Patterns of nitrogen fixation along 10°N in the tropical Atlantic

Nitrogen fixation supports new production in the oligotrophic oceans and removes dinitrogen and carbon dioxide from mixed layer waters. N‐fixation rates have been estimated in various ways but measurements are still too rare and factors limiting N‐fixation are not yet fully understood. Here we present data from a transect along 10°N through the tropical Atlantic on the Meteor Cruise 55 where N‐fixation rates between 3.7 and 255 μmol N*m−2*d−1 were recorded. The highest rates occurred off Africa in the eastern tropical North Atlantic (ETNA), and in the Amazon River plume in the West and contributed to 1–12.2% of the N‐demand of primary production. N‐fixation rates correlated with dissolved Fe concentrations, which were 20–280 times greater than the estimated demand. High atmospheric Fe inputs combined with the shallow nutricline make the ETNA a favourable environment for N‐fixers

Iron and phosphorus co-limit nitrogen fixation in the eastern tropical North Atlantic

The role of iron in enhancing phytoplankton productivity in high nutrient, low chlorophyll oceanic regions was demonstrated first through iron-addition bioassay experiments1 and subsequently confirmed by large-scale iron fertilization experiments2. Iron supply has been hypothesized to limit nitrogen fixation and hence oceanic primary productivity on geological timescales3, providing an alternative to phosphorus as the ultimate limiting nutrient4. Oceanographic observations have been interpreted both to confirm and refute this hypothesis5, 6, but direct experimental evidence is lacking7. We conducted experiments to test this hypothesis during the Meteor 55 cruise to the tropical North Atlantic. This region is rich in diazotrophs8 and strongly impacted by Saharan dust input9. Here we show that community primary productivity was nitrogen-limited, and that nitrogen fixation was co-limited by iron and phosphorus. Saharan dust addition stimulated nitrogen fixation, presumably by supplying both iron and phosphorus10, 11. Our results support the hypothesis that aeolian mineral dust deposition promotes nitrogen fixation in the eastern tropical North Atlantic

First Neutrino Observations from the Sudbury Neutrino Observatory

The first neutrino observations from the Sudbury Neutrino Observatory are presented from preliminary analyses. Based on energy, direction and location, the data in the region of interest appear to be dominated by 8B solar neutrinos, detected by the charged current reaction on deuterium and elastic scattering from electrons, with very little background. Measurements of radioactive backgrounds indicate that the measurement of all active neutrino types via the neutral current reaction on deuterium will be possible with small systematic uncertainties. Quantitative results for the fluxes observed with these reactions will be provided when further calibrations have been completed.Comment: Latex, 7 pages, 10 figures, Invited paper at Neutrino 2000 Conference, Sudbury, Canada, June 16-21, 2000 to be published in the Proceeding

Evidence for the linked biogeochemical cycling of zinc, cobalt, and phosphorus in the western North Atlantic Ocean

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 22 (2008): GB4012, doi:10.1029/2007GB003119.Many trace metals such as iron, copper, and manganese have lower concentrations in the surface waters of the North Pacific Ocean than in North Atlantic surface waters. However, cobalt and zinc concentrations in North Atlantic surface waters are often as low as those reported in the North Pacific. We studied the relationship between the distribution of cobalt, zinc, and phosphorus in surface waters of the western North Atlantic Ocean. Both metals show strong depletion in the southern Sargasso Sea, a region characterized by exceedingly low dissolved inorganic phosphorus (generally <4 nmol L−1) and measurable alkaline phosphatase activity. Alkaline phosphatase is a metalloenzyme (typically containing zinc) that cleaves phosphate monoesters and is a diagnostic indicator of phosphorus stress in phytoplankton. In contrast to the North Pacific Ocean, cobalt and zinc appear to be drawn down to their lowest values only when inorganic phosphorus is below 10 nmol L−1 in the North Atlantic Ocean. Lower levels of phosphorus in the Atlantic may contribute to these differences, possibly through an increased biological demand for zinc and cobalt associated with dissolved organic phosphorus acquisition. This hypothesis is consistent with results of a culture study where alkaline phosphatase activity decreased in the model coccolithophore Emiliania huxleyi upon zinc and cobalt limitation.This research was supported by NSF grant OCE- 0136835 to J.W.M. and S.D. R.W.J. was supported by an EPA STAR Fellowship

Desert Dust as a Source of Iron to the Globally Important Diazotroph Trichodesmium

