Soil amoeba richness drives local-scale bacterial and fungal community assembly

Protists are the primary consumers of bacteria and fungi within the soil microbial food web. However, the role of protist diversity in shaping bacterial and fungal community assembly remains unclear. To address this gap, we investigated how different levels of myxomycete (amoeboid protist) diversity affect the assembly of bacterial and fungal communities through field observations and microcosm experiments. Results indicate that myxomycete species richness significantly influences the assembly processes of fungal communities at the local scale, rather than those of bacterial communities. Increased myxomycete richness promotes deterministic fungal assembly by enhancing homogeneous selection. Additionally, myxomycete richness influences bacterial-fungal co-occurrence in inter-domain networks, indirectly driving bacterial community turnover. These findings highlight the direct top-down control exerted by amoeboid protists on fungi and the indirect trophic interactions that shape bacterial communities. They underscore the role of protist diversity as a key driver of soil microbial community assembly and provide novel insights into the complexity of microbial food webs.