Precipitation seasonality and soil carbon shape Arcellinida diversity across Bulgaria's ecosystems

Recent years have witnessed the discovery of an immense diversity of protists in almost all ecosystems on Earth. However, a major challenge consists in identifying diversity patterns and the drivers that explain the distribution of this diversity. While several works have focused on local parameters, only a few studies have addressed this question at regional scales. In this study, we characterize the diversity and distribution of Arcellinida (testate amoebae) at regional scale of a climatically contrasted country, Bulgaria. We collected 75 samples geographically evenly distributed and representative of the country's natural ecosystems and used metabarcoding to determine their diversity. Communities composition differed markedly among habitats, revealing sharp beta diversity patterns. "Mesophilic" environments such as forests and ponds were most diverse, while acidic and oligotrophic Sphagnum peatlands were poorest and hosted the highest proportion of unique taxa. Our analyses of richness-environment based on a Random Forest approach applied to all soil systems indicated that stable humidity (low silt content, low precipitation seasonality) and intermediate soil carbon stocks had a positive effect on richness. The predicted increase in continentality in the next decades in Bulgaria calls for testing climatic models to evaluate the potential risk of declining Arcellinida biodiversity.