Protists introduce distinct temporal patterns in a synthetic bacterial community consistent with known feeding traits

Through predation, protists act as strong determinants of bacterial communities, yet their role in shaping synthetic communities (SynCom) remains unclear. We present the effects of three predator protists - a ciliate (Colpoda steinii), a flagellate (Neobodo designis), and an amoeba (Acanthamoeba castellanii) on the temporal dynamics of bacterial SynCom using a microtiter plate-based design. We asked whether predation contributes to the maintenance of SynCom richness, alters trajectories, and whether these effects depend on the identity of the protist. Bacterial community structure and composition in absolute abundances were estimated by 16S rRNA gene metabarcoding. Protist 18S and bacterial 16S rRNA gene copy numbers were quantified by qPCR, respectively. Using five sampling points, we found that bacterial 16S rRNA gene abundance did not change significantly over time when cultured alone, whereas protist 18S rRNA gene abundance altered at later time points, decreasing in ciliates, remaining stable in amoebae, and increasing in flagellates. Further, using 16S rRNA gene metabarcoding, we found that protists significantly reshaped bacterial community structure across the temporal scale. Protist predation was associated with community richness and species-specific SynCom dynamics: C. steinii induced broad suppression, whereas N. designis exerted more restricted effects, both preferentially targeting initially dominant taxa, while A. castellanii showed a minor impact. These results highlight the importance of accounting for trophic interaction and point to prioritizing protists with a narrower feeding range when creating SynCom to maintain community diversity over time.