Direct and indirect effects of fungi and oomycetes on leaf litter degradation by freshwater macroinvertebrates

The interaction of microbial colonisation with leaf litter breakdown by the shredder Gammarus roeselii was studied in the littoral of large Lake Constance. In a first step we studied selective feeding of gammarids on leaf litter from three different treatments. Littoral exposed, tap water exposed, and autoclaved leaves were offered to G. roeselii in standard food-choice assays under laboratory conditions. We found highly selective feeding on littoral exposed leaf litter indicating that microbial conditioning is affecting the shredders feeding behaviour. Neither C, N, and P nor protein nor ergosterol content were positively correlated with the relative consumption rates of littoral exposed leaves. For a more detailed study of the microbial conditioning of the leaves oomycetes and fungi were isolated from the littoral exposed leaves. Based on the sequence of internal transcribed spacer (ITS) regions of rDNA, we identified single strains of Fusarium sporotrichoides, Microdochium sp., Ascomycete sp., and Cylindrocladiella parva; two strains of Cylindrocarpon sp.; and three strains of Pythium sp. Subsequently food choice assays were conducted using the isolated strains by offering autoclaved leaves and leaves colonised with a single strain. Three strains (C. parva, Cylindrocarpon sp. 94-2057 and Ascomycete sp.) were preferred by G. roeselii, while leaves with F. sporotrichoides were avoided. Leaves with each of the three oomycete isolates (Pythium sp.) were neither preferred nor rejected by G. roeselii. Our results suggest that the selective feeding behaviour of G. roeselii is affected by the relative abundance of specific fungal strains. Thus, whether microbial conditioning acts as a repellent or as an attractor for shredders depends on the microbial community on the leaves.