Response of conifer seedlings to nitrate and ammonium sources of nitrogen

Summary Differences in growth responses of Douglas fir, western hemlock, Sitka spruce, and white spruce to nitrate and ammonium N sources were examined in sand culture and artificial soil culture. Effects of the two forms of N on growth, needle area, and N uptake of three Douglas fir halb-sib progenies were examined in a second sand culture. Response of Douglas fir to the two forms of N was followed over two years in nursery soil of different pH levels. In sand culture 1 mean seedling dry weight of all species, except hemlock, was greatest when ammonium N and nitrate N were provided in equal amounts. In all species, except Sitka spruce, ammonium alone resulted in greater growth than nitrate alone. Use of ammonium N resulted in greater growth of all species, than was obtained with nitrate N, at pH values in the region 5.4 and 7.5 in artificial soil culture. Only Douglas fir showed substantial differences due to N source below pH 5. Growth of all species was greater at pH 5.4 than at 7.5 in each N source treatment. Growth of Douglas fir seedlings was greatest with ammonium N and least with nitrate N in sand culture 2. Supply of nitrate and ammonium in equal proportions resulted in intermediate growth. Leaf area/plant weight ratio was unaffected by N source. Analysis of nutrient solutions showed appreciable nitrification of ammonium N during the 7 days between solution changes. In the three greenhouse experiments, with little exception, increase in proportion of ammonium in N supply resulted in increase of seedling tissue N concentration. This effect was more pronounced in roots than shoots. Total N uptake by ammonium fed seedlings was about double the N uptake of nitrate fed seedlings in sand culture 2. Nursery grown Douglas fir seedlings showed greater growth response to ammonium sulphate than to calcium nitrate, and this appeared due entirely to form of N supply in the first year. A similar response in the second year was partly due to greater soil acidification by ammonium sulphate. Compared with calcium nitrate, ammonium sulphate increased N concentration of one-year old shoots, but this difference was not detected by foliar analysis of two-year old seedlings.     

Effects of abscisic acid and its acetylenic alcohol on dormancy, root development and transpiration in three conifer species

Abscisic acid (ABA) and a synthetic analog, the 2- cis acetylenic alcohol, were compared to evaluate their effectiveness in conditioning seedlings of Douglas-fir [ Pseudotsuga menziesii (Mirb.) Franco], Engelmann spruce ( Picea engelmannii Parry) and lodgepole pine ( Pinus contorta Dougl.). Following preconditioning with ABA and the analog, seedlings were water stressed with the osmoticum polyethylene glycol (PEG) 3350. The effects of the growth regulators on transpiration, net photosynthesis, their ratio, called water use efficiency, and cell water relations parameters were then compared in stressed and unstressed plants. The antitranspirant action of these compounds varied depending on the species, the growth regulator, and the level of stress. ABA promoted transpiration in unstressed seedlings for all 3 species seven days after application. The analog was superior to ABA as an antitranspirant in osmotically-stressed lodgepole pine and Engelmann spruce, but neither compound was effective in Douglas-fir. For Douglas-fir and Engelmann spruce, net photosynthesis remained consistently higher in ABA-treated plants during the two levels of osmotic stress, relative to control and analog treatments. Neither compound had any effect on root development or cell water relations. ABA, and to a lesser extent its analog, hastened terminal bud formation in seedlings exposed to short days and low temperatures.     

Differences in nitrate and ammonium uptake between Scots pine and European larch

In forest soils, ammonium is usually the predominant form of inorganic nitrogen. However, the capacity of trees to utilize both NO₃ ^- and NH₃ ⁺ may provide greater flexibility in responding to changes of nitrogen supply from the environment. Such capacity has been studied in seedlings of Scots pine (Pinus sylvestris L.) and European larch (Larix decidua Mill.) grown in the presence or absence of either nitrate or ammonium. Nitrate-induced plants showed a higher nitrate uptake rate than non-induced plants; this difference was almost negligible after 24 h of exposure to NO₃ ^-. Ammonium uptake in both species was consistently higher than that of nitrate, regardless of prior nitrogen provision. In both nutrient conditions, larch showed a more efficient transport system in comparison with Scots pine, with higher ammonium and nitrate uptake rates in both induced and non-induced plants. This was consistent also with the activity of nitrate reductase, measured in vivo in roots and leaves. This revised version was published online in June 2006 with corrections to the Cover Date.     

