Rapid Daphnia-mediated changes in microbial community structure: an experimental study

Abstract

Shifts in morphological and taxonomical composition of bacterioplankton communities in response to protist and metazoan grazing were studied in bottle experiments, exposing bacterioplankton from a eutrophic clear-water pond, dominated by a large population of Daphnia magna, to a Daphnia gradient, ranging from 0 to 60 individuals per liter. Prior to the first experiment, the bacterioplankton community was shaped by protist grazing, while for the second experiment, bacterioplankton was pre-adapted to Daphnia grazing. In both experiments, rapid shifts in biomass and structure of the bacterioplankton community upon exposure to Daphnia grazing were observed. High Daphnia densities suppressed protozoa, resulting in a dominance of free-living bacteria. Under low Daphnia densities, heterotrophic nanoflagellates (HNF) developed as the dominant grazers and complex morphotypes (filaments, aggregates) were abundant in the bacterial community. Denaturing gradient gel electrophoresis analysis showed that taxonomical changes accompanied the morphological differences between bacterial communities shaped by HNF or Daphnia grazing. However, comparing ciliate- and Daphnia-dominated bacterial communities, we observed a discrepancy between morphological and taxonomical shifts, indicating that other traits than mere morphological ones determine vulnerability of bacterioplankton to specific grazers. Our results illustrate the rapid, pronounced and reversible impact of grazing on the morphology and taxon composition of bacterioplankton. Our results also stress that Daphnia may, already at moderate densities, have a pronounced impact on the lake bacterioplankton, both through direct grazing on the bacteria and through grazing on protozoan bacterivores.

The hydraulic retention time on the particle removal efficiency by Daphnia magna filtration on treated wastewater

“…Despite this, the steady state was well predicted by the model. The model also overestimated the Daphnia removal efficiency when the Daphnia concentration was 35 individuals l -1 or lower and for experiments with HRT [ 6 h. Degans et al (2002) found that Daphnia concentrations below 30 individuals l -1 were not able to reduce the bacterial concentration in 24 h because the growth of bacteria during this period overcame the Daphnia filtration. The model did not consider the possible growth of bacteria, which may be an important fraction in the sludge composition.…”

Food quality of heterotrophic bacteria for Daphnia magna: evidence for a limitation by sterols

“…In particular, filter-feeding cladocerans of the genus Daphnia, the key herbivores in many freshwater ecosystems, efficiently consume heterotrophic bacteria (e.g. Gophen & Geller, 1984;Brendelberger, 1991;Pace & Cole, 1994) and, like bacterivorous protozoans, are able to shape the bacterial community structure and to suppress bacterial biomass production (Jürgens, 1994;Langenheder & Jürgens, 2001;Degans et al, 2002).…”

Top‐down impact of bacterivorous nematodes on the bacterial community structure: a microcosm study

“…(1993), and has since proven to be very useful in analysing bacterial communities originating from a large number of habitats (Muyzer, 1999). Recently, this technique has been applied successfully in a number of experimental studies (Degans et al ., 2002;Massana and Jürgens, 2003;Zöllner et al ., 2003)…”