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

In July 2009 an intense algal bloom with maximum Chl-a concentration reaching 166 mu g 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 similar to 550 mu M), which dominates the dissolved inorganic nitrogen species in the estuary. The observed delta N-15 values of particulate nitrogen ranged from -8.6 parts per thousand. to 0.parts per thousand 2., and the delta N-15 values of coexisting ammonium ranged from 4.6 to 11.9 parts per thousand. Notably the offset between delta N-15(PN) and delta N-15(NH4) (Delta delta N-15) showed significant correlation with ammonium concentration. The e-values were calculated to be between -4.7 and -16.4 parts per thousand. The range overlaps with that of previous estimates (-6.5 to -18.1 parts per thousand) based on field observations. We plotted all field observed epsilon-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 epsilon-value for ammonium uptake by phytoplankton in natural waters. The curve shows the maximum magnitude of e-value (-20 parts per thousand) at ammonium concentration around 100 mu M with decreasing isotope effect on both sides; at lower concentrations, the epsilon-value diminishes to zero; at higher concentrations, it slopes gradually towards an asymptotic value around -2 parts per thousand. More than half of the epsilon-values derived from laboratory cultures of diatoms also fall on this curve. However, a few culture-based epsilon-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. (C) 2013 Elsevier B.V. All rights reserved.