Grazing rates and functional diversity of uncultured heterotrophic flagellates

Abstract

Aquatic assemblages of heterotrophic protists are very diverse and formed primarily by organisms that remain uncultured. Thus, a critical issue is assigning a functional role to this unknown biota. Here we measured grazing rates of uncultured protists in natural assemblages (detected by fluorescent in situ hybridization (FISH)), and investigated their prey preference over several bacterial tracers in short-term ingestion experiments. These included fluorescently labeled bacteria (FLB) and two strains of the Roseobacter lineage and the family Flavobacteriaceae, of various cell sizes, which were offered alive and detected by catalyzed reporter deposition-FISH after the ingestion. We obtained grazing rates of the globally distributed and uncultured marine stramenopiles groups 4 and 1 (MAST-4 and MAST-1C) flagellates. Using FLB, the grazing rate of MAST-4 was somewhat lower than whole community rates, consistent with its small size. MAST-4 preferred live bacteria, and clearance rates with these tracers were up to 2 nl per predator per h. On the other hand, grazing rates of MAST-1C differed strongly depending on the tracer prey used, and these differences could not be explained by cell viability. Highest rates were obtained using FLB whereas the flavobacteria strain was hardly ingested. Possible explanations would be that the small flavobacteria cells were outside the effective size range of edible prey, or that MAST-1C selects against this particular strain. Our original dual FISH protocol applied to grazing experiments reveals important functional differences between distinct uncultured protists and offers the possibility to disentangle the complexity of microbial food webs.

Distinct and diverse anaerobic bacterial communities in boreal lakes dominated by candidate division OD1

“…This may simply reflect more pronounced seasonal shifts in temperature (range 0.9-18 1C in the epilimnion compared with 3.6-7.5 1C in the hypolimnion), light and nutrient concentrations in the epilimnion compared with the more stable deeper strata. Other possible reasons for higher variability in the epilimnetic community over time may be selective feeding by heteronanoflagellates (Fu et al, 2003;Massana et al, 2009) or pulsed inputs of organic compounds from allochthonous production that are more labile in the surface layers, while quenched before reaching the deeper waters via sedimentation. Additional explanations for the low variability in hypolimnetic community may arise from the fact that anaerobic metabolic pathways have a lower energy yield compared with aerobic metabolism and that the mineralization of organic compounds in anaerobic systems is a multistep process that must involve complex communities (Megonigal et al, 2005).…”

Mixotrophic haptophytes are key bacterial grazers in oligotrophic coastal waters

“…An equal large volume of diluted glutaraldehyde in a ratio 1:1 (volume fixative: volume sample) is recommended in order to minimize egestion of FLB during fixation of samples (Sherr et al, 1989). Here, we had to use formaldehyde (also in a 1:1 ratio with the sample), as we did in previous work (Massana et al, 2009), because glutaraldehyde compromises FISH detection (Medina Sánchez et al, 2005). Although the grazing rates of haptophytes might have been underestimated, the use of diluted formaldehyde should have minimized the problem.…”

“…Using the approach previously published in Massana et al (2009), we estimated here the in situ grazing rates of haptophytes by combining FLB ingestion and FISH detection and quantification. The main advantage of the FLB ingestion approach is that it allows the cell-by-cell inspection, and therefore, the estimation of grazing rates of specific groups of bacterivores (Caron, 2001).…”

Vertical structure of small eukaryotes in three lakes that differ by their trophic status: a quantitative approach

“…The cloning-sequencing approach has recently allowed the opening of the black box of unidentified small heterotrophic eukaryotes and raised the important question of the taxonomic composition and functional roles of this planktonic assemblage Lefèvre et al, 2008;Lepère et al, 2006Lepère et al, , 2008Massana et al, 2009). These molecular results provide knowledge on their distribution and specific role in aquatic systems and highlight the potential importance of the process usually left out of aquatic trophic food web functioning as parasitism.…”