[1] The broad distribution and often high densities of the cyanobacterium Trichodesmium spp. in oligotrophic waters imply a substantial role for this one taxon in the oceanic N cycle of the marine tropics and subtropics. New results from 154 stations on six research cruises in the North Atlantic Ocean show depth-integrated N 2 fixation by Trichodesmium spp. at many stations that equalled or exceeded the estimated vertical flux of NO 3 À into the euphotic zone by diapycnal mixing. Areal rates are consistent with those derived from several indirect geochemical analyses. Direct measurements of N 2 fixation rates by Trichodesmium are also congruent with upper water column N budgets derived from parallel determinations of stable isotope distributions, clearly showing that N 2 fixation by Trichodesmium is a major source of new nitrogen in the tropical North Atlantic. We project a conservative estimate of the annual input of new N into the tropical North Atlantic of at least 1.6 Â 10 12 mol N by Trichodesmium N 2 fixation alone. This input can account for a substantial fraction of the N 2 fixation in the North Atlantic inferred by several of the geochemical approaches.

Fixed nitrogen (N) often limits the growth of organisms in terrestrial and aquatic biomes, and N availability has been important in controlling the CO2 balance of modern and ancient oceans. The fixation of atmospheric dinitrogen gas (N2) to ammonia is catalysed by nitrogenase and provides a fixed N for N-limited environments. 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 balances. Biogeochemical studies and models have suggested that total N2-fixation rates may be substantially greater than previously believed but cannot be reconciled with observed Trichodesmium abundances. 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.

Nitrogen (N2)-fixing microorganisms (diazotrophs) are an important source of biologically available fixed N in terrestrial and aquatic ecosystems and control the productivity of oligotrophic ocean ecosystems. We found that two major groups of unicellular N2-fixing cyanobacteria (UCYN) have distinct spatial distributions that differ from those of Trichodesmium, the N2-fixing cyanobacterium previously considered to be the most important contributor to open-ocean N2 fixation. The distributions and activity of the two UCYN groups were separated as a function of depth, temperature, and water column density structure along an 8000-kilometer transect in the South Pacific Ocean. UCYN group A can be found at high abundances at substantially higher latitudes and deeper in subsurface ocean waters than Trichodesmium. These findings have implications for the geographic extent and magnitude of basin-scale oceanic N2 fixation rates.

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 production. 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 budget. 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.

We describe a simple, precise, and sensitive experimental protocol for direct measurement of N 2 fixation using the conversion of 15 N 2 to organic N. Our protocol greatly reduces the limit of detection for N 2 fixation by taking advantage of the high sensitivity of a modern, multiple-collector isotope ratio mass spectrometer. This instrument allowed measurement of N 2 fixation by natural assemblages of plankton in incubations lasting several hours in the presence of relatively low-level (ca. 10 atom%) tracer additions of 15 N 2 to the ambient pool of N 2. The sensitivity and precision of this tracer method are comparable to or better than those associated with the C 2 H 2 reduction assay. Data obtained in a series of experiments in the Gotland Basin of the Baltic Sea showed excellent agreement between 15 N 2 tracer and C 2 H 2 reduction measurements, with the largest discrepancies between the methods occurring at very low fixation rates. The ratio of C 2 H 2 reduced to N 2 fixed was 4.68 ؎ 0.11 (mean ؎ standard error, n ‫؍‬ 39). In these experiments, the rate of C 2 H 2 reduction was relatively insensitive to assay volume. Our results, the first for planktonic diazotroph populations of the Baltic, confirm the validity of the C 2 H 2 reduction method as a quantitative measure of N 2 fixation in this system. Our 15 N 2 protocols are comparable to standard C 2 H 2 reduction procedures, which should promote use of direct 15 N 2 fixation measurements in other systems.

The fresh water discharged by large rivers such as the Amazon is transported hundreds to thousands of kilometers away from the coast by surface plumes. The nutrients delivered by these river plumes contribute to enhanced primary production in the ocean, and the sinking flux of this new production results in carbon sequestration. Here, we report that the Amazon River plume supports N 2 fixation far from the mouth and provides important pathways for sequestration of atmospheric CO 2 in the western tropical North Atlantic (WTNA). We calculate that the sinking of carbon fixed by diazotrophs in the plume sequesters 1.7 Tmol of C annually, in addition to the sequestration of 0.6 Tmol of C yr ؊1 of the new production supported by NO 3 delivered by the river. These processes revise our current understanding that the tropical North Atlantic is a source of 2. diatom diazotroph associations ͉ nitrogen fixation ͉ new production ͉ river plumes ͉ Richelia D ownward vertical transport of organic carbon produced by phytoplankton, referred to as the biological pump, is a mechanism that transfers carbon from the surface to the deep ocean and regulates atmospheric CO 2 (1). The flux of nitrate (NO 3 ) from deep water to the photic zone can stimulate new phytoplankton production and export (2), but because the upwelling or diffusive flux of NO 3 is accompanied by a corresponding upward flux of CO 2 , its net contribution to removal of carbon from the atmosphere is much reduced. However, the sinking flux due to new production associated with nitrogenous inputs from rivers, atmospheric deposition, and N 2 fixation (diazotrophy), results in the net transport of atmospheric carbon to the deep ocean (3), or ''carbon sequestration'' (4).The Amazon River has the largest discharge of any river and accounts for 18% of all of the riverine input to the oceans. Between May and September, the Amazon plume covers up to 1.3 ϫ 10 6 km 2 with a freshwater lens of salinity Ͻ35 [supporting information (SI) Table S1], which accounts for 20% of the WTNA. Our understanding of the influence of the Amazon River on the carbon cycle in the WTNA has evolved significantly since Ryther et al. (5) first suggested that the Amazon River depressed the productivity of the region influenced by its plume. Several studies have focused on the nutrients delivered by the river to the inner shelf, the subsequent river-supported new production of 0. Fig. 1 and Table S2) complement earlier studies by examining the region of the plume starting 300 km north of the mouth of the river. We classified the stations into three categories based on sea surface salinity (SSS).¶ ¶ The ''low salinity'' group contained all of the stations with SSS Ͻ30. Stations that had SSS between 30 and 35 were classified as ''mesohaline,'' whereas those with SSS Ͼ35 were classified as ''oceanic.'' Surface NO 3 concentrations were below detection at most stations, with the highest value of 0.50 M recorded at the station with the lowest salinity of 24. DeMaster and Pope (7) found when plotting NO 3 vs...

Deep-water nitrate is a major reservoir of oceanic combined nitrogen and has long been considered to be the major source of new nitrogen supporting primary production in the oligotrophic ocean.15 N : 14 N ratios in plankton provide an integrative record of the nitrogen cycle processes at work in the ocean, and near-surface organic matter in oligotrophic waters like the Sargasso Sea is characterized by an unusually low 15 N content relative to average deep-water nitrate. Herein we show that the low ␦ 15 N of suspended particles and zooplankton from the tropical North Atlantic cannot arise through isotopic fractionation associated with nutrient uptake and food web processes but are instead consistent with a significant input of new nitrogen to the upper water column by N 2 fixation. These results provide direct, integrative evidence that N 2 fixation makes a major contribution to the nitrogen budget of the oligotrophic North Atlantic Ocean.Recent biological and geochemical studies have produced greatly increased estimates of the importance of biological N 2 fixation in supporting new production in oligotrophic areas of the ocean (Carpenter and

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