Table 1.
Basic information about the DNA samples used in this study.
Fig 1.
Map and surface hydrographic features of the stations.
The yellow arrow represents the location and direction of the Kuroshio mainstream.
Table 2.
Environmental parameters of the various sampling locations.
Fig 2.
The stable isotope ratio of nitrogen (δ15N) in particular organic matter (POM) at the different stations.
The samples collected from the deep chlorophyll maximum (DCM) layer are indicated with circles.
Fig 3.
(A). A UPGMA dendrogram showing the relationship between samples, at 0.03 cutoff. (B). Shannon diversity indices and Chao richness estimators of the diazotroph community in the samples, at 0.03 cutoff.
Fig 4.
A maximum likelihood phylogenetic tree constructed with amino acid sequences of the nifH gene.
Representative sequences of the top 24 OTUs and the affiliated reference sequences were used to construct this tree. The accession numbers of each reference sequence are shown in front of the sequence description. Bootstrap resampling was performed 1,000 times; and the values that are higher than 50% are labeled with grey circular symbols on the branches.
Fig 5.
Maximum likelihood phylogenetic trees constructed with nifH DNA sequences of (A) UCYN-A sublineages and (B) the other cyanobacterial diazotrophic phylotypes.
The corresponding accession numbers are displayed in front of the description of the reference sequences. The sequences labeled with “UCYN-A4”, “UCYN-A5” and “UCYN-A6” represent the OTUs of corresponding phylotypes in the NGS based study of Turk-Kubo et al [24], which do not have accession numbers. Bootstrap resampling was performed 1,000 times; and the values that are higher than 50% are labeled with blue circles on the branches. The OTUs identified in the Tokara Strait are in bold, and the OTUs identified from the unpublished data in the upstream region of the KC, are labeled with “upstream KC” and highlighted in blue, green, or red for winter, spring, and summer respectively.
Fig 6.
The relative abundance of the various diazotroph clades.
The cyanobacteria are labeled in shades of green, the Gammaproteobacteria are in blue, the Alphaproteobacteria are in red, the cluster-III diazotrophs are in yellow, and the Firmicutes are in purple. The abundance of the diazotroph clades are shown in (A) the surface and (B) DCM waters of all the stations, as well as (C) for two cDNA samples collected in the surface and DCM layers of ST.3.
Fig 7.
Correlation biplot based on a redundancy analysis (RDA), depicting the relationship between the relative abundances of the cyanobacterial clades and the environmental factors.
The percentage of variance explained by the environmental variables are shown in the brackets.