Heterotrophic growth and nutritional aspects of the diatom Cyclotella cryptica (Bacillariophyceae): effect of nitrogen source and concentration

To investigate the nutritional value of the diatom Cyclotella cryptica Reimann, Lewin, and Guillard as an alternative feed for the use in the aquaculture industry, the heterotrophic growth characteristics, total fatty acids, and the resultant fatty acid profile of the microalga were studied when cultivated with sodium nitrate, ammonium chloride, or urea. All three nitrogen sources supported growth under heterotrophic conditions, and their uptake affected the pH of the cultivation medium, even when buffered. The use of sodium nitrate or urea resulted in a significant increase in the pH of the cultivation media, whereas the use of ammonium chloride caused a minor decrease in the pH of the cultivation media. The maximum specific growth rate was highest when urea and ammonium chloride were supplied at a low concentration; however, the total fatty acid content was not significantly affected (P = 0.101) by the nitrogen source when supplied at 10.7 mM nitrogen. The total fatty acid content and fatty acid profile of C. cryptica was more affected by the growth phase (predominately influenced by the initial nitrogen concentration) than by the source of nitrogen.     

Biomass and Production of Lake Charr During the Acidification and PH Recovery of A Small Ontario Lake

The biomass and production of lake charr, Salvelinus namaycush, was studied in Lake 223, a lake that was intentionally acidified to pH 5.0 and then allowed to gradually recover, and in Lake 224, a natural lake of neutral pH. During the first 6 years pH decreased, biomass of Lake 223 lake charr increased, primarily due to high recruitment during the initial years of acidification. Biomass then decreased the final year of acidification. Biomass of Lake 223 lake charr remained low during the subsequent 10 years of pH recovery and 5 additional years after lake pH had returned to neutral pH. Production of lake charr decreased during acidification and increased during pH recovery. Production was still at least an order of magnitude lower at the end of this study than 22 years earlier before lake pH was lowered. The continued low production was caused primarily by low abundance during the years of pH recovery and following years. Production per individual fish in Lake 223 decreased during acidification and quickly increased during pH recovery to values similar to that prior to acidification. Production, biomass, and production per individual varied little between years for lake charr in Lake 224.     

Fluoride reabsorption by nonionic diffusion in the distal nephron of the dog

This study was done to test the hypothesis that fluoride reabsorption is extensive from the distal nephron, the major site for tubular fluid acidification, and to compare the distal nephron handling of fluoride and chloride. Ten stop-flow studies were done in five dogs anesthetized with pentobarbital. Urinary alkalinization was achieved by the intravenous infusion of sodium bicarbonate and acetazolamide or lithium chloride. Acidification was achieved by the infusion of sodium nitrate or sodium sulfate. The results indicate that the extent of fluoride reabsorption from the distal nephron is inversely correlated with urinary pH (P less than 0.001). When the urine was strongly acidified by the infusion of sodium sulfate, urine to plasma fluoride concentration ratios were less than 1.0, a finding not previously reported from studies of the renal handling of fluoride. The reabsorption of fluoride from the distal nephron was not correlated consistently with that of chloride. The results indicate that the distal nephron is an important site for the reabsorption of fluoride and they provide additional evidence that HF is the permeating moiety.     

Intracellular pH on Protein Kinase C and Ionomycin Potentiation of Isoproterenol-Stimulated Cyclic AMP and Cyclic GMP Production in Rat Pinealocytes

