Oxygen-tolerant nitrogen fixation in a marine alga-colonizing <i>Planctomycetota</i>

<i>Planctomycetota</i> are abundant colonizers of macroalgal surfaces, yet their role in nitrogen fixation has remained unresolved despite genomic evidence of nitrogenase (nif) genes. Until now, no functional validation of nitrogen fixation in pure-cultured <i>Planctomycetota</i> has been reported. Here, we isolated <i>Crateriforma</i> sp. HD03 from kelp and for the first time demonstrated its ability to fix nitrogen in pure culture, confirming this key metabolic potential in marine <i>Planctomycetota</i>. Strain HD03 overcomes oxygen stress through a combination of biofilm formation and diurnal regulation of nifH expression, allowing nitrogen fixation under aerobic conditions to cope with the algal environment's oxic nature. Furthermore, genomic surveys revealed nitrogen fixation gene clusters across multiple <i>Planctomycetota</i> clades, suggesting widespread nitrogen-fixing capability in this phylum. Collectively, these findings identify <i>Planctomycetota</i> as important nitrogen providers in the ocean.