Assessing Nitrification and Denitrification in the Seine River and Estuary Using Chemical and Isotopic Techniques

Downstream from metropolitan Paris (France), a large amount of ammonium is discharged into the Seine River by the effluents of the wastewater treatment plant at Achères. To assess the extent of nitrification and denitrification in the water column, concentrations and isotopic compositions of ammonium (δ¹⁵N–NH₄⁺) and nitrate (δ¹⁵N–NO₃⁻, δ¹⁸O–NO₃⁻) were measured during summer low-flow conditions along the lower Seine and its estuary. The results indicated that most of the ammonium released from the wastewater treatment plant is nitrified in the lower Seine River and its upper estuary, but there was no evidence for water-column denitrification. In the lower part of the estuary, however, concentration and isotopic data for nitrate were not consistent with simple mixing between riverine and marine nitrate. A significant departure of the nitrate isotopic composition from what would be expected from simple mixing of freshwater and marine nitrates suggested coupled nitrification and denitrification in the water, in spite of the apparent conservative behavior of nitrate. Denitrification rates of approximately 0.02 mg N/L/h were estimated for this part of the estuary.     

Role of the furosemide-sensitive Na+/K+ transport system in determining the steady-state Na+ and K+ content and volume of human erythrocytesin vitro andin vivo

Summary To study the physiological role of the bidirectionally operating, furosemide-sensitive Na⁺/K⁺ transport system of human erythrocytes, the effect of furosemide on red cell cation and hemoglobin content was determined in cells incubated for 24 hr with ouabain in 145mm NaCl media containing 0 to 10mm K⁺ or Rb⁺. In pure Na⁺ media, furosemide accelerated cell Na⁺ gain and retarded cellular K⁺ loss. External K⁺ (5mm) had an effect similar to furosemide and markedly reduced the action of the drug on cellular cation content. External Rb⁺ accelerated the Na⁺ gain like K⁺, but did not affect the K⁺ retention induced by furosemide. The data are interpreted to indicate that the furosemide-sensitive Na⁺/K⁺ transport system of human erythrocytes mediates an equimolar extrusion of Na⁺ and K⁺ in Na⁺ media (Na⁺/K⁺ cotransport), a 1:1 K⁺/K⁺ (K⁺/Rb⁺) and Na⁺/Na⁺ exchange progressively appearing upon increasing external K⁺ (Rb⁺) concentrations to 5mm. The effect of furosemide (or external K⁺/Rb⁺) on cation contents was associated with a prevention of the cell shrinkage seen in pure Na⁺ media, or with a cell swelling, indicating that the furosemide-sensitive Na⁺/K⁺ transport system is involved in the control of cell volume of human erythrocytes. The action of furosemide on cellular volume and cation content tended to disappear at 5mm external K⁺ or Rb⁺. Thein vivo red cell K⁺ content was negatively correlated to the rate of furosemide-sensitive K⁺ (Rb⁺) uptake, and a positive correlation was seen between mean cellular hemoglobin content and furosemide-sensitive transport activity. The transport system possibly functions as a K⁺ and waterextruding mechanism under physiological conditiosin vivo. The red cell Na⁺ content showed no correlation to the activity of the furosemide-sensitive transport system.     

Phosphorylation of the Na,K-ATPase and the H,K-ATPase

Phosphorylation is a widely used, reversible means of regulating enzymatic activity. Among the important phosphorylation targets are the Na⁺,K⁺- and H⁺,K⁺-ATPases that pump ions against their chemical gradients to uphold ionic concentration differences over the plasma membrane. The two pumps are very homologous, and at least one of the phosphorylation sites is conserved, namely a cAMP activated protein kinase (PKA) site, which is important for regulating pumping activity, either by changing the cellular distribution of the ATPases or by directly altering the kinetic properties as supported by electrophysiological results presented here. We further review the other proposed pump phosphorylations.     

Variation in the export of 13C and 15N from soybean leaf: the effects of nitrogen application and sink removal

