Jena Experiment: Drought Experiment (Oomycota, Cercozoa)

Climate change poses a threat to European grasslands, increasingly exposing them to severe summer droughts that disrupt ecosystem functioning. Concurrently, the loss of plant biodiversity is linked to declines in ecosystem stability and productivity. Our study, conducted within the framework of the long-term "Jena Experiment", investigated the interactions between plant diversity and drought on microbial pathogens. The experiment features plots with plant communities ranging from 1 to 60 species, with a sub-experiment established to simulate repeated summer droughts through the use of transparent roofs for six weeks annually from 2008 to 2016. Soil samples were collected one year after the final drought period, and amplicon sequencing was performed to analyze the diversity of two protistan taxa: Oomycota (Stramenopila) and Cercozoa (Rhizaria), both of which include important plant pathogens. Our findings revealed that the diversity and community compositions of Oomycota and Cercozoa (including plant pathogenic Phytomyxea) reacted to both plant species richness and repeated drought, but not always in the same way. Overall, no buffering effect of plant diversity against drought was observed. Plant species-specific Oomycota were upregulated in the drought treatment, and similarly, Phytomyxea and cercozoan consumers exhibited a greater number of both upregulated and downregulated OTUs in the drought treatment. These results suggest that repeated and more frequent summer droughts lower plant defense against pathogenic protists, leading to persistent shifts in microbial communities. Consequently, grassland ecosystems may face heightened challenges from plant pathogens due to increasing drought frequency and biodiversity loss, potentially undermining ecosystem stability and functioning.