The effect of some ammonium salts on nitrate reductase level,onin vivo nitrate reduction and on nitrate content in excisedpisum sativum roots

The effect of some ammonium salts on nitrate reductase (NR) level, onin vivo nitrate reduction and on nitrate content was followed in the presence of nitrate in the medium, under changing experimental conditions, in excisedPisum sativum roots, and their effect was compared with that of KNO₃, Ca(NO₃)₂ and NaNO₃ at 15 mM NO₃ ^- concentration, i.e. at a concentration which considerably exceeded the level of saturation with nitrate with respect to nitrate reductase. The effect of ammonium salts on NR level is indirect and changes from a positive one to a strongly negative one which is dependent on the time of action of the salt, on the presence of other cations, on pH of the solution of the ammonium salt and on the nature of the anion of the ammonium salt. A positive effect on the enzyme level can be observed in the presence of other cations than NH₄ ⁺ at suitable concentrations of those ammonium salts, the solutions of which have their pH values in the acid region (i.e. NH₄H₂PO₄, (NH₄)₂SO₄ and NH₄NO₃). However their positive effect is independent of the presence of NH₄ ⁺ ions, and it is obviously the result of an increased concentration of H⁺ ions. A clear-cut negative effect on NR level can be observed after 24 h in one-salt NH₄NO₃ solution where NH₄ ⁺ is not balanced with other cations and thus certainly can adversely influence many metabolic processes, and in the solutions containing neutral (pH 6.2) and dibasic ammonium phosphates in which dissolved undissociated ammonia [(NH₃). (H₂O) which can also affect many metabolic processes incl. proteosynthesis] probably has a toxic influence. Thein vivo nitrate reduction is always depressed in excised pea roots in the presence of ammonium salts in the medium, regardless of the level of nitrate reductase. Under the described conditions, no relationship could be established between the enzyme level and the so-called metabolic NO₃ ^- pool (i.e. NO₂ ^- production under anaerobic conditions), nor between NR level and the total nitrate content in the roots. One-salt solutions of NaNO₃, Ca(NO₃)₂ and KNO₃ exert different effects on the level of nitrate reductase and on the content of NO₃ ^- in the roots, but the in vivo NO₃ ^- reduction shows the same trend as NR level in the roots influenced by these salts. Cl^- ions, supplied in NH₄C1, depress both NR level and NO₃ ^- content in the roots at higher concentrations, but they do not significantly affect the in vivo nitrate reduction in comparison with other ammonium salts. These results indicate that NR level,in vivo nitrate reduction, and nitrate uptake can be regulated in pea roots independently of each other.     

Lack of end-product control of nitrite reductase level in pea roots

The effects of various ammonium salts and amino acids on nitrite reductase (NIR) induction in isolated pea roots cultured in media containing nitrate or nitrite and either exogenous sucrose or no sugar were investigated. Thg aim of these investigations was to determine if the NIR level is subject to end-product control. The results showed that even though some ammonium salts and casamino acids can depress NIR level under certain conditions this inhibition cannot be interpreted in terms of direct end-product inhibition of NIR synthesis because their effects were dependent on the character (anion) and toxicity of the respective ammonium salt, on the presence of exogenous sucrose in the induction medium, and on the inducer of NIR. NH₄HCO₃ inhibited NIR induction at those concentrations which were toxic to the roots, ammonium phosphates hampered NIR induction only in roots exposed to nitrite in media containing sucrose, while casamino acids slightly depressed NIR induction only in roots exposed to nitrate and exogenous sucrose. The results further show that the basal (noninduced) NIR level changes little even under strongly toxic conditions.     