Studies on nitrate reductase from Cyanidium caldarium

Summary A nitrate reductase from the thermophilic acidophilic alga, Cyanidium caldarium, was studied. The enzyme utilises the reduced forms of benzyl viologen and flavins as well as both NADPH₂ and NADH₂ as electron donors to reduce nitrate.Heat treatment has an activating effect on the benzyl viologen (FMNH₂, FADH₂) nitrate reductase. At 50°C the activation of the enzyme is complete in about 20 min of exposure, whereas at higher temperatures (until 75°C) it is virtually an instantaneous phenomenon. The observed increase in activity is very low in extracts from potassium nitrate grown cells, whereas it is 5 or more fold in extracts from ammonium sulphate supplied cells. The benzyl viologen nitrate reductase is stable at 60°C and is destroyed at 75°C after 3 min; the NADPH₂ nitrate reductase is destroyed at 60°C. The pH optimum for both activities was found in the range 7.8–8.2.Ammonium nitrate grown cells possess a very low level of nitrate reductase: when they are transferred to a nitrate medium a rapid synthesis of enzyme occurs. By contrast, when cells with fully induced activity are supplied with ammonia, a rapid loss of NADPH₂ and benzyl viologen nitrate reductase occurs; however, activity measured with heated extracts shows that the true level of benzyl viologen nitrate reductase is as high as before ammonium addition. It is suggested that the presence of ammonia causes a rapid inactivation but no degradation of the enzyme.Cycloheximide inhibits the formation of the enzyme; the drug is without effect on the loss of nitrate reductase activity induced by ammonium. The nitrate reductase is reactivated in vivo by the removal of the ammonium, in the absence as well as in the presence of cycloheximide.     

Response of Douglas fir seedlings to nitrate and ammonium nitrogen sources at different levels of pH and iron supply

Douglas fir seedlings were grown for two to three months in sand and soil cultures in a greenhouse to examine their growth response to nitrogen (N) source at different levels of pH and iron (Fe) supply. In the first two experiments nutrient solutions of known pH were automatically applied to the top of the sand cultures and allowed to run to waste from the bottom. Under these conditions seedlings made most growth on nitrate (NO₃–N) under acid (pH4) conditions, but most growth on ammonium (NH₄–N) under neutral (pH7) conditions. Calcium carbonate (CaCO₃) was used to create a range of pH conditions (from 4.0 to 7.2) in a peat and sand artificial soil. Over the pH range 4 to 6 NH₄–N or NO₃+NH₄–N produced larger seedlings than NO₃–N alone, but above pH6 growth on all N sources was depressed. Chemical analysis showed that seedling Ca concentration had increased and Fe concentration had decreased with increase in CaCO₃ application. Both Ca and Fe concentrations were higher in seedlings receiving NO₃–N than in those receiving NH₄ or NO₃+NH₄.In sub-irrigated sand cultures, Doughlas fir seedlings receiving NO₃–N were shown to respond to additions of Fe chelate, but seedlings receiving NH₄–N responded little to Fe chelate. At pH5 seedlings receiving NO₃–N did not grow as big as seedlings receiving NH₄–N in the absence of Fe chelate, but addition of Fe chelate resulted in NO₃-fed seedlings growing larger than NH₄-fed seedlings. The relationship between seedling Fe concentration and N nutrition is discussed.The relatively larger root dry weight and surface area of seedlings grown on NO₃–N, as compared to NH₄–N, in sand culture, was noted.     

