Protists with different feeding modes change biofilm morphology.

The effect of Dexiostoma (filter feeder), Vannella, Chilodonella (raptorial feeders), Spumella, and Neobodo (direct interception feeders) on the morphology of multispecies bacterial biofilms was investigated in small flow cells. The filter feeder Dexiostoma campylum did not alter biofilm volume and porosity but stimulated the formation of larger microcolonies compared with ungrazed biofilms. In contrast, the raptorial feeder Vannella sp. efficiently grazed bacteria from the biofilm surface, leading to smaller microcolonies and lower maximal and basal layer thickness compared with ungrazed biofilms. Microcolony formation was not stimulated in the presence of the sessile Spumella sp. Chilodonella uncinata rasped bacteria from the outer surface leading to mushroom-shaped microcolonies. In the presence of C. uncinata and Spumella sp., the biofilm volume was 2.5-6.3 times lower compared with ungrazed biofilms. However, the biofilm porosity and the ratio of biofilm surface area to biofilm volume were 1.5-3.7 and 1.2-1.8 times higher, respectively. Thus, exchange of nutrients and gases between the biofilm and its surrounding fluid should also be improved in deeper biofilm layers, hence accelerating microbial growth.