Preferential Feeding by the Ciliates Chilodonella and Tetrahymena spp. and Effects of These Protozoa on Bacterial Biofilm Structure and Composition

<jats:title>ABSTRACT</jats:title> <jats:p> Protozoa are important components of microbial food webs, but protozoan feeding preferences and their effects in the context of bacterial biofilms are not well understood. The feeding interactions of two contrasting ciliates, the free-swimming filter feeder <jats:named-content content-type="genus-species">Tetrahymena</jats:named-content> sp. and the surface-associated predator <jats:named-content content-type="genus-species">Chilodonella</jats:named-content> sp., were investigated using biofilm-forming bacteria genetically modified to express fluorescent proteins. According to microscopy, both ciliates readily consumed cells from both <jats:named-content content-type="genus-species">Pseudomonas</jats:named-content> <jats:named-content content-type="genus-species">costantinii</jats:named-content> and <jats:named-content content-type="genus-species">Serratia</jats:named-content> <jats:named-content content-type="genus-species">plymuthica</jats:named-content> biofilms. When offered a choice between spatially separated biofilms, each ciliate showed a preference for <jats:named-content content-type="genus-species">P. costantinii</jats:named-content> biofilms. Experiments with bacterial cell extracts indicated that both ciliates used dissolved chemical cues to locate biofilms. <jats:named-content content-type="genus-species">Chilodonella</jats:named-content> sp. evidently used bacterial chemical cues as a basis for preferential feeding decisions, but it was unclear whether <jats:named-content content-type="genus-species">Tetrahymena</jats:named-content> sp. did also. Confocal microscopy of live biofilms revealed that <jats:named-content content-type="genus-species">Tetrahymena</jats:named-content> sp. had a major impact on biofilm morphology, forming holes and channels throughout <jats:named-content content-type="genus-species">S. plymuthica</jats:named-content> biofilms and reducing <jats:named-content content-type="genus-species">P. costantinii</jats:named-content> biofilms to isolated, grazing-resistant microcolonies. Grazing by <jats:named-content content-type="genus-species">Chilodonella</jats:named-content> sp. resulted in the development of less-defined trails through <jats:named-content content-type="genus-species">S. plymuthica</jats:named-content> biofilms and caused <jats:named-content content-type="genus-species">P. costantinii</jats:named-content> biofilms to become homogeneous scatterings of cells. It was not clear whether the observed feeding preferences for spatially separated <jats:named-content content-type="genus-species">P. costantinii</jats:named-content> biofilms over <jats:named-content content-type="genus-species">S. plymuthica</jats:named-content> biofilms resulted in selective targeting of <jats:named-content content-type="genus-species">P. costantinii</jats:named-content> cells in mixed biofilms. Grazing of mixed biofilms resulted in the depletion of both types of bacteria, with <jats:named-content content-type="genus-species">Tetrahymena</jats:named-content> sp. having a larger impact than <jats:named-content content-type="genus-species">Chilodonella</jats:named-content> sp., and effects similar to those seen in grazed single-species biofilms. </jats:p>