Bacterivory in ciliates isolated from constructed wetlands (reed beds) used for wastewater treatment

The bacterivorous activity of ciliates, isolated from constructed wetlands employing the root zone method of wastewater treatment, was investigated by measuring their grazing rates upon fluorescently-labelled bacteria (FLB), specifically Escherichia coli. The highest mean grazing rates were recorded for Paramecium spp (1.85 FLB/cell/min), which was the largest ciliate used in the study, followed by oxytrichids (1.104 FLB/cell/min), Halteria (0.648 FLB/cell/min) and scuticociliates (0.433 FLB/cell/min), the smallest ciliates used in the study. Lowest feeding rates were found in the anaerobes Plagiopyla and Caenomorpha, although the experimental conditions are likely to have adversely affected these forms. Large variations in grazing rates between individual cells from the same populations were observed. The proportion of empty cells, i.e. those without ingested FLB, and the decrease in the proportion of empty cells with incubation time, showed large differences between taxa. The present study indicates that ciliates are capable of all observed E. coli removal from wastewaters treated in constructed wetlands using the root zone method. However, it is unlikely that ciliates in situ continuously maintain maximum feeding rates. A variety of other processes, biotic and abiotic, are thought to be involved in the removal of pathogenic and indicator bacteria such as E. coli.