Estimation of ammonium regeneration efficiencies associated with bacterivory in pelagic food webs via a 15N tracer method

Phagotrophic protists are major components of pelagic food webs,both as consumers of bacterial and phytoplankton cells, andas regenerators of inorganic nutrients. In this study, we estimatedthe efficiency of ammonium regeneration by protists feedingon bacteria within natural plank-tonic assemblages, using a¹⁵N tracer method, in which the excretion of ¹⁵N-labeled ammoniumdue to grazing on ¹⁵N pre-labeled bacteria was followed overtime. We tested this approach in experiments based on the additionof heat-killed ¹⁵N-labeled bacteria to laboratory cultures andto samples of coastal seawater. During two experiments, variationin abundance of bacterivores and bacterioplankton resulted innon-constant grazing rates. Deterministic computer models thatused abundance of bacteria and protists as variables were developedto estimate best-fit values of grazing mortality (g, h^–1)and of ammonium regeneration efficiency (R_E, fraction of theinitial ¹⁵N label in added bacteria which is released as ammonium).Estimated ammonium R_E were 0.30–0.35 for one trophic linksystems with both a monospecific culture and a mixed speciesassemblage of bacterivorous flagellates. R_E was higher for multi-trophicstep food webs: 0.60 for 5 µm pre-screened coastal seawaterand 0.90 for whole coastal seawater.