Soil protist life matters!

Soils host most biodiversity on Earth, with a major fraction of its taxonomic diversity still to be uncovered and most of its functional knowledge to be determined. Much focus has been - and still is - on bacteria, fungi and animals. Clearly, without any of those components, soils would not function as they do. However, the group that constitutes the bulk of eukaryotic diversity and plays a central role for soil functioning is missing: protists. As the main consumers of the microbiome, protists shape its composition and functioning. Other less known functions performed by protists may be equally important. Protists also include primary producers, decomposers, animal parasites and plant pathogens. We briefly review the many functions protists perform in soils and argue that soil biodiversity studies that ignore protists miss some potential mechanistic insight into the drivers of observed patterns. We highlight that the immense functional repertoire of protist affects virtually every soil process, from carbon cycling to primary production, including crop production. Therefore, we call for truly integrated biodiversity assessments including protists, without which the soil food-web and processes cannot reliably be understood: protists matter!