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

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 D O N , calculated by mass balance, was very depleted in 15N relative to NIi,' and biomass dunng exponential and early stationary growth when D O N and dissol 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.