Properties of in vivo nitrogenase activity in Beggiatoa alba

A procedure was devised for analyzing in vivo nitrogenase activity in Beggiatoa alba B18LD which involves: (1) the induction of nitrogenase in cells pre-grown on NH₄Cl, by washing the cells free of NH₄Cl and lowering their exposure to oxygen, and (2) measuring acetylene reduction by these cells. Using this induction methodology we examined the effects of pH, temperature, and nitrogenous compounds on in vivo nitrogenase induction and activity in Beggiatoa alba B18LD. Nitrate and nitrite repressed the induction of nitrogenase activity, but glutamine did not. Induction and activity had a combined pH optimum of 6.5 to 8.0, and activity had a temperature optimum of 29°C. Ammonium and urea caused immediate inhibition of nitrogenase activity, but nitrate, nitrite, glutamine, asparagine, and other amino acids did not. Ammonium-induced inhibition was transient and incomplete, and the duration of inhibition increased in direct proportion to the amount of ammonium added. Methionine sulfoximine, a glutamine synthetase inhibitor, at a final concentration of 50 μM blocked ammonium uptake by cells, but did not prevent nitrogenase inhibition if added before ammonium. Our results imply that B. alba nitrogenase inhibition by ammonium: (1) is not directly caused by ammonium assimilation products, (2) is probably not due to an enzymatic inactivation, and (3) may be related to ammonium transport.     

Photosynthetic nature of nitrate uptake and reduction in the cyanobacterium Anacystis nidulans

The photosynthetic nature of the initial stages of nitrate assimilation, namely, uptake and reduction of nitrate, has been investigated in cells of the cyanobacterium Anacystis nidulans treated with l-methionine dl-sulfoximine to prevent further assimilation of the ammonium resulting from nitrate reduction. The light-driven utilization of nitrate or nitrite by these cells results in ammonium release and is associated with concomitant oxygen evolution. Stoichiometry values of about 2 mol oxygen evolved per mol nitrate reduced to ammonium and 1.5 mol oxygen per mol nitrite have been determined in the presence of CO₂, as well as in its absence, with nitrate or nitrite as the only Hill reagent. This indicates that in A. nidulans water photolysis directly provides, without the need for carbon metabolites, the reducing power required for the in vivo reduction of nitrate and nitrite to ammonium, processes which are besides strongly inhibited when the operation of the photosynthetic noncyclic electron flow is blocked. Evidence indicating the participation of concentrative transport system(s) in the uptake of nitrate and nitrite by A. nidulans is also presented. The operation of these energy-requiring systems seems to account for the sensitivity to ATP-synthesis inhibitors exhibited by nitrate and nitrite utilization in l-methionine dl-sulfoximine-treated cells. The utilization of nitrate by A. nidulans cells, concomitant with oxygen evolution, can therefore be considered as a genuinely CO₂-independent photosynthetic process that makes direct use of photosynthetically generated assimilatory power.     

THE INFLUENCE OF AMMONIUM SALTS ON CELL REACTION

1. It may be shown by means of cells of the flowers of a hybrid Rhododendron which contain a natural indicator, by means of starfish eggs stained with neutral red, and by means of an "artificial cell" in which living frog''s skin is employed that increased intracellular alkalinity may be brought about by solutions of a decidedly acid reaction which contain ammonium salts. 2. These results are analogous to those previously obtained with the CO₂-bicarbonate system, and depend on the facts: (a) that NH₄OH is sufficiently weak as a base to permit a certain degree of hydrolysis of its salts; and (b) that living cells are freely permeable to NH₄OH (or NH₃?) and not to mineral and many organic acids, and presumably not at least to the same extent to ammonium salts as such.     

Specific binding affinity for DNA of the L phage (Salmonella typhimurium) in extracts of Escherichia coli

The repressor gene c _II of the L phage was cloned into plasmid pHC624 and expressed in E. coli. Two separate binding affinities for L phage DNA were identified during fractionation of protein extract of that strain. The activity that salts out in low concentration of ammonium sulphate belonged to the repressor, the activity that salts out in high concentrations of (NH₄)₂SO₄ was proved to be of E. coli origin. Binding sites for the two proteins are located on different fragments of the L phage genome.     

