Soil microorganisms behave like macroscopic organisms: patterns in the global distribution of soil euglyphid testate amoeba

Aim: Patterns of α- and β-diversity of soil protist communities and the factors that shape them remain largely unknown. We undertook a world-wide survey of forest litter to investigate the patterns of diversity in a group of testate amoebae. We aimed to assess: (1) whether there is a latitudinal gradient in α-diversity, and (2) whether β-diversity was correlated solely with environmental factors commonly used in soil biology research or if it was also independently explained by geographical barriers. Location: World-wide. Methods: We studied the diversity of Euglyphida, a common group of testate amoebae, in 35 samples of forest litter and moss samples from a global survey, using small subunit rRNA gene sequences. We assessed the relationship between sample α-diversity and latitude using generalized additive models (GAM). Furthermore, we determined the relationships between community composition and geographical models (distance-based Moran's Eigenvector Maps – db-MEM) using Generalized UniFrac distances (GUniFrac). We also investigated the relationship between individual measured soil parameters, WorldClim data and diversity (alpha plus beta) using both raw data and synthetic variables obtained through principal components analysis. Results: We recorded 245 phylotypes belonging to 6 out of 7 known Euglyphida families, plus four novel deep clades. Euglyphid α-diversity was positively correlated with temperature and negatively with latitude and litter C/N ratio. Euglyphida community structure was correlated with the spatial eigenvector Db-MEM31, independently of all measured environmental variables. Db-MEM31 corresponds to a natural barrier constituted by the Northern Hemisphere desert belt. Beta diversity was correlated with other environmental variables, such as pH, isothermality and temperature in the coldest month of the year. Main conclusions: Soil euglyphid α-diversity displays a latitudinal gradient, and β-diversity is not only correlated with climatic and physicochemical parameters but also with geographical barriers. Such patterns of diversity were until recently believed to be characteristic only for macroscopic organisms