A common soil flagellate (Cercomonas sp.) grows slowly when feeding on the bacterium Rhodococcus fascians in isolation, but does not discriminate against it in a mixed culture with Sphingopyxis witflariensis

Abstract

Flagellates are very important predators on bacteria in soil. Because of their high growth rates, flagellate populations respond rapidly to changes in bacterial numbers. Previous results indicate that actinobacteria are generally less suitable than proteobacteria as food for flagellates. In this study, we investigated the growth of the flagellate Cercomonas sp. (ATCC 50334) on each of the two bacteria Sphingopyxis witflariensis (Alphaproteobacteria) and Rhodococcus fascians (actinobacteria) separately and in combination. The growth rate of the flagellate was lower and the lag phase was longer when fed with R. fascians than when fed with S. witflariensis. This supports our initial hypothesis that the actinobacterium is less suitable as food than the alphaproteobacterium. However, after longer periods of growth the peak abundance of flagellates was higher on R. fascians, indicating that the food quality of bacterial prey depends on the time perspective of the flagellate-bacterial interaction. There was no evidence that the flagellates selected against the actinobacterium when feeding in mixed cultures of the two bacteria. Experiments where flagellates were fed with washed bacterial cells or with bacteria growing with different substrate concentrations suggested that the low food quality of R. fascians is related both to the intrinsic cell properties and to the extracellular metabolites.

A role for copper in protozoan grazing – two billion years selecting for bacterial copper resistance

“…The plates for D. discoideum and P. crassicauda were incubated at 22°C and at 15°C respectively. Protozoan growth was monitored by counting the number of cells at the bottom of the well using an inverted microscope with a grid in the eyepiece (Lekfeldt and Ronn, ). The experiments were stopped when D. discoideum started aggregating, or when P. crassicauda became so dense that they no longer lay in one layer on the well bottom.…”

Protozoan growth rates on secondary-metabolite-producing Pseudomonas spp. correlate with high-level protozoan taxonomy

“…5 × 10 7 cells mL −1 ) is used for standard comparison between cultures. This is because different bacterial cultures with similar cell numbers may vary considerably with regard to carbon content, because cell sizes differ (Lekfeldt & Rønn, 2008). Bacterial cell size depends on the growth medium.…”

Ecological and evolutive implications of bacterial defences against predators

“…the dominance of predation resistant, albeit unproductive species (Jiang et al ., 2008). While indigestible or toxic prey may be indiscriminately consumed by protozoa (Lekfeldt and Ronn, 2008), many protozoa show a marked food selectivity (Weekers et al ., 1993), and prey selection by flagellates increases with prey density (Boenigk et al ., 2002). High resource availability reduces the relative cost of adaptations to predation less costly, and resistant phenotypes are favoured at high resource supply (Leibold, 1996; Corno and Jurgens, 2008; Friman et al ., 2008).…”