Biogeography of soil protistan consumer and parasite is contrasting and linked to microbial nutrient mineralization in forest soils at a wide-scale

Despite their essential role in soil microbiome and the global ecological processes, large-scale biogeographical patterns and predictors of protists are poorly characterized. Investigating the diversity and distribution of protists is crucial for understanding their biogeographic patterns and underlying the drivers across phylogenetic, ecological, and functional scales. Here, we explored a wide-scale pattern of protistan communities, and linked it with soil functions, in 107 soil samples from nine forest sites along a large climatic gradient. Our results showed that the biogeography of protistan communities in forest soils generally fitted the temperature diversity gradients (TDG), metabolic niche theory (MNT) and distance-decay relationships (DDR). Strikingly, the dominant protistan phyla, Cercozoa (consumer) and Apicomplexa (parasite), followed highly different/contrasting biogeographic patterns along the climatic gradient, as a result of environmental selection and stochastic processes. Cercozoa were relatively more abundant in cold arid soils while Apicomplexa thrived in tropical wet sites. Homogenizing dispersal had a stronger effect on the distribution of the Cercozoa, while ecological drift controlled the distribution of the Apicomplexa. In addition, we found that protist network modularization explained 57.5% of the variation in soil nutrient mineralization, suggesting the critical roles of Cercozoa and Apicomplexa in nutrient cycling. Collectively, we showed the general applicability of TDG, MNT and DDR to the soil protistan communities and revealed contrasting biogeographic patterns of protistan consumer and parasite along climatic gradients. Our study highlights the crucial contribution of protistan communities to nutrient mineralization in forest soils.