Nitrogen Fixation and Diazotroph Community in the Subarctic Sea of Japan and Sea of Okhotsk

Abstract

Abstract Nitrogen‐fixing microorganisms (diazotrophs) significantly influence marine productivity by converting N 2 gas into bioavailable nitrogen. Recent studies revealed that nitrogen fixation occurs in both warm oligotroph and colder and/or N‐rich water; however, little is known about the spatial variability of nitrogen fixation activity and diazotroph diversity in cold waters. In this study, we examined the nitrogen fixation activity and diazotroph community structure in the subarctic waters around Hokkaido in northern Japan, including the Sea of Japan, the Sea of Okhotsk, and the North Pacific Ocean. Nitrogen fixation activity was detected at temperatures ranging from −1.1 to 15.6°C and was significantly related to high salinity and high temperature, which characterize the Tsushima Warm Current. The highest recorded nitrogen fixation activity (5.42 nmol N L −1 d −1 ) was comparable to that of subtropical regions. Diazotrophs usually reported in subtropical regions dominated the diazotroph communities and were likely advected from the upstream area of the Tsushima Warm Current—a tributary of the Kuroshio Current. In particular, the nifH abundance of the most dominant diazotroph—the symbiotic cyanobacterium UCYN‐A1—showed a significant positive relationship with nitrogen fixation rate. We also detected a diazotroph community and low but active nitrogen fixation in the N‐rich waters sourced from the Sea of Okhotsk, which infers the influence of both advected and indigenous diazotrophs on the regional nitrogen fixation. Although further research is needed, our study points toward the widespread distribution of diazotrophy in the Sea of Okhotsk.

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