Distinct factors drive the diversity and composition of protistan consumers and phototrophs in natural soil ecosystems

Protists are important microbial consumers and phototrophs in terrestrial ecosystems and play pivotal roles in nutrient cycling and plant performance. The predation pressure of protists significantly impacts the composition and function of microorganisms, and in turn bacteria and fungi, as the key food sources of protists, may shape the distribution of protists. However, how trophic interactions (i.e. biotic factors), together with environmental factors, regulate the community structure and functional traits of protists in natural soil ecosystems – with no or limited anthropogenic impacts - remains largely unknown. Here, we assessed the influences of biotic (vegetation types, bacteria, fungi and invertebrates) and abiotic factors (climate and soil attributes) on the core protist community using metabarcoding sequencing of 258 soil samples at a large scale. Core protists, representing abundant and ubiquitous phylotypes, were comprised of two major functional groups including consumers and phototrophs and were taxonomically dominated by supergroups Alveolata, Rhizaria and Archaeplastida. Soil invertebrate and bacterial communities were identified as the top predictors for the biogeographic pattern of core consumers, while the diversity and composition of core phototrophs were best predicted by mean annual temperature. Our study provides novel insights into the importance of biotic (trophic regulations) and abiotic factors in structuring core soil protist communities, and advances our understanding of the community assembly of consumer and phototrophic protists in the terrestrial ecosystems.