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

Aims: The permanence in aquatic systems of allochthonous bacteria coming fr om sewage effluents is a risk for public health. This work aimed to analyse the elimination of the bacteria Staphylococcus epidermidis, Enterococcus f aecalis, Escherichia coli and Klebsiella pneumoniae by a riverine ciliate c ommunity. Methods and Results: The ciliates were characterized and identified on the basis of morphological and behavioural features and grouped in nine morphot ypes. Differential grazing by each morphotype on the four allochthonous bac teria was carried out by adding fluorescently labelled bacteria to the wate r samples, and measuring their uptake along time. Conclusions: The nine morphotypes were present in all the samples but in di fferent 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 mor photypes showed in general the highest grazing rates, but the selectivity w as hardly attributable to bacterial size or type of cell wall. Significance and Impact of the Study: The elimination of allochthonous bact eria in an aquatic system may be highly different depending on the diversit y of the ciliated protistan community in the system and on the nature of th e bacterial prey.