The first assessment of cyanobacterial and diazotrophic diversities in the Japan Sea

The diversity of cyanobacteria and diazotrophs in the Japan Sea was investigated by analyzing sequences of cyanobacterial 16S rRNA genes and nitrogen fixation genes ( nifH ) from seawater sampled at depths ranging from the surface to 100 m at two stations. Of the 107 cyanobacterial 16S rRNA gene sequences obtained, 97 and three sequences were assigned to Synechococcus sub-cluster 5.1 and Prochlorococcus HL (II), respectively. Unlike other oceanic regions, at our two sampling stations the composition ratio of the sequences assignable to Synechococcus sub-cluster 5.3 was relatively high (8 %). No sequences of diazotrophic cyanobacteria were found in the cyanobacterial 16S rRNA genes. In the nifH clone library (36 sequences), ten sequences were identified as a UCYN-A group of diazotrophic cyanobacteria; the other 26 sequences (72 %) were assigned to proteobacteria. These results suggest that heterotrophic bacteria, including UCYN-A, dominate the diazotrophic community in the Japan Sea. Our study reveals the dominance of Synechococcus in cyanobacterial community and (photo)heterotrophic diazotrophs in the diazotrophic community in the Japan Sea, suggesting its unique characteristics.