25th International Meeting on Organic Geochemistry IMOG 2011

O-28
Nitrogen isotopic signatures of amino acids in microbes: culture
experiments and applications to marine sediments
Yasuhiko Yamaguchi 1,2 , Yoshinori Takano 2 , Yoshito Chikaraishi 2 , Nanako Ogawa 2 ,
Hiroyuki Imachi 2 , Hisami Suga 2 , Yusuke Yokoyama 1,2 , Naohiko Ohkouchi 2
1 Atmosphere and Ocean Research Institute (AORI), The University of Tokyo, Kashiwa, Japan, 2 Institute of
Biogeosciences, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan
(corresponding author:y-t-yamaguchi@aori.u-tokyo.ac.jp)
The microbial roles in biogeochemical cycles
remain largely unknown, mainly because of the lack
of tools to explore in situ metabolic activities of
microbes. The nitrogen isotopic composition (δ 15 N) of
individual amino acids, especially for glutamic acid
(Glu) and phenylalanine (Phe), had been
demonstrated as a promising tool for estimating the
food sources of organisms in the grazing food web
[e.g. Ref.1-5]. Applicability of this amino-acids method
to microbes or detritus food web, however, remains
uncertain, because the method has been constructed
based on the analytical results of aquatic
photoautotrophs, terrestrial higher plants, and
animals, but not chemotrophic microbes. In this study,
the δ 15 N of amino acids were investigated in 5
cultured microbes namely a fungus (Saccaromyces
cerevisiae), a bacterium (Escherichia coli) and
archaea (Sulfolobus tokodaii, Halobacterium
salinarum, and Methanothermobacter
thermautotrophicus) with controlling their nitrogen
sources.
When the microbes synthesized amino acids de
novo, the relative δ 15 N values of their amino acids
(e.g., δ 15 NGlu – δ 15 NPhe = +3.1±1.0‰, n=3) were
similar to aquatic photoautotrophs (+3.4±0.9‰, n=25;
Ref.1-5), whereas the case of the microbes
assimilated amino acids from diets, they showed 15 Nenrichment
on the amino acids (e.g., Δ 15 NGlu =
+8.2±0.8‰ and Δ 15 NPhe = +0.1 ±0.2‰, n=4) close to
that of animals (+8.0±1.1‰ and +0.4±0.4‰,
respectively, n=11; Ref.1,4,5) [Fig.1]. The results
suggest that the nitrogen-isotope fractionation
processes of amino acids are likely common among
various organisms covering the 3 domains [Fig.2] and
among various environmental conditions such as
growth temperature, pH, or salinity. Therefore, δ 15 N of
amino acids would potentially be a powerful tool to
clarify in situ microbial metabolism (amino-acids
synthesis or decomposition) and their biogeochemical
roles. In the presentation, we also show its
applications to marine sediments in various settings.
Fig.1. Conceptual diagrams of δ 15 N variation of glutamic
acids (Glu) and phenylalanine (Phe) in algae and animals
(A) [Ref.1-5], and in microbes (B) [this study]. Mean values
in each organism group are shown for relative δ 15 N values
and 15 N-enrichment factors.
Fig.2. Relative δ 15 N values and 15 N-enrichment factors of Glu
and Phe in the 3 domain including microbes, algae, and
animals. [Ref.1-5 and this study]
References
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2173-2180.
[2] McClelland, J.W. et al. (2003) Deep-Sea. Res. I 50, 849-
861
[3] McCarthy, M.D. et al. (2007) Geochim. Cosmochim. Acta
71, 4727-2744.
[4] Chikaraishi, Y. et al. (2007) Mar. Ecol. Prog. Ser. 342, 85-
90.
[5] Chikaraishi, Y. et al. (2009) Limnol. Oceanogr.: Meth 7,
740-750.
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