Soil protists spatio-temporal turnover in mountains

Biodiversity patterns along elevation gradients have long been studied for plants and animals, but only quite recently for soil microorganisms, especially protists (i.e., Eukaryotes excluding plant, animal, or fungi). Micro-organisms have shorter generation times than macro-organisms and their abundance, diversity and community structure are known to vary rapidly in response to abiotic factors. If microbial diversity varied more seasonally than spatially, this would represent a potential bias for biodiversity studies along elevation gradients, which are characterised by strong phenological contrasts, if only a single sampling campaign can be done. To address this question, we investigated the relative magnitude of spatial versus temporal diversity (alpha diversity) and community turnover (beta diversity) of soil protist communities along elevation gradients in two distant mountainous regions of Europe. We collected soil samples in forests and grasslands below the treeline along five elevation gradients in two consecutive seasons (spring and summer) in the Spanish Sierra Nevada and the Swiss Alps, to include two different biogeographic scales. Using general eukaryotic primers and amplicon sequencing (Illumina MiSeq) of soil eDNA, we decomposed total protist amplicon sequence variants (ASV) diversity into local alpha- and beta-diversity components and identified climatic and edaphic predictors (likely drivers) of biodiversity patterns using redundancy analyses (RDA). Soil protist communities varied spatially within and among transects but temporal variation was comparatively low. The best edaphic predictors of community turnover were the same in spring and summer, but their explanatory power differed between seasons. The dominant spatial component of beta-diversity suggests that patterns of soil protist communities along elevation gradients are more strongly driven by spatial heterogeneity than inter-seasonal turnover, which in turn implies that sampling at one time across an elevation gradient will not bias results due to phenological contrasts along elevation gradients. This is welcome for studies of soil protist macroecology and biogeography, but also to better document the diversity patterns of protists, one of the least known components of soil biodiversity.