggested that organic matter produced in-situ
within the lake water primarily by phytoplankton and
bacteria may contribute much to the formation of
HMW-DOM.
On the contrary, 24-ethyl-cholesta-5,22-diene-3�-ol
(stigmasterol) predominated in the neutral lipid class
of HMW-DOM and its abundance decreased
considerably from summer to winter. Under these
circumstances, compound-specific carbon and
hydrogen isotope compositions were examined to
further discriminate the sources of sterols in the
HMW-DOM from a freshwater Lake Kizaki (water
depth 29m) water, Japan. Water samples (100L) were
collected in May-Dec 2007 from around 10-15m depth,
which corresponds to the lowest layer of the
thermocline developing in summer seasons.
The technique used for condensation and isolation
of solid powder HMW-DOM was TFF (Millipore Ltd.,
Pellicon Lab Cassette, 5 ft 2 ) followed by classic
stirring ultrafiltration (Toyo Ltd., 9 cm�), both equipped
with cellulose membrane filter of nominal molecular
cut-off of 1 kDa, and finally lyophilization. The solid
HMW-DOM was directly saponified by heating in
alkaline aqueous (5%) methanol (1M KOH).
Sterols/alcohol fraction was purified by column
chromatography and quantitated by GC and GC-MS.
Stable isotope ratios, � 13 C and �D of individual sterols,
were determined by GC-IRMS, Agilent 6890N
coupled to a Thermo Fischer Deltaplus XP
(Chikaraishi et al., 2005).
Stigmasterol comprises only less than 5% in the Lake
Kizaki particulate matter (PM) and surface sediment,
whereas in HMW-DOM, it amounted to 58-63% of the
total sterols (0.83-0.88 of 1.3-1.5 �g/L) in summer and
20% (0.11 of 0.55 �g/L) in winter. Such large
variation presumably indicates that both autochtonous
and allochthonous biota should participate in the
occurrence of stigmasterol in the lake water.
Meanwhile, abundance of another C29 sterol, 24-ethylcholest-5-en-3�-ol
(�-sitosterol) was almost constant
at 0.25 �g/L in all samples. It indicates that �sitosterol
may be solely derived from terrestrial plants,
irrespective to the biological production within the lake.
Evidences supporting the assumption were
obtained by stable carbon and hydrogen isotope
analyses. Both terrestrial �-sitosterol and algal
brassicasterol fell in the narrow ranges, respectively,
while stigmasterol showed a meaningful seasonal
variation (Fig.1).
Fig. 1. � 13 C-�D plots of stigmasterol in HMW-DOM and
sediment
References
1) Yoshiyama et al. (2003) Res. Org. Geochem. 18,
37-45 (in Japanese)
2) Takahashi et al.(2007) IMOG23, P92.
3) Chikaraishi et al. (2005) Limnol. Oceanogr. 50,
1763-1770.
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