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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 NH,* by the microbial food web. Protists fed the marine bacterium Vibno natl-ieg~ns were grown in batch and continuous culture to define the isotope discrimination between NH,', microbial biornass, 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 scut~cocil~ate Uronema sp., the nitrogen isotope discriminat~on between b~omass and NII.,' was 3 to 5 "6, during exponential growth of protists corresponding to the highest rates of b~omass-specific NH,' release The 6"N of DON, calculated by mass balance, was very depleted in 15N relative to NIi,' and biomass dunng exponential and early stationary growth when DON and dissol\ied free amino acid (DFAA) concentrations increased. In contrast to batch culture results, for continuous culture of the flagellate the Isotope discrimination between biomass and NH,' and the change in the calculated 6I5N of DON were small. 1 to 2 X,. 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 NH,', suspended PON, and DON within marine ecosystems, especially those environments dominated by regenerated primary production.

Stable isotope dynamics of nitrogen recycled during interactions among marine bacteria and protists