25th International Meeting on Organic Geochemistry IMOG 2011

P-405
A novel tool for food web study: compound-specific stable
nitrogen isotope analysis of amino acids
Yoshito Chikaraishi, Nanako Ogawa, Naohiko Ohkouchi
Japan Agency for Marine-Earth Science and Technology,, Yokosuka, Japan (corresponding
author:ychikaraishi@jamstec.go.jp)
Knowledge of the trophic position (TP) of organisms
in food webs allows understanding of biomass flow
and trophic linkages in complex networks of
ecosystems. Compound-specific stable isotope
analysis (CSIA) of amino acids is a new method with
that enables TP estimates of organisms in food webs
[Fig. 1; 1-4]. This approach is based on contrasting
isotopic fractionation during metabolic processes
between two common amino acids: glutamic acid
(Glu) shows significant 15 N-enrichment of +8.0‰
during reactions (transamination, deamination) that
cleaves the carbon-nitrogen bond, whereas
phenylalanine (Phe) shows little change in δ 15 N
values (by +0.4‰) during conversion to tyrosine that
neither forms nor cleaves the carbon-nitrogen bond
[3]. In the previous studies [3,4] we established based
a number of natural and laboratory grown organisms
a general equation for estimating the TP of organisms
by CSIA of amino acids:
TP = [(δ 15 NGlu – δ 15 NPhe + β)/7.6 ] + 1
where β represents the isotopic difference between
Glu and Phe in primary producers (–3.4‰ for aquatic
cyanobacteria and algae, +8.4‰ for terrestrial C3,
and –0.4‰ for terrestrial C4 plants). This method has
three key advantages.
1. The TP is estimated based on the δ 15 N of Glu and
Phe only from the target organism. Consequently,
unlike the bulk isotopic methods, it does not require
characterization of the δ 15 N values of primary
producers or baseline consumers.
2. A small uncertainty is expected. The fractional TP
standard deviation (1ζ) is only 0.12 and 0.20 units
for aquatic and terrestrial organisms, respectively.
3. A small sample size (nanomolar amount of N) is
required for CSIA of amino acids by GC/C/IRMS.
These advantages mean that CSIA overcomes a
number of concerns associated with the bulk isotope
analysis; consequently, it has been applied in
elucidating the TP of organisms in recent studies [5].
In the presentation, we review amino acid methods
and show its application to various natural organisms
in aquatic marine and freshwater as well as terrestrial
food webs.
Fig.1. The TP (a) is clearly proportionate to difference
in δ 15 N between Glu and Phe (b).
References
[1] McClelland & Montoya (2002) Ecology 83, 2173-
2180.
[2] Popp et al. (2007) In Stable isotopes as indicators
of ecological change. Academic Press. pp 173-190.
[3] Chikaraishi et al. (2009) Limnol. Oceanogr.: Meth
7, 740-750.
[4] Chikaraishi et al. (2010) In Earth, Life, and
Isotopes. Kyoto University press. pp. 37-51.
[5] Hannides et al. (2009) Limnol. Oceanogr. 54, 50-
61.
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