Influence of ammonium and nitrate supply on growth,nitrate reductase activity and N-use efficiency in a natural hybrid pine and its parents

Aims Selection of tree species with a high capacity to assimilate N and efficiently utilize N resources would facilitate the success of initial tree seedling establishment in infertile soils. The preference for N forms was tested using three pine species (Pinus densata, Pinus tabuliformis and Pinus yunnanensis). Pinus densata is a natural diploid hybrid between P. tabuliformis and P. yunnanensis .Methods Seedlings of three pine species were supplied with nitrate-N, ammonium-N (at two different pH regimes) or combined ammonium and nitrate as a nitrogen source in perlite culture in a controlled environment.Important findings Seedlings of P. densata had higher total biomass and net photosynthesis when supplied with nitrate-N and ammonium nitrate than with ammonium-N. In parental species, total biomass and net photosynthesis for P. yunnanensis seedlings was higher in ammonium-N than in nitrate-N, whereas the other parental species P. tabuliformis had the highest total biomass among species for all treatments except ammonium with CaCO 3. Most morphological traits in P. densata seedlings were intermediate between its two parental species. However, N-use efficiency and photosynthetic N-use efficiency of P. densata significantly exceeded both parents when supplied with nitrate-N and ammonium nitrate. The results suggested that the diploid hybrid tree species P. densata has a preference for nitrate and is not well adapted to ammonium-N as a sole nitrogen source regardless of the growth medium pH. Based on changes in environmental conditions, such as predicted future temperature increases in high altitude areas associated with climate change, P. densata is likely to be increasingly competitive and have wide adaptation in high altitude regions.     

Effect of mycorrhiza and nitrate nutrition on nitrate reductase activity in Scots pine seedlings

In vivo nitrate reductase (EC 1.6.6.1) activity was measured in seedlings of Scots pine ( Pinus sylvestris L.) inoculated with Cenococcum geophilum (Sow.) Ferd. & Winge, Paxillus involutus (Batsch:Fr) Fr, Piloderma croceum Erikss, & Hjortst, and Suillus variegatus (Fr.) O. Kuntze. The activity was higher in the mycorrhizal pine roots than was previously found in the fungus symbiont alone, but lower than in the roots of nonmycorrhizal pine seedlings. The differences observed in a previous study between the fungal species under pure culture conditions were not found in the present work for mycorrhiza synthezised with the same fungal species. An increase in the nitrate concentration of the nutrient solution increased the proportion of the nitrate reductase activity in the needles. The mycorrhizal root tips had higher nitrate reductase activity than nonmycorrhizal root tips in the same root system.     

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.     

Effect of nitrate and ammonium concentration on nitrate reductase activity in five species of mycorrhizal fungi

The nitrate reducing capacity of pure cultures of Cenococcum geophilum (Sow.) Ferd. & Winge, Paxillus involutes (Batsch: Fr.) Fr. (strains 1 and 2), Piloderma croceum Erikss. & Hjortst., Suillus variegatus (Fr.) O. Kuntze (strains 1 and 2) and an ectendomycorrhizal (E-strain) fungus was measured using an in vivo nitrate reductase (EC 1.6.6.3) assay. Differences between species and strains were established. The nitrate concentration of the culture medium influenced the nitrate reductase activities of the E-strain fungus and one strain of S. variegatus. The nitrate reductase activity of certain species and strains was a function of nitrate concentration. Addition of ammonium to the growth medium did not have any significant effect on the in vivo or in vitro nitrate reductase activity. The in vivo nitrate reductase activity in the mycelia of C. geophilum and the E-strain fungus decreased during 28 day growth in modified Melin-Norkrans medium. For mycelia of Paxillus involutus, Piloderma croceum and S. variegatus grown on agar the in vitro assays showed higher nitrate reductase activity than the in vivo assays.     

A nitrate reductase gene of the cyanobacterium Synechococcus PCC6301 inferred by heterologous hybridization,cloning and targeted mutagenesis

DNA probes from the narG gene of Escherichia coli, which encodes the large polypeptide of respiratory nitrate reductase, show cross-hybridization at low stringency to a single region of the genome of the cyanobacterium Synechococcus PCC6301. This segment of cyanobacterial DNA was cloned as the insert of plasmid pDN1 and characterized. RNA complementary to pDN1 was shown to be substantially more abundant in nitrate grown cells of Synechococcus PCC6301 than in ammonium grown cells, thus parallelling the nitrate induction and ammonium repression of nitrate reductase activity in cultures of this cyanobacterium. A mutant of Synechococcus PCC6301 deficient in nitrate reductase activity was obtained after a potentially mutagenic transformation treatment using pDN1 as a donor. This mutant was restored to the wild type phenotype following stable integrative transformation with pDN1 DNA. Taken together these data suggest that pDN1 might encode a polypeptide of nitrate reductase. pDN1 is distinct from three clones of genes involved in nitrate assimilation that were isolated previously from the related cyanobacterium Synechococcus PCC7942 (Kuhlemeier et al., 1984a, J.Bact. 159, 36–41, and 1984b, Gene 31, 109–116).     

