Database of diazotrophs in global ocean: abundances, biomass and nitrogen fixation rates

Abstract

Abstract. Marine N2 fixing microorganisms, termed diazotrophs, are a key functional group in marine pelagic ecosystems. The biological fixation of dinitrogen (N2) to bioavailable nitrogen provides an important new source of nitrogen for pelagic marine ecosystems and influences primary productivity and organic matter export to the deep ocean. As one of a series of efforts to collect biomass and rates specific to different phytoplankton functional groups, we have constructed a database on diazotrophic organisms in the global pelagic upper ocean by compiling about 12 000 direct field measurements of cyanobacterial diazotroph abundances (based on microscopic cell counts or qPCR assays targeting the nifH genes) and N2 fixation rates. Biomass conversion factors are estimated based on cell sizes to convert abundance data to diazotrophic biomass. The database is limited spatially, lacking large regions of the ocean especially in the Indian Ocean. The data are approximately log-normal distributed, and large variances exist in most sub-databases with non-zero values differing 5 to 8 orders of magnitude. Lower mean N2 fixation rate was found in the North Atlantic Ocean than the Pacific Ocean. Reporting the geometric mean and the range of one geometric standard error below and above the geometric mean, the pelagic N2 fixation rate in the global ocean is estimated to be 62 (53–73) Tg N yr−1 and the pelagic diazotrophic biomass in the global ocean is estimated to be 4.7 (2.3–9.6) Tg C from cell counts and to 89 (40–200) Tg C from nifH-based abundances. Uncertainties related to biomass conversion factors can change the estimate of geometric mean pelagic diazotrophic biomass in the global ocean by about ±70%. This evolving database can be used to study spatial and temporal distributions and variations of marine N2 fixation, to validate geochemical estimates and to parameterize and validate biogeochemical models. The database is stored in PANGAEA (http://doi.pangaea.de/10.1594/PANGAEA.774851).

Biological N2 Fixation in the Upwelling Region off NW Iberia: Magnitude, Relevance, and Players

“…The higher UCYN-A2 abundance is consistent with recent findings pointing out this sublineage, although globally distributed (Cabello et al, 2015;Martínez-Pérez et al, 2016), mainly inhabit coastal regions, whereas UCYN-A1 dominates in the oligotrophic open ocean (Thompson et al, 2014;Messer et al, 2015;Turk-Kubo et al, 2017). The UCYN-A abundances detected were in the mediumlow range of global abundances reported by Luo et al (2012), Martínez-Pérez et al (2016), and Benavides and Voss (2015) in the North Atlantic. In the temperate NE American coast, Mulholland et al (2012) showed that UCYN-A was the most abundant diazotroph, reaching 3.5 × 10 7 nifH copies L −1 , among the highest abundances ever observed.…”

“…Comparing with other studies using the same technique ( 15 N 2 -gas tracer method, Montoya et al, 1996) and performed in similar nitrate-rich and temperate areas, our volumetric BNF rates were of similar magnitude to those measured by Benavides et al (2011) in Cape Silleiro (NW Iberian Peninsula) in summer 2009, but between 1 and 3 orders of magnitude lower than those measured by Mulholland et al (2012) in the temperate NE American coast during several summer and autumn cruises, and by Rees et al (2009) in the western English Channel in summer. On a global context, our BNF rates estimated in the outer part of Ría de A Coruña are similar to the lower-end described for the subtropical NE Atlantic (Luo et al, 2012). Although our rates were relatively low, our results contradict the traditional paradigm about marine BNF.…”

“…This will require further spatial-temporal studies in the zone. By calculating the geometric mean of depth-integrated BNF rates estimated during the NICANOR cruises and assuming the surface area of a 3 • × 3 • grid cell (following Luo et al, 2012), we estimated an averaged new N input in the study area of about 0.4 Gg N year −1 from planktonic BNF. In the region of the Canary Current and NW African upwelling, Benavides et al (2011) estimated an input of 0-20 Gg N year −1 , whereas Mulholland et al (2012) estimated an input of 0.5-107 × 10 3 Gg N year −1 in the temperate NE American coast (Luo et al, 2012;Benavides and Voss, 2015).…”

“…These values were in the medium-high range of global abundances reviewed by Luo et al (2012), and by Benavides and Voss (2015) specifically from studies performed in the North Atlantic. In fact, heterotrophic diazotrophs abundances higher than 10 4 nifH copies L −1 have seldom been reported in the marine environment (e.g., Moisander et al, 2008;Halm et al, 2012).…”

First-order estimate of the planktic foraminifer biomass in the modern ocean

“…Finally, intercomparison of planktic foraminifer biomass data from laboratories using different methods of biomass analyses would be needed to verify the data presented here to enlarge the so far rather limited data set. Considering all of the uncertainties, the numbers given above need to be used with care, but seem to be reasonable in comparison to the estimates of other groups of plankton, for example, diazotrophs and diatoms presented by Luo et al (2012) and Leblanc et al (2012).…”