Abstract
Intimate fungal-bacterial interactions are widespread in nature. However the main drivers for the selection of hyphae-associated bacterial communities and their functional traits in soil systems remain elusive. In the present study, baiting microcosms were used to recover hyphae-associated bacteria from two Penicillium species with different phosphorus-solubilizing capacities in five types of soils. Based on amplicon sequencing of 16S rRNA genes, the composition of bacterial communities associated with Penicillium hyphae differed significantly from the soil communities, showing a lower diversity and less variation in taxonomic structure. Furthermore, soil origin had a significant effect on hyphae-associated community composition, whereas the two fungal species used in this study had no significant overall impact on bacterial community structure, despite their different capacities to solubilize phosphorus. However, discriminative taxa and specific OTUs were enriched in hyphae-associated communities of individual Penicillium species indicating that each hyphosphere represented a unique niche for bacterial colonization. Additionally, an increased potential of phosphorus cycling was found in hyphae-associated communities, especially for the gene phnK involved in phosphonate degradation. Altogether, it was established that the two Penicillium hyphae represent unique niches in which microbiome assemblage and phosphorus cycling potential are mainly driven by soil origin, with less impact made by fungal identity with a divergent capacity to utilize phosphorus.
Highlights
In the dynamic soil environment, highly diverse bacterial and fungal communities co-exist (Nazir et al, 2013)
Ballhausen et al (2015) found differences between bacteria adhering to hyphae of Trichoderma, Mucor, and Rhizoctonia exposed to bacteria extracted from rhizosphere soil, while weaker effects of the identity of four arbuscular mycorrhizal fungi (AMF) on hyphae-associated bacterial communities have been observed using a related approach (Scheublin et al, 2010)
The presence of P. canescens (Pc) and P. janthinellum (Pj) hyphae on the surface of cover slips was observed during incubation lasting 8 days in S1 microcosms (Figure 1A and Supplementary Figure S1)
Summary
In the dynamic soil environment, highly diverse bacterial and fungal communities co-exist (Nazir et al, 2013). Approaches to identify bacteria enriched by the presence of fungal hyphae include studies of soil affected by hyphae compared to hyphae-free soils (Nazir et al, 2010, 2013; Nuccio et al, 2013; Wang et al, 2016), as well as baiting approaches for the identification of bacteria colonizing fungal hyphae in laboratory (Scheublin et al, 2010) or close-to-natural soil conditions (Ghodsalavi et al, 2017) According to these studies, environmental differentiation between the hyphosphere and the bulk soil leads to different compositions of hyphaeassociated and bulk soil bacterial communities, with genera from β-Proteobacteria, γ-Proteobacteria, and Firmicutes commonly enriched in the hyphosphere or soil influenced by fungal hyphae. Abiotic factors of the soil environment could be important for shaping fungal-associated bacterial communities (Filonow and Arora, 1987; Rousk et al, 2009), as shown for ectomycorrhizal roots where soil pH, soil carbon (C), nitrogen (N) and phosphorus (P) content as well as C:N ratios have a strong effect on bacterial communities (Simon et al, 2016; Marupakula et al, 2017)
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