The fractionation of nitrogen and oxygen isotopes in macroalgae during the assimilation of nitrate
In order to determine and understand the stable isotope fractionation of <sup>18</sup>O and <sup>15</sup>N manifested during assimilation of NO<sub>3</sub><sup>−</sup> in marine macro-benthic algae, two species (<i>Ulva</i> sp. an...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2014-11-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/11/6147/2014/bg-11-6147-2014.pdf |
| Summary: | In order to determine and understand the stable isotope fractionation of
<sup>18</sup>O and <sup>15</sup>N manifested during assimilation of NO<sub>3</sub><sup>−</sup> in
marine macro-benthic algae, two species (<i>Ulva</i> sp. and
<i>Agardhiella</i> sp.) have been grown in a wide range of NO<sub>3</sub><sup>−</sup>
concentrations (2–500 μM). Two types of experiments were
performed. The first was one in which the concentration of the NO<sub>3</sub><sup>−</sup>
was allowed to drift downward as it was assimilated by the algae, between 24
hour replacements of media. These experiments proceeded for periods of
between 7 and 10 days. A second set of experiments maintained the
NO<sub>3</sub><sup>−</sup> concentration at a low steady-state value by means of a syringe
pump. The effective fractionation during the assimilation of the NO<sub>3</sub><sup>−</sup>
was determined by measuring the δ<sup>15</sup>N of both the (i) new algal
growth and (ii) residual NO<sub>3</sub><sup>−</sup> in the free-drift experiments after 0,
12, 24 and 48 h. Modelling these data show that the fractionation during
assimilation is dependent upon the concentration of NO<sub>3</sub><sup>−</sup> and is
effectively 0 at concentrations of less than ~2 μM. The
change in the fractionation with respect to concentration is the greatest at
lower concentrations (2–10 μM). The fractionation stablizes
between 4 and 6‰ at concentrations of between 50 and 500 μM.
Although the δ<sup>18</sup>O and δ<sup>15</sup>N values of NO<sub>3</sub><sup>−</sup> in the
residual solution were correlated, the slope of relationship also varied with
respect to NO<sub>3</sub><sup>−</sup> concentration, with slopes of greater than unity at
low concentration. These results suggest shifts in the dominant fractionation
mechanism of <sup>15</sup>N and <sup>18</sup>O between concentrations of 1 and
10 μM NO<sub>3</sub><sup>−</sup>. At higher NO<sub>3</sub><sup>−</sup> concentrations
(>10–50 μM), fractionation during assimilation will lead to
δ<sup>15</sup>N values in algal biomass lower than the ambient NO<sub>3</sub><sup>−</sup>
and <sup>15</sup>N enrichments in the residual NO<sub>3</sub><sup>−</sup>. |
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| ISSN: | 1726-4170 1726-4189 |