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.

Three large diatoms, Stephanopyxispalrneriana (Greville) Grunow, Pseudoquinardia recta von Stosch, and Navicula sp. (cell volumes 1.15 X 105 to 3.83 X lo5 pm3), were isolated from the Sargasso Sea and cultivated In batch cultures under low irradiance. Growth rates / L of each species occurred In 2 phases, a n exponential phase where varied from 0.72 to 1.12 d-' and a much slower transition phase that lasted from 3 to 6 d. We suspect that diffusion limitation of nutrient transport controlled growth rates during this latter phase. Exponential growth rates were rapid enough to meet the requirements of a bloom scenario whereby total annual new production in a locale such a s the Sargasso Sea could b e met in a single 21 d bloom. The C : N : P ratio of all 3 species was close to the Redfield proportions during exponential growth. Uncoupling between photosynthesis and nutrient acquisition was evident in 1 species, S. palmeriana, with carbon accumulation, both in the form of phytoplankton carbon and dissolved organic carbon, continuing well into the stationary phase, long after nutrients were depleted from the growth medium. In fact, 5 0 % of particulate organic carbon production occurred after the culture entered the stationary phase. Clearly there is ambiquity in the traditional definition of new production which implies that over a relatively short tlme scale a balance exists between new nitrogen entering the euphotic zone and new carbon production, assuming a Redfield stoichiometry between cellular carbon and nitrogen. The excess carbon, both particulate and dissolved, could lead to large discrepancies in estimates of new production.

In a series of laboratory and field experiments with natural and cultured marine phytoplankton the shapes of the time-dependent uptake curves of 15NH: and Hy4C0? were determined. Non-linearity in ''C uptake in laboratory cultures did not seem to be a function of steady state growth rate. However, temperature did appear to affect the degree to which 14C is incorporated in a linear manner for Thalassiosira weissflogii between 8 " and 25 "C and Dunaliella tertiolecta below 10 "C, but not for the other species investigated. Deviations from linearity for both H14C0; and 15NH: uptake in both laboratory and field experiments could be correlated with NH: depletion, especially when NH: levels at the start of the incubations dld not exceed a few tenths pg at I-'. The distribution of C and N among subcellular components was also investigated during the field experiments. The results demonstrated that by analyzing compositional changes among subcellular components a much improved estimate of the metabolic state of confined phytoplankton may be obtained. Our results demonstrate that there are severe incornpatabilities between choosing a n incubation period based solely on analwcal requirements from one based on the best representation of the time scale of physiological responses by phytoplankton. Time-course experiments allow us to understand better the short-term responses by phytoplankton, environmental influences on uptake, such as light and temperature, and to identify analytical problems or bottle effects, such as nutrient depletion.

The heterotrophic flagellate Paraphysomonas imperforata, a raptorial grazer, sustained maximum specific growth rates of ca 1.5 d-' at 20°C when fed 3 phytoplankton species of different sizes and shapes (the relatively small diatom Phaeodactylum tricornutum and haptophyte Isochrysis galbana, and the larger chlorophyte Dunaliella tertiolecta), either singularly or in combinations of 2 species. When prey combinations included D , tertiolecta, the chlorophyte was grazed only after a large fraction of the other species was first grazed. Diatom and haptophyte were grazed concurrently when offered in combination. Changing the relative proportions of starting biomass of the different species in combination had no effect on the order of grazing. However, in all cases the switch to the chlorophyte occurred rapidly and the maximum ingestion rate attained after the switch was proportional to the contribution of the chlorophyte to total starting biomass. From a hydrodynamic standpoint, specific clearance rate C' increased as the ratio of predator radius to prey radius R: r decreased and C' increased as R decreased for a given value of R: r. We suggest that the preference for the 2 smaller species is governed by the ability of the flagellate to adjust its own size downward to accomodate the smaller prey in order to maintain R : r at ca 2 : 1. When sized to graze these smaller species, the flagellate simply is too small to graze the chlorophyte. Thus, although there is clear evidence that the flagellate is a non-passive grazer and will not graze certain species at all, mechanoreception clearly plays a major role in the dynamics of grazing desired species. Raptorial grazers such as P. imperforata, by having the ability to graze prey almost as big as themselves, may be effective competitors with larger protozoa for nanoplankton-size food particles and also contribute to making the food chain (web) withln the microbial loop long and complicated with high losses of energy and materials.