Community and abundance of heterotrophic diazotrophs in the northern South China Sea: Revealing the potential importance of a new alphaproteobacterium in N-2 fixation

The present study quantified nifH gene abundances of heterotrophic diazotrophs by qPCR in the northern South China Sea (SCS), a region where primary productivity and new production supported by N-2 fixation are highest in the winter and unicellular N-2-fixers contribute more to nitrogen budget than filamentous N-2-fixers. Vertical distributions and seasonal abundances of four prevailing Proteobacteria, two novel alpha-Proteobacteria (named Alpha 1 and Alpha 2), and two gamma-Proteobacteria (gamma-24774A11 and Gamma 4) were investigated. The Proteobacteria accounted for 77% of the overall nifH clone library. Both Alpha 1 and Alpha 2 were distributed between 0 and 1000 m depths, while gamma-Proteobacteria were concentrated mainly in the upper 200 m. Water-column abundances of Alpha 1, with a maximum of 10(9) copies m(-2), were 1-2 orders of magnitude greater than Alpha 2 or gamma-Proteobacteria. Alpha 1 and gamma-24774A11 were most abundant in the winter and summer, respectively. Together with UCYN-A and UCYN-B, they were the most important N-2-fixers in the northern SCS. Our results suggested that diazotrophic activity of heterotrophic Proteobacteria should be considered an integrated part of nitrogen cycle in oceanic systems in which unicellular diazotrophs prevail over filamentous diazotrophs in N-2 fixation.