Transportation of Diazotroph Community From the Upstream to Downstream of the Kuroshio

Abstract

The Kuroshio is a strong western boundary current in the North Pacific, in which N-2 fixation is an important source of new nitrogen. In order to understand the dynamics of diazotrophs in the Kuroshio, we used 454-pyrosequencing of nifH gene amplicons to unravel and compare the diazotroph communities along the main path of the Kuroshio (23.44 degrees N, 121.99 degrees E-34.3 degrees N, 140.1 degrees E) with its adjacent waters. The diazotroph communities in the geographically distanced stations along the Kuroshio mainstream were highly similar, while those in the Kuroshio adjacent waters were highly varied, indicated that diazotroph communities were actively transported in the Kuroshio. Along the Kuroshio, the UCYN-A1, UCYN-C, and Gamma-A were rather evenly distributed along the Kuroshio, whereas the UCYN-A2 and Trichodesmium were mainly distributed in the Kuroshio upstream. The diazotroph communities shifted from cyanobacteria dominated in the Kuroshio upstream to proteobacteria (mainly Bradyrhizobium and sulfate-reducing bacteria) dominated in the downstream. Among all cyanobacterial diazotrophs detected, UCYN-A1 was the most sustained group in the Kuroshio with >10(3) gene copies per L and that may explain its extended distribution to higher latitude regions as compared with other cyanobacterial diazotrophs. Considering that the Kuroshio is a hot spot of unicellular cyanobacterial diazotrophs, we have adopted DNA-directed RNA polymerase subunit beta (rpoC1) gene as a biomarker to further explore their phylogeny. Comparing the data set of nifH and rpoC1 gene fragments, we provided new insights into the phylogeny of the recently defined UCYN-B2 and poorly understood UCYN-C groups.