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

Marine N 2 fixing microorganisms, termed di-azotrophs, are a key functional group in marine pelagic ecosystems. The biological fixation of dinitrogen (N 2) to bioavailable nitrogen provides an important new source of nitrogen for pelagic marine ecosystems and influences pri-mary 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 abun-dances (based on microscopic cell counts or qPCR assays targeting the nifH genes) and N 2 fixation rates. Biomass con-version factors are estimated based on cell sizes to convert abundance data to diazotrophic biomass. The database is lim-ited 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 geomet-ric standard error below and above the geometric mean, the pelagic N 2 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 abun-dances. 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 N 2 fixation has underestimated the true rates. As a result, one can expect that future rate measurements will shift the mean N 2 fixation rate upward and may result in significantly higher estimates for the global N 2 fixation. The evolving database can never-theless be used to study spatial and temporal distributions and variations of marine N 2 fixation, to validate geochemi-cal estimates and to parameterize and validate biogeochem-ical models, keeping in mind that future rate measurements may rise in the future. The database is stored in PANGAEA (doi:10.1594/PANGAEA.774851).