| Nitrogen plays a key role in marine biogeochemistry,and the transformation of nitrogen depends on the cooperation of complex microbial networks.As important components in microbial networks,diazotrophs can convert nitrogen to bioavailable nitrogen,playing a significant role in balancing the nitrogen budget in the global ocean.The South China Sea(SCS)and western Pacific Ocean(WPO)are two areas,which contain both eutrophic and oligotrophic region.Thus,diazotrophs encounter different environmental conditions in these areas.Meanwhile,the large-scale areas may bring geographical limitation for diazotrophic dispersal,resulting in distinct biogeographic pattern.Furthermore,as two interconnected areas,how these diazotrophs distributed here and why they formed this distribution pattern is still a mystery.Although the diazotrophic flora and its nitrogen fixations in this area have been studied for many years,previous studies focused mostly on local areas,such as the Pearl River estuary.Therefore,we attempt to reveal the overall biogeographic of diazotrophs in the SCS and WPO.In this research,we also try to disentangle whether the environmental filtering or dispersal limitation dominating the community assembly process in the diazotrophic community.We collected molecular samples from various habitats including estuaries,upwellings,mesoscale eddies,ocean currents,oligotrophic sea and ocean from three voyages in SCS and WPO.The High throughput sequencing based on nifH gene and real-time fluorescence quantitative PCR were used to investigated the biogeographic diversity of diazotrophs.In addition,we investigated the diversity of the unicellular cyanobacterium Candidatus atelocyanobacterium thalassa(UCYN-A),which were neglected for many years in these associated areas The main results are as follows:(1)Cyanobacteria and Proteobacteria were dominant diazotrophs in SCS and WPO.We observed Trichodesmium bloom at Halmahera Eddy where the nifH gene copes were a thousand times more than non-bloom stations.As another predominant diazotroph,Sagittula castanea prevailed in oligotrophic ocean,especially in non-Trichodesmium-bloom stations.γ-24774A11 existed extensively in various habitats with relative lower abundance.(2)The spatial heterogeneity of diazotrophic communities were observed in SCS and WPO,reflecting the overlap or differentiation of diazotrophic niches.Except for Trichodesmium and Sagittula castanea,UCYN-B were also dominant diazotrophs in WPO with the average abundance 126 times higher that in SCS.Trichodesmium and UCYN-B present significant negative correlations,indicating the overlapping ecological niches between each other,such as the competition for nutrients,iron and light utilization.We also observed low abundances of UCYN-C(even undetectable in the coastal sea).The UCYN-A had obvious spatial heterogeneity:UCYN-A2 tended to distribute in the coastal region,while UCYN-A1 preferred oligotrophic ocean.According to the correlations among diffemet sublineages,it could be inferred that UCYN-A1 coexisted with UCYN-A3,while UCYN-A2 were exclusive with above sublineages,and UCYN-A4 may coexist with UCYN-A3.(3)Both environmental filtering and dispersal limitation exerted important effects on the assembly of diazotrophic communities.The dispersal limitation in SCS was higher than that in WPO,probably arising from stronger transportation of ocean currents in WPO.The dispersal limitation of Cyanobacteria was higher than that of Proteobacteria,indicating that small-scale microbiomes are more easily dispersed due to random dispersal,while the dispersal rates of large-scale microbiomes such as Trichodesmium and diatom symbiont were weak.In addition,we found that the distribution of UCYN-A fitted the neutral model,implying the importance of stochastic processes for diazotrophic community assembly.Interestingly,UCYN-A 1,UCYN-A2 and UCYN-A3 fitted the neutral model with the exception of UCYN-A4,suggesting that environmental filtering exerted different selection pressures on different sublineages or the hosts of UCYN-A4 were ecologically different with the hosts of other sublineages.In general,temperature,ammonium and chlorophyll a were primary environmental factors affecting the dynamics of UCYN-A. |
