Mixotrophic cyanobacteria are critical active diazotrophs in polychlorinated biphenyl-contaminated paddy soil

Biological nitrogen fixation by diazotrophs is a crucial biogeochemical process in global terrestrial ecosystems, especially in nitrogen-limited, organic-contaminated soils. The metabolic activities of diazotrophs and their ability to supply fixed nitrogen may facilitate the transformation of organic pollutants. However, the active diazotrophic communities in organic-contaminated soils and their potential metabolic functions have received little attention. In the current study, the relationship between biological nitrogen fixation and polychlorinated biphenyl (PCB) metabolism was analyzed <i>in situ</i> in paddy soil contaminated with a representative tetrachlorobiphenyl (PCB52). ¹⁵N-DNA stable isotope probing was combined with high-throughput sequencing to identify active diazotrophs, which were distributed in 14 phyla, predominantly <i>Cyanobacteria</i> (23.40%). Subsequent metagenome binning and functional gene mining revealed that some mixotrophic cyanobacteria (e.g. FACHB-36 and <i>Cylindrospermum</i>) contain essential genes for nitrogen fixation, PCB metabolism, and photosynthesis. The bifunctionality of <i>Cylindrospermum</i> sp. in nitrogen fixation and PCB metabolism was further confirmed by metabolite analyses of <i>Cylindrospermum</i> sp. from a culture collection as a representative species, which showed that <i>Cylindrospermum</i> sp. metabolized PCB and produced 2-chlorobiphenyl and 2,5-dihydroxybenzonic acid. Collectively, these findings indicate that active diazotrophs, particularly mixotrophic cyanobacteria, have important ecological remediation functions and are a promising nature-based <i>in situ</i> remediation solution for organic-contaminated environments.