Spatiotemporal heterogeneity in diazotrophic communities reveals novel niche zonation on the continental shelf of the East China Sea

Abstract. The East China Sea (ECS) continental shelf is a hotspot for studying nitrogen fixation in the marginal seas of the western Pacific, where this microbially mediated process is profoundly influenced by both the coastal and oceanic current systems. Yet, how physical forcing controls the biogeography of diazotrophs and regional nitrogen budget on the ECS shelf remains poorly characterized. Here, we carried out a cross-season survey and demonstrated dynamics in diazotrophic communities that is tightly linked to distinct water masses on the ECS shelf. An overall spatial heterogeneity among some of the major diazotrophic phylotypes was unveiled, with the filamentous cyanobacteria Trichodesmium, diatom-diazotroph symbioses (Het-1 and Het-2), the unicellular cyanobacterial diazotrophs (UCYN-B) and Haptophyta-associated nitroplasts (UCYN-A) dominating the upper 30 m of the warm, nitrogen-limited offshore region intruded by the Kuroshio and Taiwan Strait water, whereas diatom-associated putative nitroplasts (UCYN-C and γ-24774A11) were abundant both at the surface and 50 m depth. The nitrogen fixation rates were generally higher in autumn than in spring, particularly in Kuroshio-affected waters dominated by Trichodesmium, Het-1 and Het-2, suggesting a seasonal variability primarily regulated by hydrographic conditions (mainly temperature and salinity) associated with water mass movement. Modeling the distribution of diazotrophs in the water masses identified three taxon-specific niches occupied by eight distinct diazotrophic phylotypes. Taken together, our analyses provide mechanistic insights into the role of dominant forms of physical forcing in driving the spatiotemporal variability in diazotrophic distribution and activity on the ECS shelf, which is of important reference in assessing diazotrophs adaptation in a changing marine ecosystem.