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 18O and 15N manifested during assimilation of NO3− in marine macro-benthic algae, two species (Ulva sp. and Agardhiella sp.) have been grown in a wide range of NO3− concentrations (2–500 μM). Two types of experiments were performed. The first was one in which the concentration of the NO3− 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 NO3− concentration at a low steady-state value by means of a syringe pump. The effective fractionation during the assimilation of the NO3− was determined by measuring the δ15N of both the (i) new algal growth and (ii) residual NO3− 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 NO3− 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 δ18O and δ15N values of NO3− in the residual solution were correlated, the slope of relationship also varied with respect to NO3− 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 NO3−. At higher NO3− concentrations (>10–50 μM), fractionation during assimilation will lead to δ15N values in algal biomass lower than the ambient NO3− and 15N enrichments in the residual NO3−.