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 h replacements of media. These experiments proceeded for periods of between seven and ten 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. Fitting models to these data show that the fractionation during assimilation is dependent upon the concentration of NO3− and is effectively zero at concentrations of less than 1 μM. The change in the fractionation with respect to concentration is the greatest at lower concentrations (1–10 μM). The fractionation determined using the δ15N of the NO3− or the solid algal material provided statistically the same result. Therefore, at typical marine concentrations of NO3−, fractionation during assimilation can probably be considered to be negligible. Although the δ18O and δ15N of NO3− in the residual solution were correlated, the slope of the relationship varied with NO3− concentration, with slopes of greater than unity at low concentration. These results suggest shifts in the dominant fractionation mechanism between 1 and 10 μM NO3−. At typical marine concentrations of NO3−, fractionation during assimilation can be considered to be negligible. However, at higher concentrations, fractionation during assimilation will lead to both δ15N values for algal biomass lower than the NO3− source, but also 15N enrichments in the residual NO3−.