Soil amoeba richness regulates bacterial and fungal community assembly in microcosms

Protists are the main consumers of bacteria and fungi in soil microbial food-webs. How protist predation impact bacterial and fungal community assembly remains rudimentary. To address this gap, we focused on local-scale soil microbial communities, including bacteria, fungi, and amoeboid protists (myxomycete). We investigated the responses of bacterial and fungal community assembly to varying levels of myxomycete diversity by conducting field observation, microcosm experiment, and meta-analysis. We found that myxomycete species richness significantly influenced fungal community assembly process. The increasing myxomycete richness promoted deterministic fungal assembly by enhancing homogeneous selection. Our results also demonstrated for the first time that myxomycetes strengthened the bacterial-fungal interactions, which indirectly led to turnover of bacterial community. Our findings highlight direct predatory selection by myxomycetes on fungi and indirect trophic interactions shaping bacteria, underscoring protist diversity as a key driver of soil microbial community dynamics, and also providing novel insight into microbial food-web complexity.