Regulation of Nitrate Uptake in Penicillium chrysogenum by Ammonium Ion

A nitrate uptake system is induced (along with nitrate reductase) when NH₄⁺-grown Penicillium chrysogenum is incubated with inorganic nitrate in synthetic medium in the absence of NH₄⁺. Nitrate uptake and nitrate reduction are probably in steady state in fully induced mycelium, but the ratios of the two activities are not constant during the induction period. Substrate concentrations of ammonium cause a rapid decay of nitrate uptake and nitrate reductase activity. The two activities are differentially inactivated (the uptake activity being more sensitive). Glutamine and asparagine are as effective as NH₄⁺ in suppressing nitrate uptake activity. Glutamate and alanine were about half as effective as NH₄⁺. Cycloheximide interferes with the NH₄⁺-induced decay of nitrate uptake activity. The ammonium transport system is almost maximally deinhibited (or derepressed) in nitrate-grown mycelium.     

Effects of Ammonium and Non-Ammonium Salt Additions on Methane Oxidation by Methylosinus trichosporium OB3b and Maine Forest Soils

Additions of ammonium and non-ammonium salts inhibit atmospheric methane consumption by soil at salt concentrations that do not significantly affect the soil water potential. The response of soils to non-ammonium salts has previously raised questions about the mechanism of ammonium inhibition. Results presented here show that inhibition of methane consumption by non-ammonium salts can be explained in part by ion-exchange reactions: cations desorb ammonium, with the level of desorption varying as a function of both the cation and anion added; differential desorption results in differential inhibition levels. Differences in the extent of inhibition among ammonium salts can also be explained in part by the effects of anions on ammonium exchange. In contrast, only minimal effects of cations and anions are observed in liquid cultures of Methylosinus trichosporium OB3b. The comparable level of inhibition by equinormal concentrations of NH₄Cl and (NH₄)₂SO₄ and the insensitivity of salt inhibition to increasing methane concentrations (from 10 to 100 ppm) are of particular interest, since both of these patterns are in contrast to results for soils. The greater inhibition of methane consumption for NH₄Cl than (NH₄)₂SO₄ in soils can be attributed to increased ammonium adsorption by sulfate; increasing inhibition by non-ammonium salts with increasing methane concentrations can be attributed to desorbed ammonium and a physiological mechanism proposed previously for pure cultures.     

Methane oxidation in boreal peat soils treated with various nitrogen compounds

The potential methane consumption activity was examined in various plant communities of a boreal Sphagnum-dominated Bakchar bog of West Siberia. In aerobic laboratory incubations, the peat consumed methane with the maximal rates varied from 17 to 153 nmol CH₄ h^–1g^{– 1}.The highest oxidation took place in the peat from the cotton grass and dwarf shrub-cotton grass communities. The addition of different N-compounds inhibited CH₄-uptake and was not a simple influence of shift in ionic balance (`salt effect'). The introduction of sodium chloride resulted in significantly weaker inhibition effect than the same amount of nitrite and nitrate salts. The inhibition occurred at NH₄ ⁺-N concentrations exceeding 100 mg kg^–1, which was more than 200 times higher native N-content in peat. Communities with high CH₄-uptake activity were more sensitive to ammonium. The inhibition by ammonium was non-competitive. The inhibition by ammonium was mainly due to the toxic action of nitrite and/or nitrate produced by nitrifiers. A strong positive correlation was found between the potential nitrifying activity and inhibition of CH₄-uptake in ammonium-treated peat (R ²= 0.87). The oxidized N-compounds were more strong inhibitors than ammonium and their toxicity increased in the following range: NH₄ ⁺< NO₂ ^–< NO₃ ^–.     

