New insights on the impact of earthworm extract on the growth of beneficial soil fungi: species-specific alteration of the nematophagous fungal growth and limitation of an entomopathogenic fungus

: Earthworms influence soil ecosystem functions such as decomposition and regulation of nutrient cycles by burrowing, feeding, or secreting their cutaneous excreta. Earthworms can also modulate soil biodiversity and their interactions. Recent research has revealed that earthworm feeding activity or excretions can alter the performance of entomopathogenic nematodes (EPNs) and entomopathogenic fungi (EPFs), known biological control agents. Still, the impact of earthworm cadavers on the growth and reproduction of other biological control agents such as nematophagous fungus (NFs) and EPFs has been poorly explored. We suggest that earthworm body tissue could be used as a food supply by certain fungal species. We hypothesize that fungi with a high adaptive capacity as pathobionts such as NF will use earthworm cadavers to a certain extent, while the growth and reproduction of fungi with limited or specific nutrition needs, such as EPFs, will be restricted. In this study, we evaluated two NF species: a nematode-trapping fungus Arthrobotrys musiformis Drechsler (Orbiliales: Orbiliaceae) and a nematode egg or cyst parasite Purpureocillium lilacinum (Thom) (Hypocreales: Ophiocordycipitaceae), and one EPF species: Beauveria bassiana (Balsamo). We exposed each fungus to two different Aporrectodea molleri extract media at six concentrations in agar medium (n = 3 per treatment) and liquid medium (n = 5): (i) earthworms devoid of intestinal contents (EDG) (C1...C6), and (ii) fresh earthworms (FE) (C1...C6). The use of FE and EDG was designed to discriminate the effect of earthworms’ intestinal content on fungal growth. These treatments were also compared to alternative conventional media potato dextrose agar (PDA) and brain heart infusion (BHI) as the positive controls. We evaluated the vegetative growth capacity daily for 18 days, registering the reproduction at the end of the experiment. We observed that EPF B. bassiana did not grow in FE and EDG solid medium but showed germination in their liquid media. However, both NF species could grow and reproduce in both media but following a species-specific pattern. In all cases, FE and EDG reduced fungal growth compared to PDA and BHI. Our results suggest that the use of earthworm as a resource in natural conditions might depend on the expected specialization, with EPF B. bassiana with the lowest use, NF A. musiformis moderately, and P. lilacinum widely used. The study illustrates the complexity of the soil organisms’ interactions and highlights the necessity to advance the understanding of the contribution of all the beneficial soil organisms in a comprehensive manner.