Frankia assemblages associated with alnus rubra and alnus viridis are strongly influenced by host species identity

This study examined the effect of host species identity on the structure of Alnus-associated Frankia bacterial assemblages in the Pacific Northwest, United States, using two approaches. First, Frankia in nodules were sampled from six stands of Alnus rubra or Alnus viridis. Second, a bioassay was conducted where A. rubra and A. viridis seedlings were grown in different soils collected from these two hosts. Frankia genotypes were characterized with nifH sequences and bacterial assemblages were compared using taxon- and divergencebased analyses. Strong host associations were evident in the field; the dominant Frankia genotypes showed significant associations with either A. rubra or A. viridis, and there were host-associated groupings at the assemblage level as well. In the bioassay, host associations among Frankia genotypes were evident but less pronounced, reflecting an interaction between host species and other factors. Although nodule abundance varied among bioassay treatments, seedling dry mass was not strongly correlated with either nodule quantity or soil chemistry. Collectively, our observation and experimental results indicate that host identity is a major factor influencing the genotype composition and abundance of Alnus-associated Frankia assemblages.