Eisenia fetida (Oligochaeta, Lumbricidae) activates fungal growth, triggering cellulose decomposition during vermicomposting

Cellulose is the most abundant polymer in nature and constitutes a large pool of carbon for microorganisms, the main agents responsible for soil organic matter decomposition. Cellulolysis occurs as the result of the combined action of fungi and bacteria with different requirements. Earthworms influence decomposition indirectly by affecting microbial population structure and dynamics and also directly because the guts of some species possess cellulolytic activity. Here we assess whether the earthworm Eisenia fetida (Savigny 1826) digests cellulose directly (i.e., with its associated gut microbiota) and also whether the effects of E. fetida on microbial biomass and activity lead to a change in the equilibrium between fungi and bacteria. By enhancing fungal communities, E. fetida would presumably trigger more efficient cellulose decomposition. To evaluate the role of E. fetida in cellulose decomposition, we carried out an experiment in which pig slurry, a microbial-rich substrate, was treated in small-scale vermireactors with and without earthworms. The presence of earthworms in vermireactors significantly increased the rate of cellulose decomposition (0.43 and 0.26% cellulose loss day(-1), with and without earthworms, respectively). However, the direct contribution of E. fetida to degradation of cellulose was not significant, although its presence increased microbial biomass (C(mic)) and enzyme activity (cellulase and beta-glucosidase). Surprisingly, as fungi may be part of the diet of earthworms, the activity of E. fetida triggered fungal growth during vermicomposting. We suggest that this activation is a key step leading to more intense and efficient cellulolysis during vermicomposting of organic wastes.