Isotope fractionation during ammonium uptake by marine microbial assemblages

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Abstract
To determine the nitrogen isotope fractionation associated with NH4 + uptake by bacteria in seawater, the stable nitrogen isotope ratio [reported as δ15N (%o)] of NH4 + and particulate organic nitrogen (PON) were measured during the growth of three different natural bacterial assemblages. The δ15N of NH4 + and bacteria size particles (4+. Isotope fractionation during NH4 + uptake (∊u) was calculated during incubation periods with decreasing NH4 + concentrations and increasing δ15N of NH4+. For the three experiments, the values of ∊u calculated by the Rayleigh equation were ‐20, ‐8, and ‐5%o when net rates of NH4 + uptake were 16, 36, and 240 nM h‐1, respectively. These estimates of isotope fractionation are much larger than observed with pure cultures at similar NH4 + concentrations. The large estimates of ∊ufor natural assemblages of bacteria, relative to pure culture studies, may be partly biased by regeneration of NH4 + and isotope effects associated with this process. However, even when regeneration effects are accounted for, the apparent ∊u is still much larger (‐10 ± 1.7% ± SE, n = 4) than anticipated from pure culture studies. These data suggest that the demand for NH4 + relative to the ambient NH4 + concentration determines nitrogen isotope fractionation during uptake of NH4 + by bacteria.