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

[摘要] In order to determine and understand the stable isotope fractionation of¹⁸O and ¹⁵N manifested during assimilation of NO₃⁻ inmarine macro-benthic algae, two species (Ulva sp. andAgardhiella sp.) have been grown in a wide range of NO₃⁻concentrations (2–500 μM). Two types of experiments wereperformed. The first was one in which the concentration of the NO₃⁻was allowed to drift downward as it was assimilated by the algae, between 24hour replacements of media. These experiments proceeded for periods ofbetween 7 and 10 days. A second set of experiments maintained theNO₃⁻ concentration at a low steady-state value by means of a syringepump. The effective fractionation during the assimilation of the NO₃⁻was determined by measuring the δ¹⁵N of both the (i) new algalgrowth and (ii) residual NO₃⁻ in the free-drift experiments after 0,12, 24 and 48 h. Modelling these data show that the fractionation duringassimilation is dependent upon the concentration of NO₃⁻ and iseffectively 0 at concentrations of less than ~2 μM. Thechange in the fractionation with respect to concentration is the greatest atlower concentrations (2–10 μM). The fractionation stablizesbetween 4 and 6‰ at concentrations of between 50 and 500 μM.Although the δ¹⁸O and δ¹⁵N values of NO₃⁻ in theresidual solution were correlated, the slope of relationship also varied withrespect to NO₃⁻ concentration, with slopes of greater than unity atlow concentration. These results suggest shifts in the dominant fractionationmechanism of ¹⁵N and ¹⁸O between concentrations of 1 and10 μM NO₃⁻. At higher NO₃⁻ concentrations(>10–50 μM), fractionation during assimilation will lead toδ¹⁵N values in algal biomass lower than the ambient NO₃⁻and ¹⁵N enrichments in the residual NO₃⁻.

[学科分类] 地球化学与岩石