A dormant amoeba species can selectively sense and predate on different soil bacteria

1. Soil protists are the invisible majority of soil eukaryotes, which are essential but often forgotten parts of the soil ecosystem. They play key roles in microbial food webs by predating on other soil microbes. However, it is not dear how dormant soil protists sense, recognize and feed on diverse microbial prey. 2. In this study, we used a soil amoeba, Dictyostelium discoideum, to study selective discrimination and predation of 14 different bacteria. We found that discrimination and sensing of prey in D. discoideum started as early as resting spores. Dictyostelium discoideum had higher hatching rates, formed bigger amoeba plaques and preferred high nutritional value bacteria. The feeding speed of amoeba on various bacteria was constant and was not linked with sensing of prey or bacterial nutritional value. We also found that higher bacterial density decreased predation efficiency, and one species, P. fluorescens, induced a strong density-dependent inhibition of amoeba spore production. 3. In conclusion, we find that dormant D. discoideum can selectively sense and predate on different soil bacteria, a process that is likely mediated through active amoeba preference as well as bacterial inhibition. This study provides new insights into the role of protists in shaping soil bacterial communities, and future study needs to assess this in natural soil environments.