Suppression of long-day flowering by nitrogenous compounds in Lemna perpusilla 6746

The long-day flowering of Lemna perpusilla 6746 on an SH inhibitor-containingmedium was inhibited by the application of ammonium ion to themedium. Ammonium ion not only suppressed long-day flowering,but relieved the inhibition of vegetative growth caused by theinhibitors. Nitrite, casamino acids, glutamine and asparaginehad a similar effect, suggesting that the inhibition of long-dayflowering by ammonium ion is not a direct effect of the ion.Most amino acids, with the exception of glutamate and aspartate,also prevented long-day flowering, but their effects on vegetativegrowth varied. No qualitative differences in amino acid compositionwere observed among plants cultured on media containing nitrate,nitrite or NH4₄NO₃as the sole nitrogen source. However, theamounts of free and total amino acids werehigher in plants fedwith nitrite or NH₄NO₃ than in those fed with nitrate. Thissuggests that the inhibition of long-day flowering by ammoniumand nitrite can be ascribed to increased nitrogen metabolism. Though decreased activity by SH inhibitors of nitrate reductase(SH enzyme) is assumed to result in long-day flowering by loweringthe nitrogen metabolism, lowering the nitrogen level in M mediumdid not bring about floral initiation in the absence of SH inhibitors. (Received January 7, 1975; )     

Heterotrophic nitrogen removal by <Emphasis Type="Italic">Providencia rettgeri</Emphasis> strain YL

Providencia rettgeri strain YL was found to be efficient in heterotrophic nitrogen removal under aerobic conditions. Maximum removal of NH₄ ⁺–N occurred under the conditions of pH 7 and supplemented with glucose as the carbon source. Inorganic ions such as Mg²⁺, Mn²⁺, and Zn²⁺ largely influenced the growth and nitrogen removal efficiency. A quantitative detection of nitrogen gas by gas chromatography was conducted to evaluate the nitrogen removal by strain YL. From the nitrogen balance during heterotrophic growth with 180 mg/l of NH₄ ⁺–N, 44.5% of NH₄ ⁺–N was in the form of N₂ and 49.7% was found in biomass, with only a trace amount of either nitrite or nitrate. The utilization of nitrite and nitrate during the ammonium removal process demonstrated that the nitrogen removal pathway by strain YL was heterotrophic nitrification-aerobic denitrification. A further enzyme assay of nitrate reductase and nitrite reductase activity under the aerobic condition confirmed this nitrogen removal pathway.     

Nitrogen Redox Metabolism of a Heterotrophic, Nitrifying-Denitrifying Alcaligenes sp. from Soil

Metabolic characteristics of a heterotrophic, nitrifier-denitrifier Alcaligenes sp. isolated from soil were further characterized. Pyruvic oxime and hydroxylamine were oxidized to nitrite aerobically by nitrification-adapted cells with specific activities (V_max) of 0.066 and 0.003 μmol of N × min⁻¹ × mg of protein⁻¹, respectively, at 22°C. K_m values were 15 and 42 μM for pyruvic oxime and hydroxylamine, respectively. The greater pyruvic oxime oxidation activity relative to hydroxylamine oxidation activity indicates that pyruvic oxime was a specific substrate and was not oxidized appreciably via its hydrolysis product, hydroxylamine. When grown as a denitrifier on nitrate, the bacterium could not aerobically oxidize pyruvic oxime or hydroxylamine to nitrite. However, hydroxylamine was converted to nearly equimolar amounts of ammonium ion and nitrous oxide, and the nature of this reaction is discussed. Cells grown as heterotrophic nitrifiers on pyruvic oxime contained two enzymes of denitrification, nitrate reductase and nitric oxide reductase. The nitrate reductase was the dissimilatory type, as evidenced by its extreme sensitivity to inhibition by azide and by its ability to be reversibly inhibited by oxygen. Cells grown aerobically on organic carbon sources other than pyruvic oxime contained none of the denitrifying enzymes surveyed but were able to oxidize pyruvic oxime to nitrite and reduce hydroxylamine to ammonium ion.     