Assimilatory nitrate uptake in Pseudomonas fluorescens studied using nitrogen-13

The mechanism of nitrate uptake for assimilation in procaryotes is not known. We used the radioactive isotope, ¹³N as NO₃ ^-, to study this process in a prevalent soil bacterium, Pseudomonas fluorescens. Cultures grown on ammonium sulfate or ammonium nitrate failed to take up labeled nitrate, indicating ammonium repressed synthesis of the assimilatory enzymes. Cultures grown on nitrite or under ammonium limitation had measurable nitrate reductase activity, indicating that the assimilatory enzymes need not be induced by nitrate. In cultures with an active nitrate reductase, the form of ¹³N internally was ammonium and amino acids; the amino acid labeling pattern indicated that ¹³NO₃ ^- was assimilated via glutamine synthetase and glutamate synthase. Cultures grown on tungstate to inactivate the reductase concentrated NO₃ ^- at least sixfold. Chlorate had no effect on nitrate transport or assimilation, nor on reduction in cell-free extracts. Ammonium inhibited nitrate uptake in cells with and without active nitrate reductases, but had no effect on cell-free nitrate reduction, indicating the site of inhibition was nitrate transport into the cytoplasm. Nitrate assimilation in cells grown on nitrate and nitrate uptake into cells grown with tungstate on nitrite both followed Michaelis-Menten kinetics with similar K _mvalues, 7 M. Both azide and cyanide inhibited nitrate assimilation. Our findings suggest that Pseudomonas fluorescens can take up nitrate via active transport and that nitrate assimilation is both inhibited and repressed by ammonium.     

The development of nitrate reductase in Chlorella and its repression by ammonium

Summary Chlorella vulgaris, grown with ammonium sulphate as nitrogen source, contains very little nitrate reductase activity in contrast to cells grown with potassium nitrate. When ammonium-grown cells are transferred to a nitrate medium, nitrate reductase activity increases rapidly and the increase is partially prevented by chloramphenicol and by p-fluorophenylalanine, suggesting that protein synthesis is involved. The increase in nitrate reductase activity is prevented by small quantities of ammonium; this inhibition is overcome, in part, by raising the concentration of nitrate. Although nitrate stimulates the development of nitrate reductase activity, its presence is not essential for the formation of the enzyme since this is formed when ammonium-grown cells are starved of nitrogen and when cells are grown with urea or glycine as nitrogen source. It is concluded that the formation of the enzyme is stimulated (induced) by nitrate and inhibited (repressed) by ammonium.     

A comparison of ammonium, nitrate and proton net fluxes along seedling roots of Douglas-fir and lodgepole pine grown and measured with different inorganic nitrogen sources

Significant spatial variability in NH4+, NO3- and H+ net fluxes was measured in roots of young seedlings of Douglas-fir (Pseudotsuga menziesii) and lodgepole pine (Pinus contorta) with ion-selective microelectrodes. Seedlings were grown with NH4+, NO3-, NH4NO3 or no nitrogen (N), and were measured in solutions containing one or both N ions, or no N in a full factorial design. Net NO3- and NH4+ uptake and H+ efflux were greater in Douglas-fir than lodgepole pine and in roots not exposed to N in pretreatment. In general, the rates of net NH4+ uptake were the same in the presence or absence of NO3-, and vice versa. The highest NO3- influx occurred 0-30 mm from the root apex in Douglas-fir and 0-10 mm from the apex in lodgepole pine. Net NH4+ flux was zero or negative (efflux) at Douglas-fir root tips, and the highest NH4+ influx occurred 5-20 mm from the root tip. Lodgepole pine had some NH4+ influx at the root tips, and the maximum net uptake 5 mm from the root tip. Net H+ efflux was greatest in the first 10 mm of roots of both species. This study demonstrates that nutrient uptake by conifer roots can vary significantly across different regions of the root, and indicates that ion flux profiles along the roots may be influenced by rates of root growth and maturation.     

