The role of heterotrophic microflagellates in plankton communities

165

In a series of grazing experiments we found that the phagotrophic microflagellate Paraphysomonas imperforata (Lucas) (7 to 12 pm dia.) fed equally well on the diatom Phaeodactylum tricornutum and bacteria under batch conditions. Growth rates of the microflagellate were considerably higher than those of the phytoplankton prey. Regeneration of NH,' was negligible in control cultures of phytoplankton alone, bacteria alone, or phytoplankton and bacteria. However, when the microflagellate was grazing on either phytoplankton or bacteria there was considerable regeneration of NH,+ This result was a clear indication that only the microflagellate was responsible for the excretion. Rates of NH,+ excretion were highest during exponential growth of the microflagellate and slackened off considerably with onset of the stationary phase when growth of the prey was not nutrientlimited. Regeneration efficiencies, however, were lowest during exponential growth (15 to 30 %), but increased to 50 % if calculated on the basis of the entire experiment which included the stationary phase. Regeneration efficiencies during exponential growth decreased to 8 % when the prey was grown under nitrogen-limitation. Nitrogen turnover rates per body weight are generally much higher in microflagellates than in macrozooplankton, but can b e greatly influenced by many environmental factors including prey nutritional state.

Section: Ecological Implications Of Low Nutrient Regenerationsupporting

confidence: 89%

Nutrient cycling in a microflagellate food chain: I. Nitrogen dynamics

Goldman¹

1985

165

“…This is contrary to the results of several studies which have noted differences in the growth of bacterivorous and herbivorous protozoa fed different prey species (Rubin & Lee 1976, Taylor & Berger 1976, Curds & Bazin 1977, Stoecker et al 1981, Caron 1984. Cell yields were also similar for P. imperforata fed bacteria or P. tricornutum and are commensurate with the range of cell yields observed for other microflagellates (Kopylov & Moiseev 1980, Fenchel 1982a, Sherr et al 1983.…”

Section: Discussioncontrasting

confidence: 73%

“…Nevertheless, our results are consistent with anecdotal accounts of herbivorous microflagellates i n marine mass algal cultures (Raymont & Adams 1958, Goldman & Stanley 1974, Laws et al 1983) and in plankton communities (Haas 1982), and are supported by the recent isolation and culture of ecologically analogous microflagellates in freshwater (Boraas 1983, Gude 1983. The ability of some microflagellates to ingest chroococcoid cyanobacteria has also been recently established (Johnson et al 1982, Caron 1984. Collectively, these observations suggest a potentially important, but as yet unconfirmed, role for microflagellate herbivory in plankton communities.…”

Section: Discussionmentioning

confidence: 82%

“…3836 from Lament-Doherty Geological In this context, these herbivorous protozoa may be of In support of this hypothesis, it has been demonstrated that the growth rates of bacterivorous microflagellates are relatively high (Davis 1982, Fenchel 1982a, Sherr & Sherr 1983, Caron 1984 and thus able to keep pace with phytoplankton growth. In addition, microflagellates can withstand periods of starvation and yet remain responsive to the addition of food (Fenchel 1982b).…”

Section: Introductionmentioning

confidence: 94%

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Nutrient cycling in a microflagellate food chain: II. Population dynamics and carbon cycling

Caron¹,

Goldman²,

Andersen³

et al. 1985

154

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.

“…2 Our purpose in this investigation was to examine the possible relationship between bacteria and their primary consumers across a range of aquatic ecosystems. Caron (1984) tremely eutrophic waters and on marine snow particles (macroscopic detrital aggregates). Bacterial abundances ranged over approximately 3 orders of magnitude from 105 to 10' rnl-l. Log-transformed abundances of HNAN and bacteria were significantly correlated (r2 = 0.50, p < 0.001, n = 600) using the relationship:…”

Section: System Trophy and Microbial Abundancesmentioning

confidence: 99%

Relationships between bacteria and heterotrophic nanoplankton in marine and fresh waters: an inter-ecosystem comparison

Sanders¹,

Caron²,

Berninger³

1992

442

329

Despite differences in the species compositions and absolute abundances of planktonic microorganisms in fresh-and saltwater, there are broad similarities in microbial food webs across systems. Relative abundances of bacteria and nanoplanktonic protozoa (HNAN, primarily heterotrophic flagellates) are similar in marine and freshwater environments, which suggests analogous trophic relationships. Ranges of microbe abundances in marine and fresh waters overlap, and seasonal changes in abundances within an ecosystem are often as great as differences in abundances between freshwater and marine systems of similar productivities. Densities of bacteria and heterotrophic nanoplankton, therefore, are strongly related to the degree of eutrophication, and not salt per se. Data from the literature is compiled to demonstrate a remarkably consistent numerical relationship (ca 1000 bacteria: l HNAN) between bacterioplankton and HNAN from the euphotic zones of a variety of marine and freshwater systems. Based on the results of a simple food web model involving bacterial growth, bacterial removal by HNAN, predation on HNAN, and the observed relationships between bacterial and HNAN abundances in natural ecosystems, it is possible to demonstrate that bottom-up control (food supply) is more important in regulating bacteiial abundances in oligotrophic environments while top-down control (predation) is more important in eutrophic environments.