Isotope fractionation associated with ammonium uptake by a marine bacterium

Abstract

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

Enrichment and characterization of ammonia-oxidizing archaea from the open ocean: phylogeny, physiology and stable isotope fractionation

“…In the later stages of growth, the reaction may become diffusion limited (R diff oR AMO ) and the isotope effect for diffusion ( 15 e diff ) would dominate. A similar mechanism has been examined for variations in expressed isotopic fractionation during NH 4 þ assimilation by bacteria (Hoch et al, 1992) and marine algae (Pennock et al, 1996), as well as CO 2 uptake by algae (Laws et al, 1997) and plants (O'Leary, 1981). Diffusion of NH 3 in aqueous solution is estimated to have an isotope effect of 20% (Hoch et al, 1992), which is similar to the late-stage 15 e NH3 values we observed.…”

“…This was particularly apparent for CN25 and CN75 enrichments grown at low [NH 4 þ ] (o20 mM), which exhibited a considerable lag phase before the commencement of ammonia oxidation. Variable e values have been observed in many other organisms, including denitrifiers (Granger et al, 2008), methane oxidizers (Templeton et al, 2006), sulfate reducers (Habicht et al, 2005) and nitrite oxidizers (Casciotti, 2009;Buchwald and Casciotti, 2010), and during assimilation of NH 4 þ by heterotrophic bacteria (Hoch et al, 1992) but has not previously been reported in ammonia-oxidizing organisms. We explore two hypotheses that could explain a large apparent isotope effect at the early stages of growth (Figure 6).…”

“…There is likely to be some loss of NH 4 þ because of uptake for anabolic metabolism by AOA and the bacteria in the enrichments, but this sink should be small and consistent across experiments. This process would alter observed 15 e NH3 values in proportion to the amount of NH 4 þ assimilated and the isotope effect for NH 4 þ assimilation (4-27%; (Hoch et al, 1992)). However, we emphasize that this effect is likely to be small and note that relatively high estimates of 15 e NH3 , not low ones, were observed where recovery was low (Table 2).…”

The evolution of Earth's biogeochemical nitrogen cycle

“…Ammonification can be abiotic and is just included for completeness. Metal catalysts and isotopic fractionations are taken from the literature (Brunner et al, 2013;Buick, 2007a;Casciotti, 2009;Godfrey and Glass, 2011;Hoch et al, 1992;McCready et al, 1983;Nishizawa et al, 2014;Pennock et al, 1996;Waser et al, 1998;Zerkle et al, 2008;Zhang et al, 2014 Tian et al (2011) and Zahnle (1986), 10. Schoonen & Xu (2001), 11.…”

“…Non-quantitative NH 4 + assimilation imparts a fractionation of up to -27‰ when [NH 4 + ] > 20 μM, but the fractionation decreases to -4‰ at lower concentrations (Hoch et al, 1992;Pennock et al, 1996;Waser et al, 1998). Under NH 4 + replete conditions one may thus expect light biomass in underlying sediments (i.e.…”