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

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 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\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.

Environmental perturbation effects on baseline δ15N values and zooplankton trophic flexibility in the southern California Current Ecosystem

“…are known to be omnivores that opportunistically consume protozoo-and metazoan plankton (Fessenden and Cowles 1994;Ohman and Runge 1994). Trophic 15 N enrichment of protozoans with respect to phytoplankton could be low when their production, grazing, and nutrient re-mineralization is tightly coupled (Hoch et al 1996), and Hannides et al (2009) suggested this as the cause of the apparent invisibility of protozoans in TP estimates of zooplankton from the oligotrophic subtropical Pacific. This factor could explain the lack of an apparent trophic switch if we were considering protozoan prey alone.…”

Protozoans as a food source for Antarctic krill, Euphausia superba: Complementary insights from stomach content, fatty acids, and stable isotopes

“…However, distinction between various heterotrophic food sources is not possible from d 15 N values. Excretion of isotopically light nitrogen, the main process causing food web enrichment in d 15 N, is common to metazoans (Wada et al 1987) and protozoans (Hoch et al 1996). Similar to zooplankton, exponentially growing flagellates and ciliates show a 3% to 5% difference in d 15 N between excreted ammonium and biomass (Hoch et al 1996).…”

Organic matter pathways to zooplankton and benthos under pack ice in late winter and open water in late summer in the north-central Bering Sea

“…Furthermore, under such conditions, nitrogen required for bacterial growth is typically from DIN, with little turnover of DON (Kirchman et al 1991, Amon & Benner 1994. In situations of limited substrate availability, grazing protists excrete DIN and some DON that have lower δ 15 N than the original substrate, and bacteria assimilate this lighter DIN and DON (Hoch et al 1996). By these mechanisms, part of the very large pool of dissolved organic matter can be made available to macroinvertebrates via flocculation of bacteria or their exudates (Alber & Valiela 1995).…”