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 O-18 and N-15 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 mu M). 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 delta N-15 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 similar to 2 mu M. The change in the fractionation with respect to concentration is the greatest at lower concentrations (2-10 mu M). The fractionation stablizes between 4 and 6% at concentrations of between 50 and 500 mu M. Although the delta O-18 and delta N-15 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 N-15 and O-18 between concentrations of 1 and 10 mu M NO3-. At higher NO3- concentrations (> 10-50 mu M), fractionation during assimilation will lead to delta N-15 values in algal biomass lower than the ambient NO3- and N-15 enrichments in the residual NO3-.