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Nitrate uptake by heterotrophic bacteria plays an important role in marine N cycling. However, few studies have investigated the diversity of environmental nitrate assimilating bacteria (NAB). In this study, the diversity and biogeographical distribution of NAB in several global oceans and particularly in the western Pacific marginal seas were investigated using both cultivation and culture-independent molecular approaches. Phylogenetic analyses based on 16S rRNA and nasA (encoding the large subunit of the assimilatory nitrate reductase) gene sequences indicated that the cultivable NAB in South China Sea belonged to the α- Proteobacteria , γ- Proteobacteria  and CFB ( Cytophaga-Flavobacteria-Bacteroides ) bacterial groups. In all the environmental samples of the present study, α- Proteobacteria , γ- Proteobacteria  and  Bacteroidetes  were found to be the dominant  nasA -harboring bacteria. Almost all of the α- Proteobacteria  OTUs were classified into three  Roseobacter -like groups (I to III). Clone library analysis revealed previously underestimated  nasA  diversity; e.g. the  nasA  gene sequences affiliated with β- Proteobacteria , ε- Proteobacteria  and  Lentisphaerae  were observed in the field investigation for the first time, to the best of our knowledge. The geographical and vertical distributions of seawater  nasA -harboring bacteria indicated that NAB were highly diverse and ubiquitously distributed in the studied marginal seas and world oceans. Niche adaptation and separation and/or limited dispersal might mediate the NAB composition and community structure in different water bodies. In the shallow-water Kueishantao hydrothermal vent environment, chemolithoautotrophic sulfur-oxidizing bacteria were the primary NAB, indicating a unique nitrate-assimilating community in this extreme environment. In the coastal water of the East China Sea, the relative abundance of  Alteromonas  and  Roseobacter -like  nasA  gene sequences responded closely to algal blooms, indicating that NAB may be active participants contributing to the bloom dynamics. Our statistical results suggested that salinity, temperature and nitrate may be some of the key environmental factors controlling the composition and dynamics of the marine NAB communities.

Ubiquity and Diversity of Heterotrophic Bacterial nasA Genes in Diverse Marine Environments