Root nodules of red alder (Alnus rubra) and Sitka alder (Alnus viridis ssp. sinuata) are inhabited by taxonomically diverse cultivable microbial endophytes

The root nodules of actinorhizal plants are home to nitrogen‐fixing bacterial symbionts, known as Frankia, along with a small percentage of other microorganisms. These include fungal endophytes and non‐Frankia bacteria. The taxonomic and functional diversity of the microbial consortia within these root nodules is not well understood. In this study, we surveyed and analyzed the cultivable, non‐Frankia fungal and bacterial endophytes of root nodules from red and Sitka alder trees that grow together. We examined their taxonomic diversity, co‐occurrence, differences between hosts, and potential functional roles. For the first time, we are reporting numerous fungal endophytes of alder root nodules. These include Sporothrix guttuliformis, Fontanospora sp., Cadophora melinii, an unclassified Cadophora, Ilyonectria destructans, an unclassified Gibberella, Nectria ramulariae, an unclassified Trichoderma, Mycosphaerella tassiana, an unclassified Talaromyces, Coniochaeta sp., and Sistotrema brinkmanii. We are also reporting several bacterial genera for the first time: Collimonas, Psychrobacillus, and Phyllobacterium. Additionally, we are reporting the genus Serratia for the second time, with the first report having been recently published in 2023. Pseudomonas was the most frequently isolated bacterial genus and was found to co‐inhabit individual nodules with both fungi and bacteria. We found that the communities of fungal endophytes differed by host species, while the communities of bacterial endophytes did not. Plants that host nitrogen‐fixing symbionts in their root nodules play a significant role in both ecology and economy. However, the secondary members of these root nodule microbiomes have not been thoroughly studied. Our field survey conducted on Mount St. Helens revealed that both red Alder and Sitka Alder host a diverse range of cultivable microorganisms in their root nodules.