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

Subtropical‐subpolar transition water is a potential domain for N2 fixation, but the understanding of N2 fixation in such waters remains incomplete. We simultaneously examined the N2 fixation activity and community structures of diazotrophs and all prokaryotes from the surface to just above the seafloor off Patagonia in the transitional region of the eastern South Pacific Ocean. N2 fixation activity was not detected in the surface waters, but was observed sporadically and only in subpolar bathypelagic waters (>1,000 m) at very low rates (0.02–0.06 nmol N L−1 d−1). By contrast, the nifH gene, a key gene involved in N2 fixation, was detected widely from the surface to the bottom waters. The majority of diazotrophs were classified as non‐cyanobacterial diazotrophs (NCDs), and the nifH amino acid sequences of major diazotrophs were similar to sequences detected in the Southern Ocean, the aphotic zone and sediment of other oceans, and estuarine waters, suggesting that the NCDs are distributed across diverse marine environments. The overall prokaryotic communities were generally similar to those in other open ocean regions at the phylum level (class level for Proteobacteria) and differed among water depths. Diazotrophs, in contrast, showed vertical and horizontal heterogeneity below the euphotic zone and little association with water depth, indicating a lack of cohesion within the community, which may characterize diazotroph community in the transitional surface water and aphotic zones. Elucidating this community heterogeneity may provide pivotal information about N2 fixation in these waters. Plain Language Summary: Marine N2 fixation is the main source of nitrogen to the oceans but was generally considered a local process that occurs primarily in the surface layers of tropical and subtropical oligotrophic waters. However, recent studies shows that N2 fixation occurs in diverse environments, including polar regions. The subtropical‐subpolar transition region has recently been recognized as a region where N2 fixation occurs, but studies of diazotrophy in this region have only just begun and information remains incomplete. In this study, diazotrophy in the transition region off Patagonia was examined throughout the water column. N2 fixation activity was sporadic in the deep waters of this region. By contrast, diazotrophs were detected from the surface to just above the seafloor. Unlike other prokaryotic communities, diazotrophic communities had heterogenous vertical and horizontal distribution below the euphotic zone. The heterogeneity of diazotrophs may be key to understanding N2 fixation in transitional surface and aphotic waters. Key Points: N2 fixation activity was detected sporadically and only in subpolar bathypelagic waters off western PatagoniaDiazotrophs were nearly omnipresent from the surface to just above the seafloorIn contrast to other prokaryotes, the vertical and horizontal distributions of diazotrophs were heterogeneous [ABSTRACT FROM AUTHOR]