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).

No abstract

An oligonucleotide primer, NITRO821R, targeting the 16S rRNA gene of unicellular cyanobacterial N 2 fixers was developed based on newly derived sequences from Crocosphaera sp. strain WH 8501 and Cyanothece sp. strains WH 8902 and WH 8904 as well as several previously described sequences of Cyanothece sp. and sequences of intracellular cyanobacterial symbionts of the marine diatom Climacodium frauenfeldianum. This oligonucleotide is specific for the targeted organisms, which represent a well-defined phylogenetic lineage, and can detect as few as 50 cells in a standard PCR when it is used as a reverse primer together with the cyanobacterium-and plastid-specific forward primer CYA359F (U. Nübel, F. Garcia-Pichel, and G. Muyzer, Appl. Environ. Microbiol. 63:3327-3332, 1997). Use of this primer pair in the PCR allowed analysis of the distribution of marine unicellular cyanobacterial diazotrophs along a transect following the 67°E meridian from Victoria, Seychelles, to Muscat, Oman (0.5°S to 26°N) in the Arabian Sea. These organisms were found to be preferentially located in warm (>29°C) oligotrophic subsurface waters between 0 and 7°N, but they were also found at a station north of Oman at 26°N, 56°35E, where similar water column conditions prevailed. Slightly cooler oligotrophic waters (<29°C) did not contain these organisms or the numbers were considerably reduced, suggesting that temperature is a key factor in dictating the abundance of this unicellular cyanobacterial diazotroph lineage in marine environments.Microbial N 2 fixation has traditionally been considered to be of minor importance as a source of fixed nitrogen in oceanic environments (5). Contemporary mass balance estimates, however, have suggested that there is excess removal of combined forms of nitrogen, particularly from areas of the tropical ocean, and it has been proposed that biological N 2 fixation may account for the imbalances (6,12). This has led to a radically revised view of the quantitative importance of this process in the nitrogen cycle. Current estimates of global N 2 fixation are ϳ240 Tg of N year Ϫ1 and a marine contribution of ϳ80 Tg of N year Ϫ1 (4). Trichodesmium, a filamentous nonheterocystous cyanobacterium broadly distributed throughout tropical and subtropical oceans, has long been considered to be responsible for most of this marine N 2 fixation (see references 3 and 17 for recent reviews). However, recently, Zehr et al. (34) showed that unicellular cyanobacteria that are 3 to 10 m in diameter may make a significant contribution to oceanic N 2 fixation. Indeed, based on N 2 fixation data for the 0.2-to 10-m bacterioplankton size fraction and the concentrations of phycoerythrin-containing unicellular cyanobacteria, they estimated that this contribution might equal or exceed that of Trichodesmium.Several unicellular marine cyanobacterial diazotrophs belonging to the genera Synechococcus (15, 16) Cyanothece (21), and Crocosphaera (28) have previously been described. These organisms all have temporal separation of N 2 fixation and...