How Does Salinity Shape Bacterial and Fungal Microbiomes of Alnus glutinosa Roots?

Abstract

Black alder (<i>Alnus glutinosa</i> Gaertn.) belongs to dual mycorrhizal trees, forming ectomycorrhizal (EM) and arbuscular (AM) root structures, as well as represents actinorrhizal plants that associate with nitrogen-fixing actinomycete <i>Frankia</i> sp. We hypothesized that the unique ternary structure of symbionts can influence community structure of other plant-associated microorganisms (bacterial and fungal endophytes), particularly under seasonally changing salinity in <i>A. glutinosa</i> roots. In our study we analyzed black alder root bacterial and fungal microbiome present at two forest test sites (saline and non-saline) in two different seasons (spring and fall). The dominant type of root microsymbionts of alder were ectomycorrhizal fungi, whose distribution depended on site (salinity): <i>Tomentella</i>, <i>Lactarius</i>, and <i>Phialocephala</i> were more abundant at the saline site. <i>Mortierella</i> and <i>Naucoria</i> (representatives of saprotrophs or endophytes) displayed the opposite tendency. Arbuscular mycorrhizal fungi belonged to Glomeromycota (orders Paraglomales and Glomales), however, they represented less than 1% of all identified fungi. Bacterial community structure depended on test site but not on season. Sequences affiliated with <i>Rhodanobacter</i>, <i>Granulicella</i>, and <i>Sphingomonas</i> dominated at the saline site, while <i>Bradyrhizobium</i> and <i>Rhizobium</i> were more abundant at the non-saline site. Moreover, genus <i>Frankia</i> was observed only at the saline site. In conclusion, bacterial and fungal community structure of alder root microsymbionts and endophytes depends on five soil chemical parameters: salinity, phosphorus, pH, saturation percentage (SP) as well as total organic carbon (TOC), and seasonality does not appear to be an important factor shaping microbial communities. Ectomycorrhizal fungi are the most abundant symbionts of mature alders growing in saline soils. However, specific distribution of nitrogen-fixing <i>Frankia</i> (forming root nodules) and association of arbuscular fungi at early stages of plant development should be taken into account in further studies.