Food selection by bacterivorous protists: insight from the analysis of the food vacuole content by means of fluorescence in situ hybridization

Abstract

A modified fluorescence in situ hybridization (FISH) method was used to analyze bacterial prey composition in protistan food vacuoles in both laboratory and natural populations. Under laboratory conditions, we exposed two bacterial strains (affiliated with beta- and gamma-Proteobacteria -- Aeromonas hydrophila and Pseudomonas fluorescens, respectively) to grazing by three protists: the flagellates Bodo saltans and Goniomonas sp., and the ciliate Cyclidium glaucoma. Both flagellate species preferably ingested A. hydrophila over P. fluorescens, while C. glaucoma showed no clear preferences. Differences were found in the digestion of bacterial prey with B. saltans digesting significantly faster P. fluorescens compared to two other protists. The field study was conducted in a reservoir as part of a larger experiment. We monitored changes in the bacterial prey composition available compared to the bacteria ingested in flagellate food vacuoles. Bacteria detected by probe HGC69a (Actinobacteria) and R-BT065 were negatively selected by flagellates. Bacteria detected by probe CF319a were initially positively selected but along with a temporal shift in bacterial cell size, this trend changed to negative selection during the experiment. Overall, our analysis of protistan food vacuole content indicated marked effects of flagellate prey selectivity on bacterioplankton community composition.

Some Mixotrophic Flagellate Species Selectively Graze on Archaea

“…5). This observation agrees with previous studies that indicate lower grazing pressure on Actinobacteria by heterotrophic nanoflagellates (18,20) and by the mixotrophic Chrysophyceae Ochromonas sp. (47).…”

“…Prey identification can be addressed by techniques based on DNA fingerprinting (15,16), which allow ensuing taxonomic changes in prey assemblages (10,17). Linking prey and predator can be addressed using techniques based on fluorescence in situ hybridization (FISH), which allow detecting targeted prey inside protist food vacuoles (5,14,18,19). These techniques have been performed mostly under experimental conditions (18,20), and results suggest a high selectivity in the feeding of heterotrophic flagellates and some ciliate species investigated.…”

“…Linking prey and predator can be addressed using techniques based on fluorescence in situ hybridization (FISH), which allow detecting targeted prey inside protist food vacuoles (5,14,18,19). These techniques have been performed mostly under experimental conditions (18,20), and results suggest a high selectivity in the feeding of heterotrophic flagellates and some ciliate species investigated. In contrast, the few in situ measurements showed unclear ingestion patterns for lake flagellates, and even random feeding was proposed for some bacterial groups (14).…”

Grazing rates and functional diversity of uncultured heterotrophic flagellates

“…Second, each species might have different prey preferences, being adapted to consume a specific part of the bacterial assemblage. The most critical parameter to define the grazing vulnerability of a given bacteria is its cell size (González et al, 1990), but other factors such as cell viability (Landry et al, 1991), surface properties (Matz and Jü rgens, 2001), motility (Matz and Jü rgens, 2005), phylogenetic affiliation (Jezbera et al, 2005) or food quality (Shannon et al, 2007) have been also demonstrated. Finally, intrinsic physiological parameters like the functional response (relationship of grazing rates with prey concentration) or the growth efficiency (conversion of ingested food to biomass) might explain adaptations to specific environmental settings.…”

Comparative study of protozoan communities in full-scale MWTPs in Beijing related to treatment processes

“…Giving an insight into protozoan communities is hereby of great significance in expanding the holistic understanding of activated-sludge ecosystem, and thus improving the performance of the existing biological treatment processes. It is accepted that the protozoan communities will be influenced in two pathways: to be manipulated directly by virtue of their inherent sensitivities to environment, and meanwhile to be modified indirectly in accordance with the potential changes of bacterial community structures through food chain (Curds and Cockburn, 1970;Zarda et al, 1998;Jezbera et al, 2005;Nicolau et al, 2005). For building the species-habitat relationship, most previous studies have been devoted to monitoring protozoan communities for predicting effluent quality and plant performance (Curds and Cockburn, 1970;Poole, 1984;Al-Shahwani and Horan, 1991;Salvadó et al, 1995;Martín-Cereceda et al, 1996).The species of protozoa present in activated sludge have been well inventoried as important bio-indicators since last century (Curds and Cockburn, 1970;Madoni and Ghetti, 1981;Madoni et al, 1993;Nicolau et al, 2001).…”