Articles | Volume 12, issue 15
Research article
07 Aug 2015
Research article |  | 07 Aug 2015

Nitrogen fixation and the diazotroph community in the temperate coastal region of the northwestern North Pacific

T. Shiozaki, T. Nagata, M. Ijichi, and K. Furuya

Abstract. Nitrogen fixation in temperate oceans is a potentially important, but poorly understood process that may influence the marine nitrogen budget. This study determined seasonal variations in nitrogen fixation and the diazotroph community within the euphotic zone in the temperate coastal region of the northwestern North Pacific. Nitrogen fixation as high as 13.6 nmol N L−1 d−1 was measured from early summer to fall when the surface temperature exceeded 14.2 °C (but was lower than 24.3 °C) and the surface nitrate concentration was low (≤ 0.30 μM), although we also detected nitrogen fixation in subsurface layers (42–62 m) where nitrate concentrations were high (> 1 μM). Clone library analysis results indicated that nifH gene sequences were omnipresent throughout the investigation period. During the period when nitrogen fixation was detected (early summer to fall), the genes affiliated with UCYN-A, Trichodesmium, and γ-proteobacterial phylotype γ-24774A11 were frequently recovered. In contrast, when nitrogen fixation was undetectable (winter to spring), many sequences affiliated with Cluster III diazotrophs (putative anaerobic bacteria) were recovered. Quantitative PCR analysis revealed that UCYN-A was relatively abundant from early to late summer compared with Trichodesmium and γ-24774A11, whereas Trichodesmium abundance was the highest among the three groups during fall.

Short summary
We found that nitrogen fixation can proceed at high rates, depending on the season, in the temperate coastal region of the northwestern North Pacific. nifH sequences were omnipresent and recovered throughout the year. UCYN-A was a major diazotroph during summer, whereas Trichodesmium was abundant during fall, despite low temperatures. The Cluster III diazotrophs, putative anaerobic bacteria, were found at the surface throughout the year.
Final-revised paper