ISOTOPE FRACTIONATION DURING AMMONIUM UPTAKE BY MARINE MICROBIAL ASSEMBLAGES

To determine the nitrogen isotope fractionation associated with NH4+ uptake by bacteria in seawater, the stable nitrogen isotope ratio [reported as deltaN-15 (parts per thousand)] of NH4+ and particulate organic nitrogen (PON) were measured during the growth of three different natural bacterial assemblages. The deltaN-15 of NH4+ and bacteria size particles (<0.8 mum) changed by as much as 8 parts per thousand during 100 h of incubation with 2-5 muM NH4+. Isotope fractionation during NH4+ uptake (epsilon(U)) was calculated during incubation periods with decreasing NH4+ concentrations and increasing deltaN-15 of NH4+. For the three experiments, the values of epsilon(U) calculated by the Rayleigh equation were -20, -8, and -5 parts per thousand 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 epsilon(U) for 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 epsilon(U) is still much larger (-10 +/- 1.7 parts per thousand; +/- 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 uptakes of NH4+ by bateria.