The high affinity of Paracoccus denitrificans cells for nitrate as an electron acceptor. Analysis of possible mechanisms of nitrate and nitrite movement across the plasma membrane and the basis for inhibition by added nitrite of oxidase activity in permeabilised cells

(1) The apparent K_m for nitrate of the electron-transport system in intact cells of Paracoccus denitrificans was less than 5 μM. In contrast the apparent K_m for nitrate of inverted membrane vesicles oxidising NADH was greater than 50 μM. When azide, a competitive inhibitor, was present, the apparent K_m observed with the vesicles was raised to 0.64 mM, consistent with values previously reported for purified preparations of the reductase. In membrane vesicles the nitrate reductase is probably not rate-limiting for NADH-nitrate oxido-reductase activity, and thus a lower limit for K_m (NO₃⁻) is obtained. It is suggested that the very low K_m (NO₃⁻) in intact cells must arise from either a transport process or a nitrate-specific pore that allows access of nitrate directly to the active site of its reductase from the periplasm. (2) The swelling of spheroplasts has been studied under both aerobic and anaerobic conditions to probe possible mechanisms of nitrate and nitrite transport across the plasma membrane of P. denitrificans. Nitrate reductase was inhibited by azide to prevent reduction of internal nitrate. No evidence for operation of either nitrate-nitrite antiport or proton-nitrate symport was obtained. (3) Measurements from the fluorescence intensity of 8-anilino-naphthalene-1-sulphonate of the rates of decay of diffusion potentials generated by addition of potassium salts to valinomycin-treated plasma membrane vesicles from P. denitrificans showed that the permeability of the membrane to anions is SCN⁻ > NO₃⁻, NO₂⁻, pyruvate, acetate > CI⁻ > SO₄²⁻. In the presence of a protonophore the rate of decay of the diffusion potential was considerably enhanced with potassium acetate or potassium nitrite, but not with potassium salts of nitrate, chloride or pyruvate. This result indicates that HNO₂ and CH₃COOH can rapidly and passively diffuse across the cell membrane. This finding suggests that transport systems for nitrite are in general probably not required in bacteria. The failure of a protonophore to enhance the dissipation of the diffusion potential generated by potassium nitrate is evidence against the operation of a proton-nitrate symporter. (4) Low concentrations of added nitrite very strongly inhibit electron flow to oxygen in anaerobically grown cells, provided that they have been treated with Triton X-100 or an uncoupler. This inhibition is not observed with aerobically grown cells. It is concluded that the inhibitory species is a reaction product or an intermediate of the nitrite reductase reaction. The requirement for collapse of protonomotive force by uncoupler or permeabilising the plasma membrane suggests that any such species could be negatively charged. Nitroxyl anion (NO⁻) can be considered, as its conjugate acid is a postulated intermediate between nitrite and nitrous oxide; nitroxyl anion can bind to heme centres to give nitrosyl derivatives. (5) The basis for the ability of permeabilised, but not intact, cells of P. denitrificans to reduce oxygen and nitrate simultaneously is discussed.     

Factors Controlling Anaerobic Ammonium Oxidation with Nitrite in Marine Sediments

Factors controlling the anaerobic oxidation of ammonium with nitrate and nitrite were explored in a marine sediment from the Skagerrak in the Baltic-North Sea transition. In anoxic incubations with the addition of nitrite, approximately 65% of the nitrogen gas formation was due to anaerobic ammonium oxidation with nitrite, with the remainder being produced by denitrification. Anaerobic ammonium oxidation with nitrite exhibited a biological temperature response, with a rate optimum at 15°C and a maximum temperature of 37°C. The biological nature of the process and a 1:1 stoichiometry for the reaction between nitrite and ammonium indicated that the transformations might be attributed to the anammox process. Attempts to find other anaerobic ammonium-oxidizing processes in this sediment failed. The apparent K_m of nitrite consumption was less than 3 μM, and the relative importance of ammonium oxidation with nitrite and denitrification for the production of nitrogen gas was independent of nitrite concentration. Thus, the quantitative importance of ammonium oxidation with nitrite in the jar incubations at elevated nitrite concentrations probably represents the in situ situation. With the addition of nitrate, the production of nitrite from nitrate was four times faster than its consumption and therefore did not limit the rate of ammonium oxidation. Accordingly, the rate of this process was the same whether nitrate or nitrite was added as electron acceptor. The addition of organic matter did not stimulate denitrification, possibly because it was outcompeted by manganese reduction or because transport limitation was removed due to homogenization of the sediment.     

