Concentration dependent nitrogen isotope fractionation during ammonium uptake by phytoplankton under an algal bloom condition in the Danshuei estuary, northern Taiwan

• First field evidence of variable N-15/N-14 fractionation during algal NH4+ uptake
• 14N was favored over 15N during uptake, causing drop of δ15N up to 20‰ in algae.
• Isotope effect maximizing at [NH4+] around 100 μM likely due to membrane diffusion of NH3
• Findings useful in understanding δ15N of bio-organics in highly eutrophic waters

In July 2009 an intense algal bloom with maximum Chl-a concentration reaching 166 μg L− 1 occurred in the highly eutrophic Danshuei River estuary, which receives waste discharges from the densely populated Taipei metropolitan area in northern Taiwan. The estuary is often burdened with very high concentration of ammonium (up to ~ 550 μM), which dominates the dissolved inorganic nitrogen species in the estuary. The observed δ15N values of particulate nitrogen ranged from − 8.6‰ to 0.2‰, and the δ15N values of coexisting ammonium ranged from 4.6 to 11.9‰. Notably the offset between δ15NPN and δ15NNH4 (Δδ15N) showed significant correlation with ammonium concentration. The ε-values were calculated to be between − 4.7 and − 16.4‰. The range overlaps with that of previous estimates (− 6.5 to − 18.1‰) based on field observations. We plotted all field observed ε-values vs. corresponding ammonium concentrations and found a trend similar to that previously observed for marine bacterium, Vibrio harveyi, in laboratory cultures. Thus, we constructed a concentration dependent curve of the ε-value for ammonium uptake by phytoplankton in natural waters. The curve shows the maximum magnitude of ε-value (− 20‰) at ammonium concentration around 100 μM with decreasing isotope effect on both sides; at lower concentrations, the ε-value diminishes to zero; at higher concentrations, it slopes gradually towards an asymptotic value around − 2‰. More than half of the ε-values derived from laboratory cultures of diatoms also fall on this curve. However, a few culture-based ε-values fall on another curve with similar pattern but considerably larger maximum magnitude. The maximum isotope effect is probably attributed to the cumulative isotope effects from ammonium deprotonation and the subsequent membrane diffusion of ammonia. This study provides the first field observed evidence of concentration dependent nitrogen isotope fractionation during ammonium uptake by phytoplankton and reconciles partially the disparity between estimates from field observations and from laboratory cultures.