Amazon River enhances diazotrophy and carbon sequestration in the tropical North Atlantic Ocean

Abstract

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...

A novel cohabitation between two diazotrophic cyanobacteria in the oligotrophic ocean

“…(Foster et al, , 2011, respectively. These diatom-diazotroph associations are well documented and globally distributed through the oligotrophic ocean and are thought to have an important role in carbon sequestration (Venrick, 1974;Janson et al, 1999;Subramaniam et al, 2008;Karl et al, 2012;Villareal et al, 2012).…”

Metatranscriptomics of N2-fixing cyanobacteria in the Amazon River plume

“…As the highnutrient riverine water mixes with oligotrophic oceanic waters, NO 3 À and NO 2 À are rapidly taken up by microbial communities dominated by coastal diatoms (Shipe et al, 2007;Subramaniam et al, 2008;Goes et al, 2014). Further along the mixing gradient, some nutrients (Si, P and Fe) persist in relatively high concentrations, but N is depleted, providing an advantage to the diazotrophs (Foster et al, 2007;Shipe et al, 2007;Subramaniam et al, 2008;Goes et al, 2014).…”

“…As the highnutrient riverine water mixes with oligotrophic oceanic waters, NO 3 À and NO 2 À are rapidly taken up by microbial communities dominated by coastal diatoms (Shipe et al, 2007;Subramaniam et al, 2008;Goes et al, 2014). Further along the mixing gradient, some nutrients (Si, P and Fe) persist in relatively high concentrations, but N is depleted, providing an advantage to the diazotrophs (Foster et al, 2007;Shipe et al, 2007;Subramaniam et al, 2008;Goes et al, 2014). The cyanobacterium Richelia, located within the cell wall of the diatom Hemiaulus, is the most abundant N 2 fixer in transitional waters (30-35 PSU (practical salinity unit)), whereas the colony-forming, filamentous Trichodesmium is the dominant diazotroph in more oceanic waters (435 PSU) (Carpenter et al, 1999;Subramaniam et al, 2008).…”

“…Further along the mixing gradient, some nutrients (Si, P and Fe) persist in relatively high concentrations, but N is depleted, providing an advantage to the diazotrophs (Foster et al, 2007;Shipe et al, 2007;Subramaniam et al, 2008;Goes et al, 2014). The cyanobacterium Richelia, located within the cell wall of the diatom Hemiaulus, is the most abundant N 2 fixer in transitional waters (30-35 PSU (practical salinity unit)), whereas the colony-forming, filamentous Trichodesmium is the dominant diazotroph in more oceanic waters (435 PSU) (Carpenter et al, 1999;Subramaniam et al, 2008). The free-living unicellular cyanobacterium Crocosphaera, the picoeukaryotic alga-associated UCYN-A and Richelia associated with the diatom Rhizosolenia have also been detected in and around the Amazon River plume (Foster et al, 2007;Goebel et al, 2010).…”

“…A bloom of Richelia-harboring Hemiaulus in transitional waters, accompanied by Trichodesmium, accounted for an estimated input of nearly 0.5 Tg N to the surface community over just a 10-day period (Carpenter et al, 1999). Another study found that the particulate export at transitional stations was dominated by Richelia-Hemiaulus associations that were estimated to be responsible for the sequestration of 20 Tg Carbon (C) to the deep ocean annually (Subramaniam et al, 2008). These studies show the significance of the Amazon River plume diazotroph community, as a whole, but provide little information about the organisms that comprise the populations within that community.…”