Effects of Exogenously Supplied Ammonium on Root Development of Scots Pine (Pinus sylvestris L.) Seedlings

The effect of potentially toxic concentrations of ammonium on root development of Scots pine seedlings raised on Perlite was investigated during growth periods of 3 or 10 weeks after sowing. It was shown that imbalanced ammonium nutrition led to conspicuous changes of root morphology provided the pH value in the medium was allowed to decrease to 3.9 due to the NH⁺₄-dependent proton excretion into the rhizosphere. Ammonium toxicity could not be observed with seedlings treated either with ammonium nitrate or with ammonium chloride at pH 5.3 ? 6.8. While the supply of NH⁺₄ considerably inhibited root development the biomass production of the shoot was increased. Determination of the endogenous level of ammonium in roots and the leaf whorl exclude a simple causal correlation between ammonium toxicity and accumulated ammonium as has been postulated for herbaceous plants.     

Ammonium fixation by some soils in the ussr and the availability of this ion to plants

Summary The ability of various soils to fix ammonium in unexchangeable form was studied. Soils in their natural state contain unexchangeably fixed NH₄. The amount of which considerably increases with systematic fertilization and in soils with high nutrient status, but the ability of these soils for extra fixation of ammonium decreases. Chernozem soils are able to fix ammonium in unexchangeable form in considerably greater quantities than soddy-podzolic soils. All soils show a little increase of ability to fix ammonium with profile depth. The heavier is the soil the greater is its ability for fixation of NH₄ in unexchangeable form.In the conditions of a greenhouse experiment unexchangeably fixed NH₄ was used by plants very poorly. Exchangeably absorbed ammonium is available to plants, although to a lesser extent than water soluble salts. Soils fix ammonium to a greater amount from NH₄OH than from ammonium nitrate, hence the latter is a better source of nitrogen for plants than ammonia liquor solution if applied to soils with a definite abolity to fix unexchangeable ammonium.     

A possible mechanism of ammonium ion regulation of photosynthetic carbon flow in higher plants

Addition of NH₄⁺ to the photosynthesizing leaf cells of Dolichos lab lab L. var. Lignosis Prain and leaf discs of Vigna sinensis L. savi ex Hassk caused a significant increase in the flow of photosynthetic carbon toward amino acids with a concomitant decrease toward sugars without affecting the over-all photosynthetic rate. Similar diversion of photosynthetic carbon away from sugars was also observed in the photosynthesizing isolated chloroplasts of V. sinensis, but the latter differed in that they accumulated organic acids rather than amino acids. In an effort to understand the mechanism of NH₄⁺-mediated regulation, the specific and total activities of NAD(P)-glutamate dehydrogenase, glutamine synthetase, pyruvate kinase, alkaline fructose 1,6-bisphosphatase, and NAD(P)-glyceraldehyde-3-phosphate dehydrogenase of the cells of D. lab lab were checked but none was affected by the added ammonium salts even after prolonged incubation. At certain concentrations, ammonium ions abolished the light activation of NADP-glyceraldehyde-3-phosphate dehydrogenase and alkaline fructose 1,6-bisphosphatase in isolated chloroplasts from dark-adapted Vigna leaves without interfering with the basal dark activity of these enzymes. Based on these observations, a possible mechanism of action of NH₄⁺ in regulating the photosynthetic carbon flow is postulated.     

