Diversity and community composition of root-associated fungi explored by high throughput sequencing

Abstract

Interactions between plant roots and fungi are well known from most terrestrial ecosystems. Mycorrhizal association is the most prominent plant-fungi interaction, where the fungal partners increase the water and nutrient uptake of their host plants. This symbiosis might be especially important in marginal habitats like arctic and alpine environments. The structure, diversity and spatial patterns of the root-associated fungal communities are to a large extent unknown due to previous methodological limitations. The main objective in this thesis was to implement high throughput DNA sequencing to assess the community structure, richness and spatial distribution of root-associated fungal communities in arctic and alpine environments. We focused on one host plant species, namely the ectomycorrhizal herb Bistorta vivipara. Its small and condensed root system enabled us to analyze the entire fungal assemblages associated with individual root systems, using 454 pyrosequencing of ITS1 and/or ITS2 amplicons. All the five studies included in this thesis revealed that the most prominent fungal groups were well-known ectomycorrhizal fungi such as Agaricales, Sebacinales and Thelephorales. Furthermore, ascomycete fungi of the order Helotiales were also recovered frequently across all root systems. Although a high patchiness in fungal community composition generally was observed, some systematic compositional changes along gradients were observed. In a 2x2 m2 local scale study, a spatial autocorrelation was observed at small scales (<0.34 m). Furthermore, a significant compositional difference was observed between the rootassociated fungal communities and adjacent soil fungal communities. Along two primary succession gradients in arctic and alpine areas, a systematic compositional shift was observed. The fungal richness increased along the chronosequences towards the climax vegetation. In a biogeographic survey, where the root-associated fungi were analyzed across Svalbard, a compositional shift was observed that was associated with the latitudinal gradient. Moreover, the fungal richness increased westwards in the more climatic favorable habitats. Overall, the different studies indicate that stochastic processes, possibly related to aerial spore dispersal, are important during fungal community establishment. The conducted studies exemplify that high throughput sequencing is a powerful approach for analyzing complex microbial communities.