The marine cyanobacterium Trichodesmium sp. accounts for approximately half of the annual ‘new’ nitrogen introduced to the global ocean but its biogeography and activity is often limited by the availability of iron (Fe). A major source of Fe to the open ocean is Aeolian dust deposition in which Fe is largely comprised of particles with reduced bioavailability over soluble forms of Fe. We report that Trichodesmium erythraeum IMS101 has improved growth rate and photosynthetic physiology and down-regulates Fe-stress biomarker genes when cells are grown in the direct vicinity of, rather than physically separated from, Saharan dust particles as the sole source of Fe. These findings suggest that availability of non-soluble forms of dust-associated Fe may depend on cell contact. Transcriptomic analysis further reveals unique profiles of gene expression in all tested conditions, implying that Trichodesmium has distinct molecular signatures related to acquisition of Fe from different sources. Trichodesmium thus appears to be capable of employing specific mechanisms to access Fe from complex sources in oceanic systems, helping to explain its role as a key microbe in global biogeochemical cycles

Measurement of the rate of nu_e + d --> p + p + e^- interactions produced by 8B solar neutrinos at the Sudbury Neutrino Observatory

Solar neutrinos from the decay of $^8$B have been detected at the Sudbury Neutrino Observatory (SNO) via the charged current (CC) reaction on deuterium and by the elastic scattering (ES) of electrons. The CC reaction is sensitive exclusively to nu_e's, while the ES reaction also has a small sensitivity to nu_mu's and nu_tau's. The flux of nu_e's from ^8B decay measured by the CC reaction rate is \phi^CC(nu_e) = 1.75 +/- 0.07 (stat)+0.12/-0.11 (sys.) +/- 0.05(theor) x 10^6 /cm^2 s. Assuming no flavor transformation, the flux inferred from the ES reaction rate is \phi^ES(nu_x) = 2.39+/-0.34 (stat.)+0.16}/-0.14 (sys) x 10^6 /cm^2 s. Comparison of \phi^CC(nu_e) to the Super-Kamiokande Collaboration's precision value of \phi^ES(\nu_x) yields a 3.3 sigma difference, providing evidence that there is a non-electron flavor active neutrino component in the solar flux. The total flux of active ^8B neutrinos is thus determined to be 5.44 +/-0.99 x 10^6/cm^2 s, in close agreement with the predictions of solar models.Comment: 6 pages (LaTex), 3 figures, submitted to Phys. Rev. Letter

Iron availability limits the ocean nitrogen inventory stabilizing feedbacks between marine denitrification and nitrogen fixation

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 21 (2007): GB2001, doi:10.1029/2006GB002762.Recent upward revisions in key sink/source terms for fixed nitrogen (N) in the oceans imply a short residence time and strong negative feedbacks involving denitrification and N fixation to prevent large swings in the ocean N inventory over timescales of a few centuries. We tested the strength of these feedbacks in a global biogeochemical elemental cycling (BEC) ocean model that includes water column denitrification and an explicit N fixing phytoplankton group. In the northern Indian Ocean and over longer timescales in the tropical Atlantic, we find strong stabilizing feedbacks that minimize changes in marine N inventory over timescales of ∼30–200 years. In these regions high atmospheric dust/iron inputs lead to phosphorus limitation of diazotrophs, and thus a tight link between N fixation and surface water N/P ratios. Maintenance of the oxygen minimum zones in these basins depends on N fixation driven export. The stabilizing feedbacks in other regions are significant but weaker owing to iron limitation of the diazotrophs. Thus Fe limitation appears to restrict the ability of N fixation to compensate for changes in denitrification in the current climate, perhaps leading the oceans to lose fixed N. We suggest that iron is the ultimate limiting nutrient leading to nitrogen being the proximate limiting nutrient over wide regions today. Iron stress was at least partially alleviated during more dusty, glacial times, leading to a higher marine N inventory, increased export production, and perhaps widespread phosphorus limitation of the phytoplankton community. The increased efficiency of the biological pump would have contributed to the glacial drawdown in atmospheric CO2.This work was supported by grants from the U.S. National Science Foundation (OCE-0222033 and OCE-0452972). Computations supported by Earth System Modeling Facility (NSF ATM-0321380) and by the Climate Simulation Laboratory at the National Center for Atmospheric Research

Iron Deficiency Increases Growth and Nitrogen-Fixation Rates of Phosphorus-Deficient Marine Cyanobacteria

