The bacterial-fungal energy channel concept challenged by enormous functional versatility of soil protists

•The soil food web contains two separated energy channels at lower trophic levels.•Soil protists represent key bacterivores controlling the bacterial energy channel.•However, mycophagous, nematophagous, parasitic and pathogenic protists are common.•Several protists exhibit omnivorous feeding on bacteria, fungi and other organisms.•This functional diversity questions current soil food web and energy channel models.

Protists (=protozoa) are commonly treated as bacterivores that control the bacterial energy channel in soil food webs. This ecologist's perspective is, however, challenged by taxonomic studies showing that a range of protists feed on fungi, other protists and even nematodes. Recently, it was revealed that obligate and facultative mycophagous protists are common soil inhabitants, while others are facultative nematophagous. Furthermore, protists act as parasites and pathogens of plants and animals. This neglected functional diversity of protists, that is similarly prevalent for other groups of soil organisms, reveals that current food web model models are oversimplified.Facultative feeding of various protist taxa on bacteria and fungi, the source of both major energy channels, strongly implies that a clear split of the energy channels at lower trophic levels does not exist and that more complex energy flows prevail in soil food webs. Future efforts should therefore target ecological functioning of protists and other groups of soil organisms, on a species-specific level, to create more meaningful functional units that then need incorporation in modified soil food web models. Such efforts will help disentangling the structure, diversity and resulting functioning of complex soil systems, including energy flows through the soil food web.

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