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

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 feederTetrahymena sp. and the surface-associated predatorChilodonella sp., were investigated using biofilm-forming bacteria genetically modified to express fluorescent proteins. According to microscopy, both ciliates readily consumed cells from bothPseudomonas costantinii andSerratia plymuthica biofilms. When offered a choice between spatially separated biofilms, each ciliate showed a preference forP. costantinii biofilms. Experiments with bacterial cell extracts indicated that both ciliates used dissolved chemical cues to locate biofilms.Chilodonella sp. evidently used bacterial chemical cues as a basis for preferential feeding decisions, but it was unclear whetherTetrahymena sp. did also. Confocal microscopy of live biofilms revealed thatTetrahymena sp. had a major impact on biofilm morphology, forming holes and channels throughoutS. plymuthica biofilms and reducingP. costantinii biofilms to isolated, grazing-resistant microcolonies. Grazing byChilodonella sp. resulted in the development of less-defined trails throughS. plymuthica biofilms and causedP. costantinii biofilms to become homogeneous scatterings of cells. It was not clear whether the observed feeding preferences for spatially separatedP. costantinii biofilms overS. plymuthica biofilms resulted in selective targeting ofP. costantinii cells in mixed biofilms. Grazing of mixed biofilms resulted in the depletion of both types of bacteria, withTetrahymena sp. having a larger impact thanChilodonella sp., and effects similar to those seen in grazed single-species biofilms.