AME 12:131-138 (1997)  -  doi:10.3354/ame012131

Impact of metazoan and protozoan grazers on bacterial biomass distribution in microcosm experiments

Jürgens K, Arndt H, Zimmermann H

The interactions between different metazooplankters (cladocerans, rotifers, copepods), protozoans (ciliates, nanoflagellates) and bacteria (mixed lakewater culture) were examined in laboratory microcosm experiments. Growth of bacteria was stimulated in batch cultures by the addition of glucose. During 24 h experiments most of the bacterial production was consumed by nanoflagellates which achieved high growth rates. Moderate biomasses of copepods, rotifers or small cladocerans (Bosmina longirostris, Ceriodaphnia reticulata) had no or only weak effects on protozoan development. In contrast, Daphnia (D. galeata, D. magna) in higher densities consumed protozoans and bacteria simultaneously. Bacterial abundance was strongly reduced in all treatments but bacteria were either mainly consumed by protozoans (nanoflagellates) or by metazoans (Daphnia). This resulted in striking differences in the bacterial morphology of the remaining bacteria after 24 h. A shift in bacterial size structure towards the appearance of large aggregates and long filaments correlated to predation by protozoans. These protozoan-resistant morphotypes dominated ungrazed bacterial biomass after 24 h. When bacteria grew without predation or when Daphnia suppressed protozoan growth, the original bacterial size structure largely remained, and freely dispersed rods and cocci predominated. These model experiments illustrate the different impacts of metazoan and protozoan grazers which could both control bacterial production but produced a very different bacterial biomass distribution.

Heterotrophic flagellates · Ciliates · Bacteria · Metazoan grazing · Grazing resistance · Microbial food web

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