Spatiotemporal heterogeneity in diazotrophic communities reveals novel niche zonation in the East China Sea

Abstract. The East China Sea (ECS) 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 in the ECS 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 in the ECS. An overall spatial heterogeneity among some of the major diazotrophic ecotypes was unveiled, with the filamentous cyanobacteria Trichodesmium and diatom-diazotroph symbioses (Het-1 and Het-2) dominating the upper 30 m of the warm, saline, N-limited offshore water intruded by the Kuroshio and Taiwan warm current, whereas the unicellular cyanobacterial diazotrophs (UCYN-A, UCYN-B and UCYN-C) and the non-cyanobacterial diazotroph (γ-24774A11) extending their distribution further down to 60 m of the Kuroshio surface and subsurface waters. The diazotrophic abundances and nitrogen fixation rates were generally higher in autumn than in spring, 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 five taxon-specific niches occupied by eight distinct diazotrophic ecotypes. 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 in the ECS, which is of important reference in assessing diazotrophs adaptation in a changing marine ecosystem.