Acanthamoeba castellanii alone is not a growth promoter for Hordeum vulgare.

Protists are important key players in the microbial loop and influence their environment by grazing, which leads to the return of nutrients into the soil and reduces pathogen pressure on plants. Specifically, protists on and around plant roots are important for plants' development and growth. For this study, the fourth most important crop in the world, Hordeum vulgare, was selected. Seeds of H. vulgare were inoculated with Acanthamoeba castellanii alone or with additional soil bacteria at the beginning and during the experiment. The germination of the seeds and the growth of the plants in pouches were monitored over 3 weeks. No differences were found in leaf growth, root growth, root and leaf nitrogen content or ammonia content of the liquid from the pouches. In contrast, the relative increase in root and leaf dry weight showed a small difference compared to the controls. The results of this experiment demonstrated that seed inoculation with A. castellanii alone or with additional unidentified soil bacteria did not have a major effect on the growth and development of barley. Nevertheless, small changes in plant development were detected, indicating that A. castellanii should be considered for further investigation of co-inoculations with plant growth-promoting bacteria and additional nutrients.

Control of Blumeria graminis f. sp. hordei on Barley Leaves by Treatment with Fungi-Consuming Protist Isolates.

The obligate biotrophic fungal pathogen Blumeria graminis causes the powdery mildew disease of cereals, which results in large crop losses. Control of B. graminis in barley is mainly achieved by fungicide treatment and by breeding resistant varieties. Vampyrellid amoebae, just like mycophagous protists, are able to consume a variety of fungi. To reveal the impact of some selected fungus-consuming protists on Blumeria graminis f. sp. hordei (Bgh), and to evaluate the possibility of using these protists as biological agents in the future, their feeding behaviour on B. graminis spores on barley leaves was investigated. An experiment was carried out with five different protist isolates (Leptophrys vorax, Platyreta germanica, Theratromyxa weberi U 11, Theratromyxa weberi G7.2 and Acanthamoeba castellanii) and four matched controls, including the food sources of the cultures and the medium. Ten-day-old leaves of barley (Hordeum vulgare cv. Golden Promise) were first inoculated with Blumeria graminis (f. sp. hordei race A6) spores, then treated with protists and fungal colonies on the leaf surfaces were counted under the microscope after 5 days. The isolates L. vorax, P. germanica, and T. weberi U11 did not show a significant reduction in the number of powdery mildew colonies whereas the isolates T. weberi G7.2 and A. castellanii significantly reduced the number of powdery mildew colonies on the leaf surfaces compared to their respective controls. This indicates that these two isolates are capable of reducing B. graminis colonies on barley leaves and are suitable candidates for further investigation for possible use as biological agents. Nevertheless, the susceptibility to dryness and the cell division rate should be considered during the optimisation of the next steps like application procedure and whole plant treatment.

Relative importance of evolutionary dynamics depends on the composition of microbial predator-prey community.

Community dynamics are often studied in subsets of pairwise interactions. Scaling pairwise interactions back to the community level is, however, problematic because one given interaction might not reflect ecological and evolutionary outcomes of other functionally similar species interactions or capture the emergent eco-evolutionary dynamics arising only in more complex communities. Here we studied this experimentally by exposing Pseudomonas fluorescens SBW25 prey bacterium to four different protist predators (Tetrahymena pyriformis, Tetrahymena vorax, Chilomonas paramecium and Acanthamoeba polyphaga) in all possible single-predator, two-predator and four-predator communities for hundreds of prey generations covering both ecological and evolutionary timescales. We found that only T. pyriformis selected for prey defence in single-predator communities. Although T. pyriformis selection was constrained in the presence of the intraguild predator, T. vorax, T. pyriformis selection led to evolution of specialised prey defence strategies in the presence of C. paramecium or A. polyphaga. At the ecological level, adapted prey populations were phenotypically more diverse, less stable and less productive compared with non-adapted prey populations. These results suggest that predator community composition affects the relative importance of ecological and evolutionary processes and can crucially determine when rapid evolution has the potential to change ecological properties of microbial communities.