In rat pinealocytes, alpha 1-adrenergic activation, which leads to cytoplasmic alkalinization, also potentiates the beta-adrenergic stimulated cyclic AMP (cAMP) and cyclic GMP (cGMP) responses. Both elevation of intracellular calcium ([Ca2+]i) and activation of protein kinase C are involved in the potentiation mechanism. Recently, intracellular pH has also been found to modulate the adrenergic-stimulated cyclic nucleotide responses, suggesting intracellular pH may also affect the potentiation mechanism. This possibility was examined in the present study. Cytoplasmic alkalinization by ammonium chloride had an enhancing effect on the isoproterenol and ionomycin-stimulated cAMP and cGMP accumulation. In comparison, cytoplasmic acidification by sodium propionate reduced the isoproterenol and ionomycin-stimulated cAMP and cGMP responses. Direct measurement of [Ca2+]i indicated that neither ammonium chloride nor sodium propionate had an effect on the ionomycin-stimulated elevation of [Ca2+]i, suggesting their effects on cyclic nucleotide responses may be independent of [Ca2+]i. In cells stimulated by isoproterenol and an activator of protein kinase C, ammonium chloride had an enhancing effect on both cAMP and cGMP responses, whereas sodium propionate had no effect. Taken together, these results suggest that a site distal to elevation of [Ca2+]i and activation of protein kinase C, of importance to the potentiation mechanism, is modulated by intracellular pH.     

The effect of changes in pH on phosphate and potassium uptake by Monascus rubiginosus ATCC 16367 in submerged shaken culture.

Monascus rubiginosus ATCC 16367 was cultivated aerobically in media containing ammonium and nitrate as nitrogen source. The pH of the medium was adjusted at different times, the pH of the nitrate medium being lowered to the pH of the ammonium medium and the pH of the ammonium medium raised to that of the nitrate. More phosphate was taken up on the nitrate medium, but potassium uptake did not start until 24h. On the ammonium medium, both were taken up in parallel from the beginning, but the amount of phosphate taken up never reached the same level as on nitrate medium. When the pH was adjusted, the uptakes changed, especially on the ammonium medium where a great increase in phosphate uptake was observed. More conidia were formed on the nitrate medium and more pigment on the ammonium medium. When the pH of either media was adjusted, the development of conidia and pigment production changed to that of the other control medium where the pH evolved normally in the direction of the change, regardless of the source of nitrogen. The reasons for the development of conidia on nitrate medium or where the pH is high, and the production of pigment on ammonium medium or at low pH is discussed.     

Cl-/HCO3- exchange at the apical membrane of Necturus gallbladder

The hypothesis of Cl-/HCO3- exchange across the apical membrane of the epithelial cells of Necturus gallbladder was tested by means of measurements of extracellular pH (pHo), intracellular pH (pHi), and Cl- activity (alpha Cli) with ion-sensitive microelectrodes. Luminal pH changes were measured after stopping mucosal superfusion with a solution of low buffering power. Under control conditions, the luminal solution acidifies when superfusion is stopped. Shortly after addition of the Na+/H+ exchange inhibitor amiloride (10(-3) M) to the superfusate, alkalinization was observed. During prolonged (10 min) exposure to amiloride, no significant pHo change occurred. Shortly after amiloride removal, luminal acidification increased, returning to control rates in 10 min. The absence of Na+ in the superfusate (TMA+ substitution) caused changes in the same direction, but they were larger than those observed with amiloride. Removal of Cl- (cyclamate or sulfate substitution) caused a short-lived increase in the rate of luminal acidification, followed by a return to control values (10-30 min). Upon re-exposure to Cl-, there was a transient reduction of luminal acidification. The initial increase in acidification produced by Cl- removal was partially inhibited by SITS (0.5 mM). The pHi increased rapidly and reversibly when the Cl- concentration of the mucosal bathing solution was reduced to nominally 0 mM. The pHi changes were larger in 10 mM HCO3-Ringer's than in 1 mM HEPES-Ringer's, which suggests that HCO3- is transported in exchange for Cl-. In both HEPES- and HCO3-Ringer's, SITS inhibited the pHi changes. Finally, intracellular acidification or alkalinization (partial replacement of NaCl with sodium propionate or ammonium chloride, respectively) caused a reversible decrease or increase of alpha Cli. These results support the hypothesis of apical membrane Cl-/HCO3- exchange, which can be dissociated from Na+/H+ exchange and operates under control conditions. The coexistence at the apical membrane of Na+/H+ and Cl-/HCO3- antiports suggests that NaCl entry can occur through these transporters.     

