Protists, like Arcellinida (i. e. lobose testate amoebae), play a considerable role in the maintenance of the global ecosystem. In soil, Arcellinida are important predators, playing a key role in shaping microbial communities, carbon fixation and nutrient cycling amongst others. However, their diversity and its drivers still remain poorly understood, particularly the effects of local climate. The cedar forests of Lebanon are well preserved ecosystems situated at different altitudes and constitute a perfect scenario to test macroecological hypotheses, such as the water-energy theory. We collected 123 samples from four cedar forests (Bsharre, Tannourine, Shouf, Ehden) situated along an elevational gradient. We determined their diversity using an Arcellinida-specific metabarcoding approach based on the cytochrome oxidase subunit I gene. Our study shows that Arcellinida richness and phylogenetic diversity follow a unimodal distribution, peaking at mid-elevations. Precipitation and actual evapotranspiration were identified as key drivers of community variation, supporting the water-energy balance hypothesis. The most humid forest (Shouf) was the most diverse and had also the most heterogeneous communities. It also presented the highest number of exclusive operational taxonomic units. Conversely, the sites situated at the lower humidity (Bsharre) had a diversity nested within other forests. These results suggest Bsharre communities are more vulnerable because their lower diversity implies a lower functional redundancy. In a nutshell, under Mediterranean climates, warmer and wetter climate support a higher diversity of Arcellinida, while high or low altitude sites host generalists. This is particularly important in a context of climate change where these ecosystems face significant threats from reduced water availability and biodiversity loss.
