What Drives the Assembly of Plant-associated Protist Microbiomes? Investigating the Effects of Crop Species, Soil Type and Bacterial Microbiomes

In a field experiment we investigated the influence of the environmental filters soil type (i.e. three con-trasting soils) and plant species (i.e. lettuce and potato) identity on rhizosphere community assembly of Cercozoa, a dominant group of mostly bacterivorous soil protists. Plant species (14%) and rhizo-sphere origin (vs bulk soil) with 13%, together explained four times more variation in cercozoan beta diversity than the three soil types (7% explained variation). Our results clearly confirm the existence of plant species-specific protist communities. Network analyses of bacteria-Cercozoa rhizosphere com-munities identified scale-free small world topologies, indicating mechanisms of self-organization. While the assembly of rhizosphere bacterial communities is bottom-up controlled through the resource supply from root (secondary) metabolites, our results support the hypothesis that the net effect may depend on the strength of top-down control by protist grazers. Since grazing of protists has a strong impact on the composition and functioning of bacteria communities, protists expand the repertoire of plant genes by functional traits, and should be considered as 'protist microbiomes' in analogy to 'bacterial microbiomes'.(c) 2022 The Authors. Published by Elsevier GmbH. This is an open access article under the CC BY-NC -ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

An experiment in the field examined the effects of soil type and plant species on the assembly of soil protist communities. The results confirmed the presence of plant species-specific protist communities and identified mechanisms of self-organization.