pH Changes Associated with Iron-Stress Response

When Fe-inefficient T3238fer and Fe-efficient T3238FER tomatoes were supplied iron, and nitrogen as nitrate, they increased the pH of the nutrient culture. When they were supplied nitrogen as ammonium, they decreased the pH. When Fe supply was limited, Fe-stress response developed in T3238FER that opposed the usual nitrate response and decreased, rather than increased, the pH. A “reductant” which reduced Fe³⁺ to Fe²⁺ was released from the roots of these plants and lowered the pH; and there was a tremendous increase in the uptake of Fe. T3238fer did not produce “reductant” in response to Fe-stress; the pH increased, and the plants developed Fe-deficiency when nitrogen was supplied as nitrate. Nitrogen nutrition and iron-stress response are important factors associated with iron chlorosis in plants. Release of hydrogen ions from roots of Fe-stressed plants is caused by more than response to imbalanced uptake of cations and anions.     

Effect of ammonium or nitrate nutrition on net photosynthesis,growth, and activity of the enzymes nitrate reductase and glutamine synthetase in blueberry,raspberry and strawberry

Blueberry, raspberry and strawberry may have evolved strategies for survival due to the different soil conditions available in their natural environment. Since this might be reflected in their response to rhizosphere pH and N form supplied, investigations were carried out in order to compare effects of nitrate and ammonium nutrition (the latter at two different pH regimes) on growth, CO2 gas exchange, and on the activity of key enzymes of the nitrogen metabolism of these plant species. Highbush blueberry (Vaccinium corymbosum L. cv. 13–16–A), raspberry (Rubus idaeus L. cv. Zeva II) and strawberry (Fragaria × ananassa Duch. cv. Senga Sengana) were grown in 10 L black polyethylene pots in quartz sand with and without 1% CaCO3 (w: v), respectively. Nutrient solutions supplied contained nitrate (6 mM) or ammonium (6 mM) as the sole nitrogen source. Compared with strawberries fed with nitrate nitrogen, supply of ammonium nitrogen caused a decrease in net photosynthesis and dry matter production when plants were grown in quartz sand without added CaCO3. In contrast, net photosynthesis and dry matter production increased in blueberries fed with ammonium nitrogen, while dry matter production of raspberries was not affected by the N form supplied. In quartz sand with CaCO3, ammonium nutrition caused less deleterious effects on strawberries, and net photosynthesis in raspberries increased as compared to plants grown in quartz sand without CaCO3 addition. Activity of nitrate reductase (NR) was low in blueberries and could only be detected in the roots of plants supplied with nitrate nitrogen. In contrast, NR activity was high in leaves, but low in roots of raspberry and strawberry plants. Ammonium nutrition caused a decrease in NR level in leaves. Activity of glutamine synthetase (GS) was high in leaves but lower in roots of blueberry, raspberry and strawberry plants. The GS level was not significantly affected by the nitrogen source supplied. The effects of nitrate or ammonium nitrogen on net photosynthesis, growth, and activity of enzymes in blueberry, raspberry and strawberry cultivars appear to reflect their different adaptability to soil pH and N form due to the conditions of their natural environment. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dissimilatory nitrate reduction to nitrate, nitrous oxide, and ammonium by Pseudomonas putrefaciens

The influence of redox potential on dissimilatory nitrate reduction to ammonium was investigated on a marine bacterium, Pseudomonas putrefaciens. Nitrate was consumed (3.1 mmol liter-1), and ammonium was produced in cultures with glucose and without sodium thioglycolate. When sodium thioglycolate was added, nitrate was consumed at a lower rate (1.1 mmol liter-1), and no significant amounts of nitrite or ammonium were produced. No growth was detected in glucose media either with or without sodium thioglycolate. When grown on tryptic soy broth, the production of nitrous oxide paralleled growth. In the same medium, but with sodium thioglycolate, nitrous oxide was first produced during growth and then consumed. Acetylene caused the nitrous oxide to accumulate. These results and the mass balance calculations for different nitrogen components indicate that P. putrefaciens has the capacity to dissimilate nitrate to ammonium as well as to dinitrogen gas and nitrous oxide (denitrification). The dissimilatory pathway to ammonium dominates except when sodium thioglycolate is added to the medium.     

