Grazing rates of diverse morphotypes of bacterivorous ciliates feeding on four allochthonous bacteria

Aims: The permanence in aquatic systems of allochthonous bacteria coming from sewage effluents is a risk for public health. This work aimed to analyse the elimination of the bacteria Staphylococcus epidermidis, Enterococcus faecalis, Escherichia coli and Klebsiella pneumoniae by a riverine ciliate community. Methods and Results: The ciliates were characterized and identified on the basis of morphological and behavioural features and grouped in nine morphotypes. Differential grazing by each morphotype on the four allochthonous bacteria was carried out by adding fluorescently labelled bacteria to the water samples, and measuring their uptake along time. Conclusions: The nine morphotypes were present in all the samples but in different proportions, being the most abundant the small scuticociliates and hipotrichs. The smallest morphotypes showed grazing rates lower than 20 FLB ciliate(-1) h(-1), with a preference towards K. pneumoniae. The larger morphotypes showed in general the highest grazing rates, but the selectivity was hardly attributable to bacterial size or type of cell wall. Significance and Impact of the Study: The elimination of allochthonous bacteria in an aquatic system may be highly different depending on the diversity of the ciliated protistan community in the system and on the nature of the bacterial prey.