Regulation by ammonium of nitrate and nitrite assimilation in Chlamydomonas reinhardtii

The inhibitor of mRNA synthesis, 6-methylpurine, inhibited nitrate reductase derepression in either ammonium-grown or methylammonium-treated wild-type cells of Chlamydomonas reinhardtii, but not in nitrogen-starved cells. In contrast, 6-methylpurine did not inhibit nitrate reductase synthesis in the methylammonium-resistant mutant 2170 (ma-1) either grown on ammonium, treated with methylammonium or nitrogen starved, but did inhibit the continuous synthesis of nitrate reductase, which required the presence of nitrate in the media. In both wild-type and mutant 2170 grown on ammonium and transferred to nitrate media, cycloheximide immediately prevented nitrate reductase derepression when added either at the beginning or at different times of induction treatment. Unlike wild-type cells, mutant 2170 was able to take up either nitrate or nitrite simultaneously with ammonium in whose presence nitrate and nitrite reductases were synthesized. However, synthesis of nitrate reductase was progressively inhibited in the mutant cells when the intracellular ammonium levels were raised as a result of an increase in either the external pH or the extracellular ammonium concentrations. The results rule out the existence of maturase-like proteins in Chlamydomonas and indicate that ammonium has a double effect on the regulation of nitrate reductase synthesis: (a) it prevents nitrate reductase mRNA production; and (b) it controls negatively the expression of this mRNA.     

Regulation of nitrate reductase inRhodobacter capsulatus E1F1

The photosynthetic purple non-sulfur nitrate-assimilating bacteriumRhodobacter capsulatus E1F1 has an adaptive nitrate reductase activity inducible by either nitrate or nitrite and molybdenum traces. Nitrate reductase induction by nitrate did not occur in media with nitrate and ammonium, which showed no effect if nitrite was the inductor instead of nitrate or in the presence ofl-methionine-dl-sulfoximine (MSX) plus nitrate. In vivo, tungstate inhibited nitrate reductase activity, and this was not recovered upon addition of molybdenum unless de novo protein synthesis took place. Nitrate reductase was also repressed in nitrogen-starved cells or after the addition of azaserine to cells growing phototrophically with nitrate. Moreover, higher rates of nitrate reductase induction and nitrite excretion were found in illuminated cells grown with nitrate under air than in those grown under argon.     

Dissimilatory nitrate reduction by a strain ofClostridium butyricum isolated from estuarine sediments

Nitrate dissimilation in chemostat grown cultures ofClostridium butyricum SS6 has been investigated. Sucrose limited cultures grown on nitrate produced nitrite as the principal end-product of nitrate reduction whilst under nitrate-limiting conditions ammonia accumulated in the spent media. Nitrate reduction was accompanied by the synthesis of a soluble nitrate reductase (123 nmol·NADH oxidised · min^-1 · mg protein^-1) and in addition, under N-limiting conditions, a soluble nitrite reductase (56 nmol NADH oxidised min^-1 · mg protein^-1). Corresponding ammonia grown cultures synthesised neither enzyme. Concurrent with the dissimilation of nitrate to nitrite and ammonia cell population densities increased by 18% (C-limitation) and 32% (N-limitation). Spent media analyses of the fermentation products from ammonia and nitrate grown cells showed the accumulation of acetate in nitrate dissimilating cultures. Molar ratios of acetate/butyrate increased by a factor of 5 (C-limitation) to 12 (N-limitation) upon adding nitrate to the growth medium. In C-limited cultures, grown on nitrate, hydrogenase activity was 340 nmol · min^-1 · mg protein^-1 and under N-limitation this increased to 906 nmol · min^-1 · mg protein^-1. Since N-limited cultures are electron acceptor limited, the increase in hydrogenase activity enables excess electrons to be spilled by this route.     