Dissimilatory nitrate reduction to nitrate, nitrous oxide, and ammonium by Pseudomonas putrefaciens.

The influence of redox potential on dissimilatory nitrate reduction to ammonium was investigated on a marine bacterium, Pseudomonas putrefaciens. Nitrate was consumed (3.1 mmol liter-1), and ammonium was produced in cultures with glucose and without sodium thioglycolate. When sodium thioglycolate was added, nitrate was consumed at a lower rate (1.1 mmol liter-1), and no significant amounts of nitrite or ammonium were produced. No growth was detected in glucose media either with or without sodium thioglycolate. When grown on tryptic soy broth, the production of nitrous oxide paralleled growth. In the same medium, but with sodium thioglycolate, nitrous oxide was first produced during growth and then consumed. Acetylene caused the nitrous oxide to accumulate. These results and the mass balance calculations for different nitrogen components indicate that P. putrefaciens has the capacity to dissimilate nitrate to ammonium as well as to dinitrogen gas and nitrous oxide (denitrification). The dissimilatory pathway to ammonium dominates except when sodium thioglycolate is added to the medium.     

Extra- and Intracellular pH and Membrane Potential Changes Induced by K, Cl, H(2)PO(4), and NO(3) Uptake and Fusicoccin in Root Hairs of Limnobium stoloniferum

Short-term ion uptake into roots of Limnobium stoloniferum was followed extracellularly with ion selective macroelectrodes. Cytosolic or vacuolar pH, together with the electrical membrane potential, was recorded with microelectrodes both located in the same young root hair. At the onset of chloride, phosphate, and nitrate uptake the membrane potential transiently decreased by 50 to 100 millivolts. During Cl⁻ and H₂PO₄⁻ uptake cytosolic pH decreased by 0.2 to 0.3 pH units. Nitrate induced cytosolic alkalinization by 0.19 pH units, indicating rapid reduction. The extracellular medium alkalinized when anion uptake exceeded K⁺ uptake. During fusicoccin-dependent plasmalemma hyperpolarization, extracellular and cytosolic pH remained rather constant. Upon K⁺ absorption, FC intensified extracellular acidification and intracellular alkalinization (from 0.31 to 0.4 pH units). In the presence of Cl⁻ FC induced intracellular acidification. Since H⁺ fluxes per se do not change the pH, recorded pH changes only result from fluxes of the stronger ions. The extra- and intracellular pH changes, together with membrane depolarization, exclude mechanisms as K⁺/A⁻ symport or HCO₃⁻/A⁻ antiport for anion uptake. Though not suitable to reveal the actual H⁺/A⁻ stoichiometry, the results are consistent with an H⁺/A⁻ cotransport mechanism.     

Effects of ammonium on intracellular pH in rat medullary thick ascending limb: mechanisms of apical membrane NH4+ transport