Enhancement of cyanobacterial salt tolerance by combined nitrogen

Presence of certain nitrogenous compounds in the growth medium significantly enhanced the salt tolerance of the fresh-water cyanobacterium Anabaena sp. strain L-31 as well as the brackish water cyanobacterium Anabaena torulosa. Among these, nitrate, ammonium, and glutamine were most effective followed by glutamate and aspartate. These nitrogenous compounds also inhibited Na⁺ influx in both Anabaena spp. with the same order of effectiveness as that observed for protection against salt stress. The inhibition of Na⁺ influx on addition of the nitrogenous substances was rapid; nitrate and ammonium inhibited Na⁺ influx competitively. Proline and glycine did not affect Na⁺ influx and also had no influence on the salt tolerance of either Anabaena sp. The observed protection was not consequent to a stimulatory effect of combined nitrogen on growth per se. Uptake of NO₃⁻ and NH₄⁺ increased during salt stress but was not correlated with growth. Intracellular levels of NO₃⁻ and NH₄⁺ were found to be inadequate to constitute a major component of the internal osmoticum. The results suggest that inhibition of Na⁺ influx by combined nitrogen is a major mechanism for protection of cyanobacteria against salt stress.     

Nitrate Reduction to Nitrite,Nitric Oxide and Ammonia by Gut Bacteria under Physiological Conditions

The biological nitrogen cycle involves step-wise reduction of nitrogen oxides to ammonium salts and oxidation of ammonia back to nitrites and nitrates by plants and bacteria. Neither process has been thought to have relevance to mammalian physiology; however in recent years the salivary bacterial reduction of nitrate to nitrite has been recognized as an important metabolic conversion in humans. Several enteric bacteria have also shown the ability of catalytic reduction of nitrate to ammonia via nitrite during dissimilatory respiration; however, the importance of this pathway in bacterial species colonizing the human intestine has been little studied. We measured nitrite, nitric oxide (NO) and ammonia formation in cultures of Escherichia coli, Lactobacillus and Bifidobacterium species grown at different sodium nitrate concentrations and oxygen levels. We found that the presence of 5 mM nitrate provided a growth benefit and induced both nitrite and ammonia generation in E.coli and L.plantarum bacteria grown at oxygen concentrations compatible with the content in the gastrointestinal tract. Nitrite and ammonia accumulated in the growth medium when at least 2.5 mM nitrate was present. Time-course curves suggest that nitrate is first converted to nitrite and subsequently to ammonia. Strains of L.rhamnosus, L.acidophilus and B.longum infantis grown with nitrate produced minor changes in nitrite or ammonia levels in the cultures. However, when supplied with exogenous nitrite, NO gas was readily produced independently of added nitrate. Bacterial production of lactic acid causes medium acidification that in turn generates NO by non-enzymatic nitrite reduction. In contrast, nitrite was converted to NO by E.coli cultures even at neutral pH. We suggest that the bacterial nitrate reduction to ammonia, as well as the related NO formation in the gut, could be an important aspect of the overall mammalian nitrate/nitrite/NO metabolism and is yet another way in which the microbiome links diet and health.     

Oxygen transfer efficiency and environmental osmolarity response to neutralizing agents on l-lactic acid production efficiency by Lactobacillus paracasei

The effects of Ca(OH)₂, NH₄OH and NaOH as neutralizing agents on the efficiency of l-lactic acid production by Lactobacillus paracasei were investigated in this study. Fermentation performance with Ca(OH)₂ was superior to NH₄OH and NaOH because it had the highest oxygen transfer rate (OTR) and lowest environmental osmotic pressure. Much smaller bubbles were generated using a calcium lactate solution compared with those generated using ammonium lactate and sodium lactate solutions, indicating that Ca(OH)₂ had the highest OTR. Moreover, experiments demonstrated that ammonium lactate and sodium lactate caused more severe osmotic stress on cell growth than calcium lactate. In conclusion, the effects of neutralizing agents on l-lactic acid production efficiency could be ascribed to the contribution of lactates to OTR and environmental osmotic stress.