ABSTRACT: Variation in the distribution of stable nitrogen isotopes among different size classes of particulate organic nitrogen (PON) in aquatic environments may be partly explained by isotope effects associated with regeneration of NH₄⁺ by the microbial food web. Protists fed the marine bacterium Vibrio natriegens were grown in batch and continuous culture to define the isotope discrimination between NH₄⁺, microbial biomass, and dissolved organic nitrogen (DON) for culture systems closed or open to exogenous nutrient inputs. In batch cultures of a flagellate Pseudobodo sp. and a scuticociliate Uronema sp., the nitrogen isotope discrimination between biomass and NH₄⁺ was 3 to 5 o/oo during exponential growth of protists corresponding to the highest rates of biomass-specific NH₄⁺ release. The delta¹⁵N of DON, calculated by mass balance, was very depleted in ¹⁵N relative to NH₄⁺ and biomass during exponential and early stationary growth when DON and dissolved free amino acid (DFAA) concentrations increased. In contrast to batch culture results, for continuous culture of the flagellate the isotope discrimination between biomass and NH₄⁺ and the change in the calculated delta¹⁵N of DON were small: 1 to 2 o/oo. Based on rates of thymidine incorporation and turnover of DFAA, protist excretion and bacterial uptake of DON were less coupled for exponentially growing protists fed starved bacteria in batch cultures than for the flagellate fed active bacteria growing in a 2-stage continuous culture. Coupling between release and utilization of DON, as well as isotope effects associated with protist metabolism, are proposed to constrain the nitrogen isotope dynamics of NH₄⁺ suspended PON, and DON within marine ecosystems, especially those environments dominated by regenerated primary production.

KEY WORDS: Stable nitrogen isotopes . Protist grazing . Marine bacteria . Ammonium regeneration . Dissolved organic nitrogen

Published in MEPS Vol. 132 (1996) on February 29
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1996