The fractionation of nitrogen and oxygen isotopes in macroalgae during the assimilation of nitrate

Swart, P. K.; Evans, S.; Capo, T.; Altabet, M. A.

In order to determine and understand the stable isotope fractionation of 18O and 15N manifested during assimilation of NO 3 in marine macro-benthic algae, two species ( Ulva sp. and Agardhiella sp.) have been grown in a wide range of NO 3 concentrations (2–500 μM). Two types of experiments were performed. The first was one in which the concentration of the NO 3 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 3 concentration at a low steady-state value by means of a syringe pump. The effective fractionation during the assimilation of the NO 3 was determined by measuring the δ 15N of both the (i) new algal growth and (ii) residual NO 3 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 3 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 18O and δ 15N values of NO 3 in the residual solution were correlated, the slope of relationship also varied with respect to NO 3 concentration, with slopes of greater than unity at low concentration. These results suggest shifts in the dominant fractionation mechanism of 15N and 18O between concentrations of 1 and 10 μM NO 3. At higher NO 3 concentrations (>10–50 μM), fractionation during assimilation will lead to δ 15N values in algal biomass lower than the ambient NO 3 and 15N enrichments in the residual NO 3.

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Swart, P. K. / Evans, S. / Capo, T. / et al: The fractionation of nitrogen and oxygen isotopes in macroalgae during the assimilation of nitrate. 2014. Copernicus Publications.

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