Translocation of carbon and nitrogen within a single source-sink unit, comprising a trifoliated leaf, the axillary pod and the subtending internode, and from this unit to the rest of the plant was examined in soybean (Glycine max L. cv. Akishirome) plant by feeding ¹³CO₂ and ¹⁵NO₃. The plants were grown at two levels of nitrogen in the basal medium, i.e. low-N (2 g N m^–2) and high-N (35 g N m^–2) and a treatment of depodding was imposed by removing all the pods from the plant, except the pod of the source sink unit, 13 days after flowering. The plants at high-N accumulated more biomass in its organs compared to low-N and pod removal increased the weight of the vegetative organs. When the terminal leaflet of the source-sink unit was fed with ¹³CO₂, almost all of the radioactive materials were retained inside the source-sink unit and translocation to rest of the plants was insignificant under any of the treatments imposed. Out of the¹³C exported by the terminal leaflet, less than half went into the axillary pod, as the lateral leaflets claimed equal share and very little material was deposited in the petiole. Pod removal decreased ¹³C export at high-N , but not at low-N. Similar to ¹³C, the source-sink unit retained all the ¹⁵N fed to the terminal leaflet at high-N. At low-N, the major part of ¹⁵N partitioning occurred in favour of the rest of the plant outside the source-sink unit, but removal of the competitve sinks from the rest of the plants nullified any partitioning outside the unit. Unlike the situation in ¹³C, no partitioning of ¹⁵N occurred in favour of the lateral leaflets from the terminal leaflet inside the unit. It is concluded that sink demand influences partitioning of both C and N and the translocation of carbon is different from that of nitrogen within a source-sink unit. The translocation of the N is more adjustive to a demand from other sink units compared to the C.     

The symbiotic relationship between the hydrocoral Millepora dichotoma and the barnacle Savignium milleporum

Translocation of radioactive ¹⁴C and ³²P between the pyrgomatine barnacle Savignium milleporum and the hydrocoral Millepora dichotoma in the Red Sea was investigated in order to discover any mutual nutritional benefits. Translocation of photosynthetic products from endosymbiotic zooxanthellae to the hydrocoral was demonstrated. There was no evidence that carbon was further translocated to the barnacle. However, hydrocorals bearing barnacles accumulated significantly more ¹⁴C and ³²P than those with no barnacles. The possibility that the hydrocorals recycle substances excreted by the barnacles is discussed in the context of the oligotrophic environment of the Red Sea.     

Importance of the seryl and threonyl residues of the fifth transmembrane domain to the substrate specificity of yeast plasma membrane Na+/H+ antiporters

The Zygosaccharomyces rouxii Na⁺/H⁺ antiporter Sod2-22p is a member of the subfamily of yeast plasma membrane Nha/ Sod antiporters that do not recognize potassium as their substrate. A functional study of two ZrSod2-22p mutated versions that improved the tolerance of a S. cerevisiae alkali-metal-cation sensitive strain to high extracellular concentration of KCl identified two polar non-charged amino-acid residues in the fifth transmembrane domain, Thr141 and Ser150, as being involved in substrate recognition and transport in yeast Nha/Sod antiporters. A reciprocal substitution of amino-acid residues with a hydroxyl group at these positions, T141S or S150T, produced a broadened cation selectivity of the antiporter for K⁺, in addition to Na⁺ and Li⁺. Site-directed mutagenesis of Ser150 showed that while the replacement of Ser150 with a small hydrophobic (valine) or negatively charged (aspartate) amino acid did not produce a significant change in ZrSod2-22p substrate specificity, the introduction of a positive charge at this position stopped the activity of the antiporter. This data demonstrates that the amino-acid composition of the fifth transmembrane domain, mainly the presence of amino acids containing hydroxyl groups in this part of the protein, is critical for the recognition and transport of substrates and could participate in conformational movements during the binding and/or cation transport cycle in yeast plasma membrane Na⁺/H⁺ antiporters.     

Phospholemman expression is high in the newborn rabbit heart and declines with postnatal maturation

Phospholemman (PLM) is a small sarcolemmal protein that modulates the activities of Na(+)/K(+)-ATPase and the Na(+)/Ca(2+) exchanger (NCX), thus contributing to the maintenance of intracellular Na(+) and Ca(2+) homeostasis. We characterized the expression and subcellular localization of PLM, NCX, and the Na(+)/K(+)-ATPase alpha1-subunit during perinatal development. Western blotting demonstrates that PLM (15kDa), NCX (120kDa), and Na(+)/K(+)-ATPase alpha-1 (approximately 100kDa) proteins are all more than 2-fold higher in ventricular membrane fractions from newborn rabbit hearts (1-4-day old) compared to adult hearts. Our immunocytochemistry data demonstrate that PLM, NCX, and Na(+)/K(+)-ATPase are all expressed at the sarcolemma of newborn ventricular myocytes. Taken together, our data indicate that PLM, NCX, and Na(+)/K(+)-ATPase alpha-1 proteins have similar developmental expression patterns in rabbit ventricular myocardium. Thus, PLM may have an important regulatory role in maintaining cardiac Na(+) and Ca(2+) homeostasis during perinatal maturation.