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

Abstract

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