Carbon cycling in a 3-member food web containing a diatom (Phaeodactylurn tricornutum), bacteria, and a herbivorous/bacterivorous microflagellate (Paraphysornonas jmperforata) was examined. Ingestion of prey by the microflagellate was the primary mechanism for remineralization of particulate organic material. Approximately 65 % of the particulate organic carbon (POC) initially present was lost over the course of the 8 d experiments in cultures containing microflagellates. No significant increase in remineralization was observed when bacteria were present. Bacteria were responsible for the uptake of dissolved organic carbon (DOC), but their overall contribution to carbon cycling was small relative to that of the microflagellate. Microflagellates incorporated diatom and bacterial biomass with equal efficiency (44 %) during exponential growth. Only 10 % of the POC ingested by microflagellates was released as DOC while 10 % was released as egested POC. The relatively high weight-specific respiration rate of the microflagellates (X = 2.67 X nl O2 h-') coupled with their relatively small release of DOC indicates that herbivory by heterotrophic microflagellates may be a major mechanism for the regeneration of nutrients from living phytoplankton which circumvents bacterial decomposition.

As part of a series of grazing experiments in batch cultures, we found that the phagotrophlc microflagellate Paraphysomonas imperforata, while grazing on the diatom Phaeodactylum tn'cornutum or bacteria, was responsible for the bulk of phosphorus regeneration. Regeneration of soluble reactive phosphorus (SRP) and dissolved organic phosphorus (DOP) was negligible in control cultures of the diatom alone, bacteria alone, or the 2 microbes together. When the m~croflagellate grazed on prey organisms that had been precultured with excess nutrients or under nitrogen limitation there was considerable regeneration of total dissolved phosphorus (TDP = SRP + DOP). Rates of TDP regeneration were greatest during exponential growth of the microflagellate and then decreased through the transition and stationary phases. Overall, up to 70 % of the phosphorus initially incorporated into prey biomass was regenerated through the stationary phase. Total excretion of DOP was about 15 to 20% of TDP, although DOP excretion made up a larger fraction of total phosphorus excretion for short periods during the exponential phase of growth. When the prey were phosphoruslimited virtually no TDP was excreted throughout the entire growth cycle of the microflagellate. Our results indicate that Protozoa have higher weight-specific rates of phosphorus excretion than do Metazoa. Although metabolic activity is not the sole indicator of the role Protozoa play in the nutrient regeneration process, our results, together with those from size-fractionation studies on nutrient regeneration, point toward a major role for Protozoa in pelagic waters where they constitute a large fraction of the zooplankton biomass.