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

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)

Effects of testing and storage environments on mechanical properties of Ni-plated and bare U-3/4 wt% Ti

It was found that storage environments with an adequate supply of oxygen can effectively minimize moisture corrosion of bare U-3/4 Ti. In particular, 0.75 cm/sup 3/ of dry air is calculated to protect 1 cm/sup 2/ of U-3/4 Ti for 20 years storage at room temperature. Consideration of the geometric details of U-3/4 Ti alloy specimens and the free volumes of air (and hence O/sub 2/) available can satisfactorily explain discrepancies in corrosion behavior between recent tests and previously reported data. The storage environment at 70/sup 0/C produces a minor strength increase in bare samples with increasing time. Decreases in ductility are observed for testing conditions of low temperature, low strain rate, and/or high humidity. Surface cracks occur under the same conditions conducive to corrosion, i.e., moderate temperatures, low strain rates, and high humidity. Significant increases in strength result under low-temperature and high-strain-rate conditions of tensile testing. Residual chloride contamination may be responsible for the occasional and otherwise unexplained large scatter in ductility for nominally similar specimens and test conditions. Nickel plating is observed to cause a statistically significant decrease in tensile strength, but no effect on the yield strength or ductility was observed and the presence of high explosive during the aging of tensile bars was observed to have no effect on mechanical properties