Isotopic fractionation of ammonium and nitrate during uptake by Skeletonema costatum: Implications for δ15N dynamics under bloom conditions

Abstract: Isotopic fractionation of ammonium (NH,+ ) and nitrate (NO,-) during uptake by phytoplankton was examined in batch culture experiments with the diatom SkeZetonema costatum under nitrogen-enriched conditions (5-100 PM). The fractionation factor (E) for NOJ- uptake by Skeletonema was -9.Of0.7%0 and was concentration-independent. For NH,+, E was more variable and dependent on ambient NH,+ concentration. For NH4+ concentration ranges of 100-50, 50-20, and 20-5 PM, E was -24.6k5.5, -27.2-+ 1.6, and -7.8+3.0%. In these cultures, isotopic fractionation by phytoplankton caused variations in 615N of up to 50?& for NH4+, 1 ~%XI for NO,-, and 25% for particulate N. Similar variability in the 615N of both dissolved inorganic and particulate organic N pools should be expected during phytoplankton blooms in nature. As a result, phytoplankton-mediated isotopic variability must be considered when isotopic data are used to examine biogeochemical and physical processing of organic matter in marine ecosystems, particularly when biosynthesis and loss processes are decoupled in either space or time during bloom conditions