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Unicellular cyanobacteria fix N2 in the subtropical North Pacific Ocean


Fixed nitrogen (N) often limits the growth of organisms in terrestrial and aquatic biomes1,2, and N availability has been important in controlling the CO2 balance of modern and ancient oceans3,4. The fixation of atmospheric dinitrogen gas (N2) to ammonia is catalysed by nitrogenase and provides a fixed N for N-limited environments2,5. The filamentous cyanobacterium Trichodesmium has been assumed to be the predominant oceanic N2-fixing microorganism since the discovery of N2 fixation in Trichodesmium in 1961 (ref. 6). Attention has recently focused on oceanic N2 fixation because nitrogen availability is generally limiting in many oceans, and attempts to constrain the global atmosphere–ocean fluxes of CO2 are based on basin-scale N balances7,8,9. Biogeochemical studies and models have suggested that total N2-fixation rates may be substantially greater than previously believed7,8 but cannot be reconciled with observed Trichodesmium abundances8,9. It is curious that there are so few known N2-fixing microorganisms in oligotrophic oceans when it is clearly ecologically advantageous. Here we show that there are unicellular cyanobacteria in the open ocean that are expressing nitrogenase, and are abundant enough to potentially have a significant role in N dynamics.

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Figure 1: Expression of nifH genes in water samples collected at the Hawaii Ocean Time-series station ALOHA in May 2000 (HOT cruise 115).
Figure 2: Phylogenetic tree showing the relationships among representative cyanobacterial nifH gene sequences.
Figure 3: Photomicrographs of marine N2-fixing unicellular cyanobacteria.


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We thank the crews and participants of the HOT programme, especially L. Tupas and J. Dore for field support. We also thank L. Campbell for providing flow cytometer data. This work was supported by NSF Division of Ocean Sciences grants to J.P.Z., J.P.M. and D.M.K.

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Correspondence to Jonathan P. Zehr.

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Zehr, J., Waterbury, J., Turner, P. et al. Unicellular cyanobacteria fix N2 in the subtropical North Pacific Ocean. Nature 412, 635–638 (2001).

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