Size-selective grazing of coastal bacterioplankton by natural assemblages of pigmented flagellates, colorless flagellates, and ciliates

Abstract

Fluorescently-labelled bacteria (FLB) were used to study the feeding strategies of a natural assemblage of estuarine protozoans and to examine whether the protozoan grazing could account for the in situ size structure of the bacterioplankton. The FLB, DTAF-stained enterococci, ranging in volume from 0.01 to 0.30 × 10(-1) µm(3), were added to a natural planktonic assemblage at a density of 5.5% of the natural bacterioplankton. Initial densities (individuals ml(-1)) were as follows: total natural bacteria, 2.2 × 10(6); FLB, 1.2 × 10(5); pigmented flagellates, 300; colorless flagellates, 250; and ciliates, 30. FLB consumption rates were determined by examining the contents of protozoan food vacuoles, and the long-term effect of grazing (over a period of 100 hours) was determined by monitoring the decline in the FLB density in experimental vessels. The average consumption rates of FLB by pigmented flagellates were similar to those by flagellates that lacked chloroplasts (0.9 and 0.6 FLB protozoan(-1) hour(-1), respectively). The ciliates consumed bacteria at an average rate that was 17-fold higher (per cell) than flagellates, and they displayed a greater preference for larger bacteria than did the flagellates. FLB of the mid-size classes (0.025-0.100 µm(3)) were heavily grazed by the entire protozoan assemblage; the smallest (<0.025 µm(3)) and the largest (>0.100 µm(3)) FLB escaped protozoan grazing. This had a profound effect on the resulting size distribution of FLB. At the end of a 100-hour incubation, the percentage of mid-size FLB (0.025 to 0.100 µm(3)) decreased 2.0-2.2-fold, while the percentage of the smallest and the largest FLB increased 2.0-2.5-fold. Resultant densities of FLB were consistent with initial clearance rates determined for the protozoan groups. The grazing rates of protozoans on FLB were species-specific; whereas some species consumed FLB, others did not demonstrate bacterivory. The results suggest that protozoan grazing has a major effect on the size distribution of coastal bacterioplankton. By selectively feeding on a particular size-class of bacteria, planktonic ciliates may consume 15-90% day(-1) of the standing stock of largest size classes of bacterioplankton. Thus, ciliates, which were present in low abundance in the field, could not balance the production of the entire bacterial community, but they may strongly influence the portion of the bacterial community represented by the largest bacterial class. The direct effect of flagellates (e.g., grazing) was limited to smaller bacteria.

Ciliategrazing on picoplankton in a eutrophic reservoir during the summer phytoplankton maximum: A study at the species and community level

“…Our results generally support previous reports of size-sclectivc grazing of natural ciliate communities on bacterioplankton (Gonzalez et al 1990;Epstein and Shiaris 1992). However, our data show that small scuticociliates and oligotrichs are cspccially likely to have the strongest grazing impact on the size structure of picoplankton.…”

Effects of Experimental Lead Pollution on the Microbial Communities Associated with Sphagnum fallax (Bryophyta)

“…For this, a study of each group alone and manipulative experiments with different combinations or organisms would be needed as a complement to the community approach we used. For example, specific functional bacterial groups (i.e., appropriate bacterial size for ciliates' feeding) could be studied, as suggested by Epstein et al [10].…”

Effects of turbulence on bacterial growth mediated through food web interactions

“…Furthermore. it seems that protozoans, and especially small flagellates, generally prefer larger particles (Andersson et al 1986, Gonzalez et al 1990, Epstein & Shiaris 1992, Peters 1994) and particles with high nutritional value (Verity 1991). If encounter rates of bacteria and small algae with microflagellates were increased through shear, the flagellates could have chosen to prey more on the larger and more nutritious particles (algae) and reduce the grazing pressure on the less preferred smaller particles (bacteria).…”