The renal medullary thick ascending limb (MTAL) actively reabsorbs ammonium ions. To examine the effects of NH4+ transport on intracellular pH (pHi) and the mechanisms of apical membrane NH4+ transport, MTALs from rats were isolated and perfused in vitro with 25 mM HCO3(-)-buffered solutions (pH 7.4). pHi was monitored using the fluorescent dye BCECF. In the absence of NH4+, the mean pHi was 7.16. Luminal addition of 20 mM NH4+ caused a rapid intracellular acidification (dpHi/dt = 11.1 U/min) and reduced the steady state pHi to 6.67 (delta pHi = 0.5 U), indicating that apical NH4+ entry was more rapid than entry of NH3. Luminal furosemide (10(-4) M) reduced the initial rate of cell acidification by 70% and the fall in steady state pHi by 35%. The residual acidification observed with furosemide was inhibited by luminal barium (12 mM), indicating that apical NH4+ entry occurred via both furosemide (Na(+)-NH4(+)-2Cl- cotransport) and barium- sensitive pathways. The role of these pathways in NH4+ absorption was assessed under symmetric ammonium conditions. With 4 mM NH4+ in perfusate and bath, mean steady state pHi was 6.61 and net ammonium absorption was 12 pmol/min/mm. Addition of furosemide to the lumen abolished net ammonium absorption and caused pHi to increase abruptly (dpHi/dt = 0.8 U/min) to 7.0. Increasing luminal [K+] from 4 to 25 mM caused a similar, rapid cell alkalinization. The pronounced cell alkalinization observed with furosemide or increasing [K+] was not observed in the absence of NH4+. In symmetric 4 mM NH4+ solutions, addition of barium to the lumen caused a slow intracellular alkalinization and reduced net ammonium absorption only by 14%. Conclusions: (a) ammonium transport is a critical determinant of pHi in the MTAL, with NH4+ absorption markedly acidifying the cells and maneuvers that inhibit apical NH4+ uptake (furosemide or elevation of luminal [K+]) causing intracellular alkalinization; (b) most or all of transcellular ammonium absorption is mediated by apical membrane Na(+)- NH4(+)-2Cl- cotransport; (c) NH4+ also permeates a barium-sensitive apical membrane transport pathway (presumably apical membrane K+ channels) but this pathway does not contribute significantly to ammonium absorption under physiologic (symmetric ammonium) conditions.     

Role of vacuolar adenosine triphosphatase in the regulation of cytosolic pH in hepatocytes

The responses of the cytosolic pH of hepatocytes in suspension to agents affecting the activity of vacuolar adenosine triphosphatase (V-ATPase) and Na/H exchange have been studied. Changes of cytosolic pH were determined both with dual-wavelength excitation (500/440 nm) of the fluorescence of 2,7-bis-(2-carboxyethyl)-5(and 6)-carboxyfluorescein and from the distribution of ¹⁴C-dimethyloxazolidinedione; both methods gave very similar results. Changes of vesicular pH were determined by comparing the fluorescence of fluorescein isothiocyanate-dextran and rhodamine B isothiocyanate-dextran taken up by endocytosis. Nitrate, which inhibits V-ATPase in isolated organelles, induced a concentration-dependent acidification of the cytosol and alkalinization of vesicles, with maximal effects at 25–37.5 mm in each case, indicating that V-ATPase contributes to removal of cytosolic protons. On continued exposure to nitrate, the acidification underwent an amiloride-inhibitable reversal. At the higher concentrations of NO ₃ ^– , both cytosolic acidification and vesicular alkalinization were reduced or absent. Bafilomycin A₁ caused alkalinization of vesicular pH; cytosolic acidification was not observed, possibly because of other ionic exchanges. Recovery of cytosolic pH from an acid load (2 min exposure to 5% CO₂) was sensitive to both 25 mm NO ₃ ^– and to ouabain. The pH dependence of the nitrate effect was tested with media of different pH; the activity was negligible at cytosolic pH 6.2 and rose to a maximum at cytosolic pH 7.3. Treatment of hepatocytes with 0.5–1.0 mm ouabain resulted in an initial alkalinization (0.5–2 min duration) of the cytosol, followed by a spontaneous reversal and, on occasion, further acidification. The alkalinization was blocked by 25 mm NO ₃ ^– , but not by 25 mm gluconate. The results suggest that the cytosolic alkalinization is caused by a stimulation of H⁺ uptake by V-ATPase activity. We conclude that V-ATPases make an important contribution to the regulation of the cytosolic pH of hepatocytes.This work was supported in part by National Institutes of Health B.R.S. Grant 507 RR05417 to Temple University.     

