Nitrogen fixation is the primary source of reactive nitrogen in the ocean. Most ecological models do not predict nitrogen fixation in the Antarctic Ocean because of the low availability of iron and high abundance of nitrogen. Here we extensively examined nitrogen fixation in the Antarctic Ocean, and found substantial nitrogen fixation (maximum: 44.4 nmol N l−1 d−1) near the Antarctic coast, especially around ice-covered regions. The nitrogenase gene (nifH) was detected at all coastal stations, including stations where no nitrogen fixation was found. At the stations where nitrogen fixation was detected, the nitrogen-fixing cyanobacterium UCYN-A (Candidatus ‘Atelocyanobacterium thalassa’) dominated nifH gene expression, and the nifH sequence was identical to that of the major oligotype in tropical and subtropical oceans. Our results suggest that marine nitrogen fixation is a ubiquitous process in the global ocean, and that UCYN-A is the keystone species for making it possible.
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We thank the captain, crew and participants of the 60th Japanese Antarctic Research Expedition (JARE-60) for their cooperation at sea and on land. We also thank H. Endo and C. Deutsch for helpful discussions, N. Takeda for help with the nutrient analyses and K. Turk-Kubo for providing the sequence data for UCYN-A. This research was supported financially by JSPS KAKENHI grant numbers JP19H04263 (T.S.), 20H04985 (T.S.), 17H01852 (F.H.) and JP15H05712 (N.H.) and the Simons Foundation (Simons Postdoctoral Fellowship in Marine Microbial Ecology) under award number 544338 (K.I.).
The authors declare no competing interests.
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Extended Data Fig. 2 Nonmetric multidimensional scaling (nMDS) plot derived from a Bray–Curtis distance matrix of the diazotroph community at each sampling station.
Beach (Sta. N and T), fast-ice (Sta. A–C), ice-edge (Sta. D, E, and EL), and ice-free (Sta. BP, CD0, and CD1) areas are shown in yellow, red, green, and blue, respectively.
The major phylotypes in the Southern Ocean (≥10% of total reads for each sample) are shown in red. The nifH sequences from isolated culture are shown in bold. Bootstrap values (> 50%) determined from 1,000 iterations are shown as purple circles, with the area proportional to the value.
Sampling locations, nitrogen fixation, diazotroph community composition and satellite data in the Antarctic Ocean. Background satellite-derived data are available in the UTokyo Repository (http://hdl.handle.net/2261/00079465).
The depth-integrated rate of nitrogen fixation and maximum abundance of UCYN-A in the world ocean.
Nitrate, total inorganic nitrogen/phosphate ratio and UCYN-A in the Antarctic Ocean.
Sequence variant table used for the nMDS analysis.
Sequence data used to construct the phylogenetic tree.
Maximum abundance of UCYN-A in the world ocean.
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Shiozaki, T., Fujiwara, A., Inomura, K. et al. Biological nitrogen fixation detected under Antarctic sea ice. Nat. Geosci. 13, 729–732 (2020). https://doi.org/10.1038/s41561-020-00651-7
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