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-.
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics
- Earth-Surface Processes