2′‐Deoxymugineic acid promotes growth of rice (Oryza sativa L.) by orchestrating iron and nitrate uptake processes under high pH conditions

Poaceae plants release 2′‐deoxymugineic acid (DMA) and related phytosiderophores to chelate iron (Fe), which often exists as insoluble Fe(III) in the rhizosphere, especially under high pH conditions. Although the molecular mechanisms behind the biosynthesis and secretion of DMA have been studied extensively, little information is known about whether DMA has biological roles other than chelating Fe in vivo. Here, we demonstrate that hydroponic cultures of rice (Oryza sativa) seedlings show almost complete restoration in shoot height and soil‐plant analysis development (SPAD) values after treatment with 3–30 μm DMA at high pH (pH 8.0), compared with untreated control seedlings at normal pH (pH 5.8). These changes were accompanied by selective accumulation of Fe over other metals. While this enhanced growth was evident under high pH conditions, DMA application also enhanced seedling growth under normal pH conditions in which Fe was fairly accessible. Microarray and qRT‐PCR analyses revealed that exogenous DMA application attenuated the increased expression levels of various genes related to Fe transport and accumulation. Surprisingly, despite the preferential utilization of ammonium over nitrate as a nitrogen source by rice, DMA application also increased nitrate reductase activity and the expression of genes encoding high‐affinity nitrate transporters and nitrate reductases, all of which were otherwise considerably lower under high pH conditions. These data suggest that exogenous DMA not only plays an important role in facilitating the uptake of environmental Fe, but also orchestrates Fe and nitrate assimilation for optimal growth under high pH conditions.     

Agar and ammonium nitrate influence hyperhydricity,tissue nitrate and total nitrogen content of serviceberry (Amelanchier arborea) shoots in vitro

Shoot tip cultures of Amelanchier arborea Michx.f. were grown on Murashige & Skoog or Woody Plant (WP) medium containing 4.4 M benzyladenine and various concentrations of agar. Increases in agar concentration affected various culture growth variables, decreased culture hyperhydricity and increased tissue nitrate concentration. Additions of ammonium nitrate to cultures grown on WP medium containing 0.4% agar increased all growth variables measured except percent dry weight. Hyperhydricity and tissue nitrate concentration also increase in response to increasing ammonium nitrate in the medium. Since hyperhydricity was shown to be both positively and negatively correlated with increases in tissue nitrate content, it is unlikely that tissue nitrate level alone directly affects hyperhydricity.Abbreviations BA benzyladenine - MS Murashige & Skoog - WP Woody Plant     

Respiratory nitrate ammonification by Shewanella oneidensis MR-1

Anaerobic cultures of Shewanella oneidensis MR-1 grown with nitrate as the sole electron acceptor exhibited sequential reduction of nitrate to nitrite and then to ammonium. Little dinitrogen and nitrous oxide were detected, and no growth occurred on nitrous oxide. A mutant with the napA gene encoding periplasmic nitrate reductase deleted could not respire or assimilate nitrate and did not express nitrate reductase activity, confirming that the NapA enzyme is the sole nitrate reductase. Hence, S. oneidensis MR-1 conducts respiratory nitrate ammonification, also termed dissimilatory nitrate reduction to ammonium, but not respiratory denitrification.     

Effect of nitrate,ammonia and nitrogen starvation on the regulation of nitrate reductase in Cyanidium caldarium

Summary Cells of Cyanidium caldarium grown with ammonia or ammonium nitrate as nitrogen source do not contain appreciable nitrate reductase activity. The alga develops the capacity to synthesize the enzyme when it is transferred from the ammonium medium to a nitrogen-free medium. Nitrate is not needed as an inducer and no enhancement in the rate of enzyme synthesis is observed when it is present. By contrast, whereas the synthesis of the enzyme in nitrogen-free medium proceeds at an increasing rate, in the nitrate medium it attains a stationary level after a short time.Nitrate grown cells possess variable amount of inactive nitrate reductase (from 9 to 60%) whereas in nitrogen-free medium the enzyme occurs principally in a fully active form. Addition of ammonia inactivates reversibly the preexisting enzyme. The inactive enzyme is measurable in the crude extract after activation by heating.It is suggested that in Cyanidium the inactivating effect of ammonia, which is the end product of nitrate reduction, in association with the repression of enzyme controls the level of nitrate reductase activity.