STABLE-ISOTOPE DYNAMICS OF NITROGEN RECYCLED DURING INTERACTIONS AMONG MARINE-BACTERIA AND PROTISTS

Variation in the distribution of stable nitrogen isotopes among differ ent size classes of particulate organic nitrogen (PON) in aquatic envi ronments may be partly explained by isotope effects associated with re generation 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 op en to exogenous nutrient inputs. In batch cultures of a flagellate Pse udobodo sp. and a scuticociliate Uronema sp., the nitrogen isotope dis crimination between biomass and NH4+ was 3 to 5 % during exponential g rowth of protists corresponding to the highest rates of biomass-specif ic NH4+ release. The delta(15)N of DON, calculated by mass balance, wa s very depleted in N-15 relative to NH4+ and biomass during exponentia l 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 be tween biomass and NH4+ and the change in the calculated delta(15)N Of DON were small: 1 to 2 %. Based on rates of thymidine incorporation an d 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 utilizat ion of DON, as well as isotope effects associated with protist metabol ism, are proposed to constrain the nitrogen isotope dynamics of NH4+, suspended PON, and DON within marine ecosystems, especially those envi ronments dominated by regenerated primary production.