Gromiaohio!oxy Journal, Volume 12. pp. 113-127 () :49·(J45l!94 S IOJX) + Pdmcd in the UK. All rights reserved. Copyright © 1994 Tay10f & Franci, Isotope Fractionation During Ammonium Uptake by Marine Microbial Assemblages MATTHEW P. HOCH University of Delaware College of Marine Studies Lewes, Delaware, USA MARILYN L. FOGEL Carnegie Institution of Washington Geophysical Laboratory Washington, DC, USA DAVID L. KIRCHMAN University of Delaware College of Marine Studies Lewes, Delaware. USA To determine the nitrogen isotope fractionation associated with NH/ uptake by bac· teria ill seawater, the stable nitrogen isotope ratio I reported as o 15 N ('lee)] of NIl./ and particulate organic nitrogen (PON) were measured during the growth of three different natural bacterial assemblages. The O'IW of Nil: and bacteria size particles «0.8 pm) changed by as much as 8%0 during 100 h of inCllbation with 2-5 pM Nil" +. isotope fractionation during Nil" + uptake (ou) was calculated dining incubation peri- ods with decreasing Nil; concentrations and increasing Ol5N of Nil" +. For the three experiments, the values of 0u calculated by the Rayleigh equation were -20, -8, and -5%0 when net rates of NH; uptake were i6, 36, and 240 nM h- I , respectively. These estimates of isotope fractionation are much larger than observed with pure cultures at similar NH; concentrations. The large estimates of fu for natural assemblages of bacteria, relative to pure culture studies, may be partly biased by regeneration of Nil; and isotope effects associated with this process. However, even when rp"pnprrl_ tion effects are accountedfor, the apparent is still much larger (-10 ± 1. ± SE, n = 4) than anticipated from pure CIIlwre These data suggest that the demand Received 27 October 1993; accepted 25 lvlay 1994. We thank R. G. Keil and K. Verde for their technical help; 1. H. Sharp for the use of his mass spectrometer for analyses of 15N-tracer addition samples; two anonymous reviewers for their con- structive comments; and L. A. Cifuentes and R. A. Snyder for discussion of an early draft. This re- search was supported in part by the United States Nmional Science Foundation (NSF) 9103842 to D. L. Kirchman, NSF OCE-8601616 to M. L. Fogel, and a predoctoral fellowship from the Carnegie Institution of Washington, Geophysical Laboratory. Present address for Matthew P. Hoch is Texas A&M University, Oceanography Department. % U.S. EPA-GBERL, Gulf Breeze, FL 32561, USA. Address correspondence to Dr. Matthew P. Hoch, Texas A&M University, Oceanography De- partment, % USA-GBERL. Gulf Breeze, FL 32561, USA. 113