Diatoms,lake acidification and the Surface Water Acidification Programme (SWAP): a review

The Surface Water Acidification Programme (SWAP) was set up as collaborative research project involving scientists from Norway, Sweden and the UK. Its aim was to evaluate the factors responsible for fish decline in acid streams and lakes. A substantial sub-project was concerned with the palaeolimnological evidence for acidification and its causes. The central technique used was diatom analysis. In order to harmonise methodology between the seven diatomists from four laboratories in three countries a programme of taxonomic quality control was organised, involving slide exchanges, ‘blind’ counting, and regular workshops. In addition a calibration data-set of surface sediment diatoms and water chemistry from 170 lakes was constructed and archived on DISCO, the UCL diatom database. This data-set was used to generate diatom-chemistry transfer functions for pH, DOC and total Al using a weighted averaging technique. Application of the pH transfer function to sediment cores from a range of lakes demonstrated a dose-response relationship between lake sensitivity to acidification (as represented by mean Ca²⁺ values) and acid deposition (g S m ^-2 yr ^-1), indicating the overwhelming importance of acid deposition as the cause of lake acidification.     

COMPARATIVE pH DEPENDENT METAL INHIBITION OF NUTRIENT UPTAKE BY SCENEDESMUS QUADRICAUDA(CHLOROPHYCEAE)1,2

Cadmium and copper inhibition of nutrient uptake by the green alga Scenedesmus quadricauda is highly pH dependent in an inorganic medium; both metals are less toxic at low pH. The alga was grown in chemostats with both N and P approaching limiting levels; it was then possible to study metal toxicity to the nitrate, ammonium, and phosphate uptake systems of algae in an identical physiological state. When the logarithm of the Cd concentration causing 25% inhibition of nitrate, ammonium, and phosphate uptake was regressed against pH almost perfect linear relationships were obtained. This was also true at the 50% inhibition level, except for a smaller than predicted increase in Cd toxicity to ammonium uptake at pH 8, which may be due to the beginning of Cd precipitation at this pH. Cu²⁺ toxicity was linearly related to pH for ammonium and phosphate uptake and although, its toxicity for nitrate uptake also increased with pH, the increase was not perfectly linear. The toxicity of total Cu showed no linear relationship to pH. Cd²⁺ and Cu²⁺ toxicity increased by up to four orders of magnitude from pH 5 to 8. Competition between free metal and hydrogen ions for uptake sites on the cell surface is suggested as a mechanism increasing the toxicity of free metal, ions as the hydrogen ion content decreases (i.e. at higher pH).     

Heterogeneity of the changes in cytoplasmic pH upon serum stimulation of quiescent fibroblasts

Addition of mitogens to quiescent cells results in rapid ionic changes in the cytoplasm, including pH. We studied the changes in cytoplasmic pH in single Swiss 3T3 cells upon serum stimulation using fluorescence ratio imaging microscopy. Quiescence was attained using two approaches, serum deprivation of subconfluent cells and confluence. All measurements were made in the presence of bicarbonate and the absence of other organic buffers. We also used BCECF coupled to dextran to avoid several artifacts associated with using BCECF-AM, including leakage and phototoxicity. Analysis of the changes in cytoplasmic pH demonstrated a dramatic heterogeneity in the responses of single cells. There were six basic classes of responses, 1) a fast alkalinization, reaching a maximum pH in approximately 2-5 min; 2) a slow alkalinization, reaching a maximum pH in 10-20 min; 3) a very slow alkalinization, not reaching a plateau pH within the measurement time; 4) no apparent change in pH during the measurement time; 5) an early transient acidification, followed by either a fast or slow alkalinization; and 6) an acidification, followed by alkalinization and then by a decrease to some intermediate pH. Subconfluent cells exhibited greater heterogeneity in response than confluent cells, with no single dominant class of response. The dominant (55%) response for confluent cells was a gradual alkalinization of approximately 0.01 pH units/min. A larger proportion (52%) of subconfluent cells exhibited an early transient acidification compared to confluent cells (7%). A significant proportion of both types of cells (23% subconfluent, 36% confluent) exhibited no change in cytoplasmic pH upon stimulation. In general, the kinetics of changes in cytoplasmic pH were significantly different from the published results with population averaging methods.     

