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

Abstract

Biological nitrogen fixation by specialized microorganisms (diazotrophs) reduces N 2 gas into ammonia and is a major source of bioavailable nitrogen in the ocean. The oceanic reservoir of bioavailable nitrogen is mainly controlled by the balance between the gains from nitrogen fixation and the losses from denitrification (Gruber & Galloway, 2008). Based on geochemical estimates, the gain and loss of nitrogen are considered roughly balanced (Brandes & Devol, 2002;Deutsch et al., 2004), while direct measurements identified significantly higher denitrification rates compared with nitrogen fixation (Codispoti, 2007). This discrepancy suggests a significant underestimation of marine nitrogen fixation in the field, which may be partly explained by methodological uncertainty. Recent studies have incorporated a revised dissolution method 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. Plain Language SummaryThe oceanic reservoir of bioavailable nitrogen is mainly controlled by the balance between the gains from nitrogen fixation and the losses from denitrification. However, direct measurements have identified significantly higher denitrification than nitrogen fixation, suggesting the lack of understanding of diazotroph diversity and abundance. Recent studies have shown that diazotrophs occur over a much wider habitat range than previously assumed warm oligotrophic waters. Here, we examined for the first time the nitrogen fixati...

Low Nitrogen Fixation Related to Shallow Nitracline Across the Eastern Indian Ocean

“…Both the first and second PCRs were performed under the same conditions in the following order: 94°C for 2 min, 35 cycles of 94°C for 30 s, 52°C for 1 min, 72°C for 1 min, and finally 72°C for 7 min. Product purification and quantification were done as previously reported (Sato et al., 2021). Products were sequenced using a MiSeq Reagent Kit v3 (600 cycles; Illumina) with a Phix control v3 (Illumina).…”

Global biogeography of N2-fixing microbes:nifHamplicon database and analytics workflow

“…Switching the trimming approach from one based on individual read quality profiles (using truncQ in Table 3) to fixed-length trimming based on overall quality profiles of the forward and reverse reads (using truncLen.fwd and truncLen.rev in Table 2) resulted in more reads being retained for some studies (Sato et al, 2021; Selden et al, 2021; Hallstrøm et al, 2022b; Gradoville et al, 2020). However, fixed-length trimming would have required the selection of trim lengths based on visual, qualitative assessments of hundreds of FASTQ quality plots which is difficult to accomplish in a systematic manner.…”

Heterogeneous Diazotroph Communities in the Subtropical‐Subantarctic Transition and Aphotic Zones Off the Coast of Patagonia, Eastern South Pacific Ocean

“…In the subtropical surface layer, diazotrophic communities were dominated by cyanobacterial diazotrophs including Trichodesmium and UCYN‐A as well as NCDs including Gamma‐A, and were relatively uniform (Chen et al., 2019; Gradoville et al., 2020; Shiozaki et al., 2017). By contrast, communities in the transition region tend to be horizontally heterogenous (Gradoville et al., 2020; Sato et al., 2021; Shiozaki et al., 2017). Studies of diazotrophic community structure in the aphotic zone are limited, and thus the biogeography of diazotrophs in the aphotic zone remains unclear.…”

“…N 2 fixation has long been thought to occur mainly in oligotrophic surface waters in the tropics and subtropics due to its high energy requirements compared to other N assimilation processes (Falkowski, 1983), and because well‐known cyanobacterial diazotrophs prefer high‐temperature regions (Luo et al., 2014). However, recent studies indicate that N 2 fixation also occurs in subtropical‐subarctic transition regions (Gradoville et al., 2020; Sato et al., 2021; Shiozaki et al., 2017), N‐rich environments (Knapp, 2012; Mills et al., 2020), polar regions (Blais et al., 2012; Shiozaki, Fujiwara, et al., 2018, 2020), and aphotic zones (Benavides et al., 2018; Moisander et al., 2017). Moreover, diazotrophs include not only cyanobacteria but also a wide variety of non‐cyanobacteria, which can adapt to various environments due to their diverse metabolic functions (Delmont et al., 2018, 2022; Shiozaki et al., 2023; Turk‐Kubo et al., 2022).…”