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| Titel |
The Fractionation of Nitrogen Isotopes in Macroalgae during the Assimilation of Nitrate and Ammonium as a Function of Concentration |
| VerfasserIn |
Peter Swart, Sam Evans, Mark Altabet |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250079906
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| Zusammenfassung |
| In order to determine the amount of fractionation of 15N and 18O during the assimilation of
NO3- in marine macro-benthic algae, two species (Ulva sp. and Agardhiella sp.) were grown
in a wide range of concentrations of NO3- (2-500 μM). Two types of experiments were
performed, one in which the concentration of NO3- was allowed to decrease as it was
assimilated by the algae over a 24 hours period and the water and nutrients then restored to
the original concentrations and a second set in which the concentration was maintained at a
low steady state value by means of a syringe pump. The effective fractionation factor during
assimilation was determined by measuring the δ15N of the (i) new algal growth produced,
and (ii) measuring the δ15N and δ18O of the NO3- in the free drift experiments which were
left for 0, 12, 24, and 48 hours. The fractionation of 15N and 18O was modeled using a
Rayleigh distillation model. The results indicate that the fractionation factor during
assimilation for NO3- is dependent upon the concentration of NO3- in both species of
algae and approaches unity at concentrations of less than 10 μM. The change in the
fractionation factor with respect to concentration is the greatest at lower concentrations
(1-10 μM). There is also evidence that the fractionation factor falls below unity
at very low concentrations. Determinations of the assimilation factor made using
the δ15N of the NO3- and the solid algal material provided statistically the same
result.
At normal marine concentrations of NO3-, fractionation during assimilation can be
considered to be negligible. However, at higher concentrations, fractionation during
assimilation will lead to isotopic enrichment of the δ15N of the NO3- pool regardless of the
δ15N of the source. In addition, to changing the relationship in fractionation relative to
concentration, the relationship between the δ15N and δ18O also varies, with values close to
unity at higher concentrations and decreasing at lower concentrations. These results
suggest different fractionation mechanisms at high and low NO3- concentrations. |
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