Nitrate uptake and extracellular alkalinization by the green alga Hydrodictyon reticulatum in blue and red light

Nitrate uptake and the medium alkalinization related to it were studied with nets of the coenocytic, giant cell, green alga Hydrodictyon reticulatum. A comparison of red, blue and white light irradiation showed no special control of nitrate uptake and of the corresponding alkalinization of the external medium by light quality, but rather a response as expected for the photosynthetic apparatus. In the dark, nitrate uptake rates amounted to one-fifth of those in saturating white light. This is in contrast to the chlorococcal microalga Monoraphidium braunii, where blue light specifically switched on nitrate uptake-dependent alkalinization and where uptake and reduction of nitrate strongly depended on blue light; the rates in pure red light and in the dark being very low. The stoichiometric ratio between nitrate taken up and extracellular alkalinization was close to 1 (0.86) in air with CO2 but close to 2 (1.84) in N2 for nitrate pre-loaded cells. In the absence of any carbon source, a high proportion of the absorbed and reduced nitrogen is released, most of it as ammonium which causes the excess alkalinization and some as nitrite, which lowers the ratio. Nitrite and ammonium release rates under anaerobic, CO2-free conditions were also independent of red or blue light and continued for several hours when the medium was buffered at pH 6. The data indicate that nitrate uptake, but less its reduction, is regulated differently in vacuolate, coenocytic algae from microalgae. In Hydrodictyon, nitrate uptake and reduction seems to be controlled by energy supply; in various microalgae, in addition, it is controlled specifically by blue light.     

冬小麦等4种作物对铵,硝态氮的吸收能力

采用水培试验探讨了冬小麦、大豆、油菜和莴笋４种作物对硝、铵态氮的相对吸收能力以及这两种氮源对它们生长发育的影响。试验表明：（１）不同氮源对供试作物的生长发育影响极大。供给硝态氮,这些作物生长发育良好,供给等量的ＮＯ＾－３和ＮＨ＾－４（１：１）时,蔬菜作物莴笋生长量下降幅度最大;供给铵态氨,莴笋和大豆极为敏感,供给ＮＯ＾－３时莴笋吸氮量显著高于供给等氮量ＮＯ＾－３和ＮＨ＾＋４,莴上麦供给等量ＮＯ＾－     

Protons and calcium alter gating of the hyperpolarization-activated cation current (I(h)) in rod photoreceptors

We investigated the effects of protons and calcium ions on the voltage-dependent gating of the hyperpolarization-activated, nonselective cation channel current, I(h), in rod photoreceptors. I(h) is a cesium-sensitive current responsible for the peak-plateau sag during the rod response to bright light. The voltage dependence of I(h) activation shifted about 5 mV per pH unit, with external acidification producing positive shifts and alkalinization producing negative shifts. Increasing external [Ca(2+)] from 3 to 20 mM resulted in a large (approximately 17 mV) positive shift in I(h) activation. External [Ca(2+)] (20 mM) blocked pH-induced shifts in activation. Cytoplasmic acidification produced by 25 mM sodium acetate led to a negative shift in inactivation (-9 mV) and internal alkalinization produced with 20 mM ammonium chloride resulted in a positive shift (+6 mV). Surface charge binding and screening theory (Gouy-Chapman-Stern) accounted for the observed shifts in I(h) activation, with the best fit achieved when protons and calcium ions were assumed to bind to distinct sites on the membrane. Since light induces changes in the retinal ionic environment, these results permit us to gauge the degree to which rod light responses could be modified via alterations in I(h) activation.     

湖泊氮素氧化及脱氮过程研究进展

自然界中氮的生物地球化学循环主要由微生物驱动,由固氮作用、硝化作用、反硝化作用和氨化作用来完成。过去数十年间,随着异养硝化、厌氧氨氧化和古菌氨氧化作用的发现,人们对环境中氮素循环认识逐步深入,提出了多种脱氮途径新假说。对湖泊生态系统中氮素的输入、输出及其在水体、沉积物和水土界面的迁移转化过程进行了概括,对湖泊生态系统中反硝化和厌氧氨氧化脱氮机理及脱氮效率的最新研究进展进行了探讨,并对以后的氮素循环研究进行了展望。