A Bradyrhizobium isolate from a marine diatom induces nitrogen-fixing nodules in a terrestrial legume

Biological nitrogen fixation converts atmospheric nitrogen into ammonia, essential to the global nitrogen cycle. While cyanobacterial diazotrophs are well characterized, recent studies have revealed a broad distribution of non-cyanobacterial diazotrophs (NCDs) in marine environments, although their study is limited by poor cultivability. Here we report a previously uncharacterized Bradyrhizobium isolated from the marine diatom Phaeodactylum tricornutum. Phylogenomic analysis places the strain within photosynthetic Bradyrhizobium, suggesting evolutionary adaptations to marine and terrestrial niches. Average nucleotide identity supports its classification as a previously undescribed species. Remarkably, inoculation experiments showed that the isolate induced nitrogen-fixing nodules in the Aeschynomene indica legume, pointing to symbiotic capabilities across ecological boundaries. Pangenome analysis and metabolic predictions indicate that this isolate shares more features with terrestrial photosynthetic Bradyrhizobium than with marine NCDs. Overall, these findings suggest that symbiotic interactions could evolve across different ecological niches, and raise questions about the evolution of nitrogen fixation and microbe–host interactions. A non-cyanobacterial diazotroph previously isolated from a marine eukaryotic phytoplankton triggers formation of nitrogen-fixing root nodules when inoculated onto a terrestrial legume.