Bacteria can account for a large fraction of total NH,' uptake in both pelagic and benthic marine environments. 15N natural abundance was examined to understand the effect of NH,' uptake by bacteria on nitrogen isotope biogeochemistry. Isotope fractionation (E) for NH,' uptake by the marine bacterium Vibrio harveyi changed from -4 to -277~ when cells were grown on 23-l 82 PM NH,+ and then from -27 to -14Ya when the NH;' concentration increased to 23.3 mM. Changes in fractionation correlated with a switch in the pathway of NH,' uptake from membrane diffusion of NH, and assimilation catalyzed by glutamate dehydrogcnase at millimolar NH,' to active ammonium transport (Amt) and assimilation catalyzed by glutamine synthetase (GS) at micromolar NH,+. This transition occurred between 0.1 and I mM NH,+. Within this concentration range, cellular N demand was no longer supported by Fickian diffusion of NH:, and Amt activity increased. The isotope fractionation of whole cells with the highest GS activity (E = -4%) and that measured for the GS-catalyzed reaction in vitro (E = -8%; pH = 7

Uptake of NH4+ by heterotrophic bacteria and the relative importance of NH,+ and dissolved free amino acids (DFAA) as nitrogen sources for bacterial production were examined in the Delaware estuary and adjacent coastal waters during 1988 and 1990. Although total uptake of NH,+ and bacterial production were -4-fold higher in 1988 than in 1990, percent NH,+ uptake by bacteria in the upper and lower estuary was similar for both years. Bacterial uptake rates were highest at the mouth of the bay in summer and represented lo-25% of total uptake of NH,+. Less than 5% of NHd+ uptake was by bacteria at salinities <207& In contrast to NH4+ uptake, DFAA uptake was greatest in the upper estuary and often exceeded nitrogen requirements for bacterial growth. Bacteria accounted for 15-35% of total NH,+ uptake at coastal and offshore stations in 1990. About 50% of bacterial nitrogen demand was supported by NH4+ at the mouth of the bay and in coastal waters during summer, when DFAA concentrations were generally lowest. Although DFAA concentration and uptake did not explain all variability, they appeared to explain large-scale features in NH,+ uptake by heterotrophic bacteria. Ammonium uptake by bacteria was lowest in the estuary, where DFAA concentrations and uptake were highest; at an offshore station, where DFAA concentrations and uptake were low, relative NH,+ uptake by bacteria was highest. These and other results suggest that NH,+ uptake by bacteria is relatively high in oligotrophic water and low in eutrophic systems, which has important implications on the role of heterotrophic bacteria in the N cycling of marine environments.

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