Nitrogen fixation by Trichodesmium spp.: An important source of new nitrogen to the tropical and subtropical North Atlantic Ocean

Abstract

[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.

Spatial variation in N<sub>2</sub>-fixation rate and diazotroph activity in the Tropical Atlantic

“…1) using methods described in detail elsewhere (Capone et al, 2005;Voss et al, 2004). In brief, rate measurements were carried out under simulated in situ conditions in shipboard incubators using either isolated colonies of Trichodesmium or unconcentrated water samples collected with a CTD-rosette.…”

“…Areal rates of N 2 fixation by Trichodesmium on all cruises and by small diazotrophs on cruise ME55 were calculated by integrating rate and biomass measurements from different depths through the upper water column as described in detail elsewhere (Capone et al, 2005;Voss et al, 2004). For cruises MP01 and MP03, areal rates were derived for most stations by integrating discrete rate measurements from a series of depths in the upper water column.…”

“…Trichodesmium often forms dense surface blooms of large spatial extent and can clearly make a significant contribution to the local and global budget of oceanic nitrogen (Capone et al, 2005;Karl et al, 2002). Because of its size and prominence at sea, Trichodesmium has received a great deal of attention in both field (Capone et al, 1997;Carpenter, 1992;Capone et al, 2005) and laboratory studies (Ohki et al, 1986;Chen et al, 1996;Mulholland and Capone, 1999;Mulholland et al, 2004). As a result, we now have a much better understanding of the environmental factors and physiological underpinnings of N 2 -fixation by Trichodesmium than we do for any other marine diazotroph.…”

“…As a result, we now have a much better understanding of the environmental factors and physiological underpinnings of N 2 -fixation by Trichodesmium than we do for any other marine diazotroph. Compilation of extensive data from multiple cruises also provides substantial information on the spatial distribution of N 2 -fixation by Trichodesmium within the North Atlantic: Trichodesmium biomass and N 2 -fixation rates both tend to be highest in the western part of the basin (Capone et al, 2005), as is the overall contribution of N 2 -fixation to the water column N budget (Capone et al, 2005;Montoya et al, 2002;McClelland et al, 2003) Despite its clear importance, N 2 fixation by Trichodesmium cannot account for the total rate of N 2 fixation implied by geochemical budgets for the oligotrophic ocean in general or the North Atlantic in particular (Gruber and Sarmiento, 1997;Deutsch et al, 2001;Capone et al, 2005). Other diazotrophs must be making a significant contribution to N 2 -fixation on both a basin and a global scale.…”

Interactions between CCM and N2 fixation in Trichodesmium

“…As a key species in the marine ecosystem, Trichodesmium was used in several studies on the regulation of N 2 fixation (e.g., Berman-Frank et al 2001a, b, 2007Capone et al 2005;Kana 1993;Mulholland et al 2004). Unlike other diazotrophs, this species has evolved special features allowing N 2 fixation to occur during the photoperiod.…”