Isotope fractionation associated with ammonium uptake by a marine bacterium

Bacteria can account for a large fraction of total NH 4 + uptake in both pelagic and benthic marine environments. 15 N natural abundance was examined to understand the effect of NH 4 uptake by bacteria on nitrogen isotope biogeochemistry. Isotope fractionation ( ε ) for NH 4 + uptake by the marine bacterium Vibrio harveyi changed from −4 to −27‰ when cells were grown on 23–182 µ M NH 4 + and then from −27 to −14‰ when the NH 4 + concentration increased to 23.3 mM. Changes in fractionation correlated with a switch in the pathway of NH 4 + uptake from membrane diffusion of NH 3 and assimilation catalyzed by glutamate dehydrogenase at millimolar NH 4 + to active ammonium transport (Amt) and assimilation catalyzed by glutamine synthetase (GS) at micromolar NH 4 + . This transition occurred between 0.1 and 1 mM NH 4 + . Within this concentration range, cellular N demand was no longer supported by Fickian diffusion of NH 3 and Amt activity increased. The isotope fractionation of whole cells with the highest GS activity ( ε = −4‰) and that measured for the GS‐catalyzed reaction in vitro ( ε = −8%; pH = 7.1) differed by 5‰ , which suggests that the GS reaction was not the rate‐limiting step during NH 4 + uptake. Isotope fractionation associated with NH 4 + uptake by marine bacteria appears to depend on the membrane transport mechanism and the ammonium assimilation enzymes. Depending on NH 4 + concentrations, marine bacteria using NH 4 + for growth may partly account for variation in the isotope composition of NH 4 + and particulate organic N in the water column and sediments of marine environments.