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. 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 (52–73) Tg N yr−1 and the pelagic diazotrophic biomass in the global ocean is estimated to be 2.1 (1.4–3.1) Tg C from cell counts and to 89 (43–150) Tg C from nifH-based abundances. Reporting the arithmetic mean and one standard error instead, these three global estimates are 140 ± 9.2 Tg N yr−1, 18 ± 1.8 Tg C and 590 ± 70 Tg C, respectively. Uncertainties related to biomass conversion factors can change the estimate of geometric mean pelagic diazotrophic biomass in the global ocean by about ±70%. It was recently established that the most commonly applied method used to measure N2 fixation has underestimated the true rates. As a result, one can expect that future rate measurements will shift the mean N2 fixation rate upward and may result in significantly higher estimates for the global N2 fixation. The evolving database can nevertheless 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, keeping in mind that future rate measurements may rise in the future. The database is stored in PANGAEA (doi:10.1594/PANGAEA.774851).
Highlights
N2 fixation is the biological conversion of dinitrogen (N2) gas into two molecules of ammonia by diazotrophic organisms
The authors reported that a massive bloom, which seemed to be associated with a front, was encountered and large streaks with Trichodesmium colonies were floating at the surface
The first global database of oceanic diazotrophic measurements has been constructed with sub-databases for N2 fixation rates, cell counts for diazotrophs and nifH-based abundances
Summary
N2 fixation is the biological conversion of dinitrogen (N2) gas into two molecules of ammonia by diazotrophic organisms. It is generally accepted that cyanobacteria are the major N2-fixing microorganisms in the ocean (Karl et al, 2002; Zehr, 2011). There are three major types of marine planktonic diazotrophic cyanobacteria (Sohm et al, 2011b; Zehr, 2011): (1) non-heterocystous filamentous cyanobacteria; (2) heterocystous cyanobacteria; and (3) unicellular cyanobacteria. Most field and laboratory research has focused on non-heterocystous filamentous Trichodesmium (Oscillatoriales) (Dugdale et al, 1961; Carpenter and Romans, 1991; Capone et al, 1997). Trichodesmium is an abundant diazotroph in the open ocean (Capone et al, 2005; LaRoche and Breitbarth, 2005), which can be found as large colonies
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