Fungivorous nematodes drive microbial diversity and carbon cycling in soil

Background/Question/Methods

Terrestrial biogeochemical cycling is intimately linked with the soil microbiome. Despite this, relatively little is known about the interactions occurring within microbial communities and how these interactions are related to soil functions. Notably, it remains unclear how the structure and function of soil bacterial and fungal communities are influenced by higher trophic levels (e.g., nematodes). In plants and animals, predators can have far reaching impacts on ecosystem biodiversity and function by affecting both prey and non-prey groups. It is unclear if predators can have similar cascading effects in microbial ecosystems. To investigate the effect of nematode fungivory on bacterial and fungal community composition and function, we constructed a simplified food web in a controlled microcosm experiment. We hypothesized that predation on fungi would cascade to affect bacteria by altering the interactions between bacteria and fungi. Our simplified food web consisted of defaunated forest soil colonized by native bacteria and fungi in the presence and absence of a common fungal-feeding nematode, Aphelenchus avenae. We analyzed bacterial and fungal biomass and community composition (via whole community DNA sequencing) as well as biogeochemical parameters important for C and N cycling in ecosystems.

Results/Conclusions

Biogeochemical cycling was stimulated by fungivore presence, as indicated by increases in soil ammonium concentrations and microbial respiration rates. Increased respiration was accompanied by a decrease in microbial carbon use efficiency (CUE). Carbon use efficiency, indicative of the amount of C allocated to biomass accrual versus energy generation, is an important parameter for understanding the fate of soil C. Bacterial and fungal diversity decreased with fungivore presence, and trends in PLFA analysis revealed that fungivory influences bacterial stress biomarkers. Bacterial richness was correlated with CUE, suggesting that predator impacts can cascade through microbial communities and alter the fate of soil C. Taken together, our results suggest cascading effects of fungal trophic interactions on bacterial communities which in turn impact the cycling of soil C and N. This is amongst the first experimental evidence of the cascading impacts of a selective microbial predator on the function of non-prey groups in a microbial context, and further experiments are warranted explore the diverse trophic interactions in microbial systems.