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High rates of N2 fixation by unicellular diazotrophs in the oligotrophic Pacific Ocean


The availability of nitrogen is important in regulating biological productivity in marine environments. Deepwater nitrate has long been considered the major source of new nitrogen supporting primary production in oligotrophic regions of the open ocean, but recent studies have showed that biological N2 fixation has a critical role in supporting oceanic new production1,2,3,4,5,6,7. Large colonial cyanobacteria in the genus Trichodesmium and the heterocystous endosymbiont Richelia have traditionally been considered the dominant marine N2 fixers, but unicellular diazotrophic cyanobacteria and bacterioplankton have recently been found in the picoplankton and nanoplankton community of the North Pacific central gyre, and a variety of molecular and isotopic evidence suggests that these unicells could make a major contribution to the oceanic N budget8. Here we report rates of N2 fixation by these small, previously overlooked diazotrophs that, although spatially variable, can equal or exceed the rate of N2 fixation reported for larger, more obvious organisms. Direct measurements of 15N2 fixation by small diazotrophs in various parts of the Pacific Ocean, including the waters off Hawaii where the unicellular diazotrophs were first characterized, show that N2 fixation by unicellular diazotrophs can support a significant fraction of total new production in oligotrophic waters.

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Figure 1: Volumetric rate of N2 fixation by unicellular organisms in the North Pacific Ocean and near Australia.
Figure 2: Time course of N2 fixation in incubations under simulated in situ conditions.
Figure 3: Summary of rate measurements as a function of depth at stations along a transect in the eastern subtropical Pacific from Hawaii to San Diego (cruise Cook-25).


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We thank D. Karl and the entire HOT team for support in Hawaii; A. Gibson for collecting samples for molecular characterization on cruise Cook-25; C. Payne, K. Rathbun, S. Patel, K. Ghanouni, P. Davoodi and G. Stewart for their assistance in the laboratory analyses; and the captains and crews of the RV Ewing and RV Melville for their assistance at sea. This research was supported by grants from the National Science Foundation.

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Correspondence to Joseph P. Montoya.

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Montoya, J., Holl, C., Zehr, J. et al. High rates of N2 fixation by unicellular diazotrophs in the oligotrophic Pacific Ocean. Nature 430, 1027–1031 (2004).

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