ISOTOPE FRACTIONATION ASSOCIATED WITH AMMONIUM UPTAKE BY A MARINE BACTERIUM

Bacteria can account for a large fraction of total NH2+ uptake in both pelagic and benthic marine environments. N-14 natural abundance was examined to understand the effect of NH4+ uptake by bacteria on nitrogen isotope biogeochemistry. Isotope fractionation (epsilon) for NH4+ uptake by the marine bacterium Vibrio harveyi changed from -4 to -27 parts per thousand when cells were grown on 23-182 muM NH4+ and then from -27 to -14 parts per thousand when the NH4+ concentration increased to 23.3 mM. Changes in fractionation correlated with a switch in the pathway of NH4+ uptake from membrane diffusion of NH, and assimilation catalyzed by glutamate dehydrogenase at millimolar NH4+ to active ammonium transport (Amt) and assimilation catalyzed by glutamine synthetase (GS) at micromolar NH4+. This transition occurred between 0.1 and 1 mM NH4+. Within this concentration range, cellular N demand was no longer supported by Fickian diffusion of NH3 and Amt activity increased. The isotope fractionation of whole cells with the highest GS activity (epsilon = -4 parts per thousand) and that measured for the GS-catalyzed reaction in vitro (e = -8 parts per thousand; pH = 7.1) differed by 5 parts per thousand, which suggests that the GS reaction was not the rate-limiting step during NH4+ uptake. Isotope fractionation associated with NH4+ uptake by marine bacteria appears to depend on the membrane transport mechanism and the ammonium assimilation enzymes. Depending on NH4+ concentrations, marine bacteria using NH4+ for growth may partly account for variation in the isotope composition of NH4+ and particulate organic N in the water column and sediments of marine environments.