Biogeochemical signatures of nitrogen fixation in the eastern North Atlantic

Abstract

Stable nitrogen isotopic determination of particulate organic matter over the eastern North Atlantic in spring 2000 reveal a region of low natural abundance of 15N relative to 14N between 26°N and 32°N along 20°W. This light isotopic signal, together with phytopigment data and persistently elevated nitrate to phosphate ratios in the upper thermocline, suggest that nitrogen fixation provides a local dominant supply of nitrogen to phytoplankton over part of the eastern North Atlantic. These independent biogeochemical proxies are coincident with a region of enhanced atmospheric dust deposition, as suggested by an atmospheric transport model. Hence, the atmospheric dust events may spatially and temporally constrain the distribution of N2 fixers.

N<sub>2</sub> fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)

“…New N input by N 2 fixation has thus been recognized as a significant process influencing global oceanic productivity, and can eventually fuel CO 2 sequestration through the N 2 -primed prokaryotic carbon (C) pump . Low δ 15 N signatures of particles from sediment traps in the tropical North Pacific (Karl et al, 1997Scharek et al, 1999a, b) and Atlantic (Altabet, 1988;Bourbonnais et al, 2009;Knapp et al, 2005;Mahaffey et al, 2003) suggest that at least part of the recently fixed N is ultimately exported out of the photic zone. Knapp et al (2008) and Bourbonnais et al (2009) also observed a low δ 15 N of NO − 3 (relative to δ 18 O-NO − 3 ) in surface waters in the western and eastern subtropical Atlantic Ocean, supporting the role of N 2 fixers in these regions.…”

Spatial variability in Florida Bay particulate organic matter composition: combining flow cytometry with stable isotope analyses

“…Isotopic depletion during a cyanobacterial bloom opposes the expectation that high primary productivity and surface nutrient utilization induces isotopic enrichment, as has been reported in many studies of oceanic and lacustrine POM (Ostrom et al, 1997;Altabet et al, 1999;Teranes & Bernasconi, 2000;Schubert & Calvert, 2001). Here, the 0.1-50 lm POM had d 15 N values characteristic of nitrogen fixing bacteria/cyanobacteria, which are habitually near 0& in terms of d 15 N, reflecting the isotopic signature of atmospheric nitrogen (Gu & Alexander, 1993;Liu et al, 1996;Mahaffey et al, 2003). This 15 N-depletion may suggest that nitrogen fixation represents a significant nutrient source for this population, which is consistent with several studies of potential aerobic N-fixation in Synechococcus species, including S. elongatus (Duerr, 1981;Phlips et al, 1989;Luo & Matsui, 1994;Phlips & Badylack, 1996;Church, 2005).…”

Biogeochemical budgets in the eastern boundary current system of the North Atlantic: Evidence of net heterotrophy and nitrogen fixation

“…in the eastern North Atlantic between 0uN and 15uN, but also significant concentrations up to 30uN. Citing Mahaffey et al (2003), this apparent inconsistency might be due to use of inorganic nutrients by Trichodesmium spp., the presence of other N 2 -fixing cyanobacteria, and the temporal variability in N 2 fixation. On the contrary, Neuer et al (2002) did not detect a high N : P ratio in the seasonal evolution of nutrients of the upper 200 m at the ESTOC station (29u109N, 15u309W near the Canary Islands); this fact along with the high d 15 N isotope values in suspended organic matter collected with the sediment trap at ESTOC led them to conclude that N 2 fixation cannot be the dominant source of nitrogen for primary production in the region.…”

“…Regarding nitrogen fixation, a recent work by Mahaffey et al (2003) justified the low abundance of 15 N relative to 14 N between 26uW and 32uN along 20uW on the basis of the existence of N 2 fixers, but did not quantify any rate. Lipschultz and Owens (1996) calculated a N 2 fixation rate of about 1.142 kmol s 21 between 31uN and 50uN, proportionally about 0.2 kmol s 21 within the MedBox.…”

“…In general, there are few direct determinations of N 2 fixing by diazotrophs; to our knowledge, only two recent works show direct evidence of N 2 fixation in the eastern North Atlantic: (1) Mahaffey et al (2003) found particulate organic matter with a low abundance of 15 N relative to 14 N between 26uN and 32uN along 20uW, which associates with a predominance of phytopigments from cyanobacteria and prochlorophytes and elevated NO 3 to PO 4 ratios in the upper thermocline; (2) Tyrrell et al (2003) found high abundances of Trichodesmium spp. in the eastern North Atlantic between 0uN and 15uN, but also significant concentrations up to 30uN.…”