Picoplankton feeding by the ciliate Vorticella similis in comparison to other peritrichs emphasizes their significance in the water purification process

Abstract

We measured the feeding and growth rates of the sessile peritrich ciliate Vorticella similis on seven picocyanobacterial strains and their accompanying bacterial flora in microcosm experiments. This ciliate and several closely related Vorticella species are important indicators in the activated sludge of water treatment plants and contribute significantly to the water purification process. The picocyanobacteria were isolated from oligo-mesotrophic subalpine lakes in Austria, central Europe, and represent different pigment types (phycoerythrin-rich, PE, and phycocyanin-rich, PC) and phylogenetic clusters. In contrast to most previous studies with peritrich ciliates, we calculated growth and grazing rates of V. similis from relatively long-term incubations (48 h). Ingestion and clearance rates were calculated from the rate of disappearance (ROD) of prey in experimental containers with ciliates, relative to controls without ciliates. The results confirmed the hypothesis that V. similis is an indiscriminative suspension feeder that would not select for prey features other than cell size. Ciliate ingestion rates depended on picoplankton abundance and biomass, but the type of the functional response of peritrich ciliates awaits further study. In particular, it needs to be tested if their grazing activity declines strongly at moderate to low food levels. The ROD method combined with the precise assessment of picoplankton abundances by flow cytometry that we used yielded moderately higher estimates of the ciliate feeding activity as previous short-term experiments using fluorescently labeled prey. The picocyanobacterial and bacterial ingestion rates that we obtained are the highest values thus far reported for sessile peritrich ciliates. Clearance rates of V. similis were also high but comparable to those of two other peritrich ciliate species. Our results can be used to quantify the tremendous significance of peritrich ciliates in the water purification process.