Marine dinitrogen (N2)-fixing cyanobacteria have large impacts on global biogeochemistry as they fix carbon dioxide (CO2) and fertilize oligotrophic ocean waters with new nitrogen. Iron (Fe) and phosphorus (P) are the two most important limiting nutrients for marine biological N2 fixation, and their availabilities vary between major ocean basins and regions. A long-standing question concerns the ability of two globally dominant N2-fixing cyanobacteria, unicellular Crocosphaera and filamentous Trichodesmium, to maintain relatively high N2-fixation rates in these regimes where both Fe and P are typically scarce. We show that under P-deficient conditions, cultures of these two cyanobacteria are able to grow and fix N2 faster when Fe deficient than when Fe replete. In addition, growth affinities relative to P increase while minimum concentrations of P that support growth decrease at low Fe concentrations. In Crocosphaera, this effect is accompanied by a reduction in cell sizes and elemental quotas. Relatively high growth rates of these two biogeochemically critical cyanobacteria in low-P, low-Fe environments such as those that characterize much of the oligotrophic ocean challenge the common assumption that low Fe levels can have only negative effects on marine primary producers. The closely interdependent influence of Fe and P on N2-fixing cyanobacteria suggests that even subtle shifts in their supply ratio in the past, present and future oceans could have large consequences for global carbon and nitrogen cycles

Common Risk Factors for Urinary House Soiling (Periuria) in Cats and Its Differentiation: The Sensitivity and Specificity of Common Diagnostic Signs

Urinary house soiling (periuria) in the home is a common but serious behaviour problem in cats. Although many specific risk factors and triggers have been postulated, their importance is largely unknown. This study assessed: (1) the significance of purported risk factors for periuria as well as specifically marking and latrine behaviour in the home; (2) the specificity and sensitivity of signs commonly used to differentiate latrine and marking behaviour. Owner responses to an internet survey (n = 245) were classified into three groups: control, marking and latrine behaviour, along with 41 potential risk factors and 15 predictors used to diagnose marking and latrine problems. Univariate statistical analyses and non-parametric tests of association were used to determine simple associations. In addition the sensitivity and specificity of four cardinal signs (posture to urinate, attempt to cover soiled area, surface chosen and volume of urine deposited) were calculated. Significant potential risk factors were: age (marking cats were older than the other two groups); multi-cat household (increased risk of marking and latrine behaviours); free outside access and cat flaps in the house (higher frequency of marking); outside access in general (lower prevalence of latrine behaviour); defecation outside the litter box (higher frequency of latrine behaviour); a heavy dependence by the cat on its owner (lower frequency of latrine behaviour) and a relaxed personality (lower risk of marking behaviour). Litterbox attributes and disease related factors were not significant. Individual cardinal signs were generally not good predictors of diagnosis. This study challenges the poor quality of evidence that has underpinned some of the hypotheses concerning the causes of periuria in cats. The results, in particular, highlight the general importance of the social environment, with the presence of other cats in the household, the cat-owner bond and personality related factors, alongside factors like the use of a cat flap which might also alter the social environment, all implicated as significant risk factors. While the physical environment may be important in specific cases, it seems this is less important as a general risk factor. The findings quantify the risk of misdiagnosis if a single sign is considered sufficient for diagnosis

Abundances of Iron-Binding Photosynthetic and Nitrogen-Fixing Proteins of Trichodesmium Both in Culture and In Situ from the North Atlantic

Marine cyanobacteria of the genus Trichodesmium occur throughout the oligotrophic tropical and subtropical oceans, where they can dominate the diazotrophic community in regions with high inputs of the trace metal iron (Fe). Iron is necessary for the functionality of enzymes involved in the processes of both photosynthesis and nitrogen fixation. We combined laboratory and field-based quantifications of the absolute concentrations of key enzymes involved in both photosynthesis and nitrogen fixation to determine how Trichodesmium allocates resources to these processes. We determined that protein level responses of Trichodesmium to iron-starvation involve down-regulation of the nitrogen fixation apparatus. In contrast, the photosynthetic apparatus is largely maintained, although re-arrangements do occur, including accumulation of the iron-stress-induced chlorophyll-binding protein IsiA. Data from natural populations of Trichodesmium spp. collected in the North Atlantic demonstrated a protein profile similar to iron-starved Trichodesmium in culture, suggestive of acclimation towards a minimal iron requirement even within an oceanic region receiving a high iron-flux. Estimates of cellular metabolic iron requirements are consistent with the availability of this trace metal playing a major role in restricting the biomass and activity of Trichodesmium throughout much of the subtropical ocean