Stimulated Nitrogenase Activity Research Articles

The Influence of Bio-Stimulants and Foliar Fertilizers on Yield, Plant Features, and the Level of Soil Biochemical Activity in White Lupine (Lupinus albus L.) Cultivation

The aim of this study is to assess the effect of two biostimulators (Titanit, Rooter) and six foliar fertilizers (Optysil, Metalosate Potassium, Bolero Bo, ADOB 2.0 Zn IDHA, ADOB B, ADOB 2.0 Mo) on white lupine. In addition, we evaluated the enzymatic activity of dehydrogenase, acid, and alkaline phosphatases, catalase, the level of biological nitrogen fixation, yield, plant biometric, chlorophyll fluorescence and chlorophyll content. A field experiment was conducted between 2016 and 2018 at the Gorzyń Experimental and Educational Station, Poznań University of Life Sciences in Poland. The best effects in plant yield were obtained after the application of Optysil or ADOB Zn IDHA. The three years results of dehydrogenase (DHA), alkaline phosphatase (PAL), and the biological index of soil fertility (BIF), show that the bio-stimulants and most of the foliar fertilizers used did not always stimulate the activity of these enzymes and index in the white lupine crops, as compared with the control treatment. Analysis of the results of the acid phosphatase activity (PAC) shows that during the entire white lupine growing season the foliar fertilizers and bio-stimulants decreased the activity of this enzyme. This effect was not observed when the Metalosate potassium foliar fertilizer was applied. The field analyses of biological nitrogen fixation showed that the fertilizers and bio-stimulants significantly stimulated nitrogenase activity under the white lupine plantation. The best effects in plant yield were obtained after application Optysil or ADOB Zn IDHA.

Changes in Biological Nitrogen Fixation and Natural-Abundance N Isotopes of Sugarcane Under Molybdenum Fertilization

Molybdenum (Mo) can stimulate nitrogenase activity (NTA), increase biological nitrogen (N) fixation (BNF) and reduce natural-abundance N isotopes (δ15N). Our hypothesis is that this stimulus depends on genotype and N availability for sugarcane. The objective of this study was to evaluate the effect of the Mo fertilization on BNF, NTA, δ15N and the accumulation of N in different genotypes (RB867515 and RB92579) of sugarcane fertilized with N. The cultivars were subjected to two rates of N fertilization (without N fertilization and 60 kg ha−1) and two rates of Mo fertilization (without Mo fertilization and 200 g ha−1). NTA, δ15N and BNF were evaluated throughout the cycle of the sugarcane. Both leaf and root NTA was maximal at approximately 100 DAP. Mo fertilization stimulated NTA in RB867515 but reduced in RB92579. The bacteria associated with the sugarcane fixed atmospheric-N2 and the Mo fertilization stimulated this fixation, especially without N fertilization. Mo fertilization promoted greater N accumulation and should be encouraged for fertility management, especially in soils with low N content or as an alternative for reducing the use of N fertilizers in sugarcane.

Role of nitrogen fixation in the autecology of Polaromonas naphthalenivorans in contaminated sediments

Polaromonas naphthalenivorans strain CJ2 is a Gram-negative betaproteobacterium that was identified, using stable isotope probing in 2003, as a dominant in situ degrader of naphthalene in coal tar-contaminated sediments. The sequenced genome of strain CJ2 revealed several genes conferring nitrogen fixation within a 65.6 kb region of strain CJ2's chromosome that is absent in the genome of its closest sequenced relative Polaromonas sp. strain JS666. Laboratory growth and nitrogenase assays verified that these genes are functional, providing an alternative source of nitrogen in N-free media when using naphthalene or pyruvate as carbon sources. Knowing this, we investigated if nitrogen-fixation activity could be detected in microcosms containing sediments from the field site where strain CJ2 was isolated. Inducing nitrogen limitation with the addition of glucose or naphthalene stimulated nitrogenase activity in amended sediments, as detected using the acetylene reduction assay. With the use of fluorescence microscopy, we screened the microcosm sediments for the presence of active strain CJ2 cells using a dual-labelling approach. When we examined the carbon-amended microcosm sediments stained with both a strain CJ2-specific fluorescent in situ hybridization probe and a polyclonal fluorescently tagged antibody, we were able to detect dual-labelled active cells. In contrast, in sediments that received no carbon addition (showing no nitrogenase activity), no dual-labelled cells were detected. Furthermore, the naphthalene amendment enhanced the proportion of active strain CJ2 cells in the sediment relative to a glucose amendment. Field experiments performed in sediments where strain CJ2 was isolated showed nitrogenase activity in response to dosing with naphthalene. Dual-label fluorescence staining of these sediments showed a fivefold increase in active strain CJ2 in the sediments dosed with naphthalene over those dosed with deionized water. These experiments show that nitrogen fixation may play an important role in naphthalene biodegradation by strain CJ2 and contribute to its ecological success.

A Membrane-Associated Mn-Superoxide Dismutase Protects the Photosynthetic Apparatus and Nitrogenase from Oxidative Damage in the Cyanobacterium Anabaena sp. PCC 7120

We investigated the functions of a membrane-associated manganese superoxide dismutase (MnSOD) of the heterocystous cyanobacterium Anabaena sp. PCC 7120. The gene sodA encoding MnSOD was inactivated by interposon mutagenesis and it was confirmed by Southern hybridization and immunoblotting. The strain A17, lacking sodA, grew more slowly than the wild type, and the difference in growth rates between the two strains became larger with an increase in growth light intensity. More severe inhibition of growth of A17 was observed when the cells were grown in the absence of combined nitrogen. Complementation of A17 with a full copy of the sodA gene restored the wild-type phenotypes. Strain A17 produced more malondialdehyde than did the wild type, especially under high light intensity, indicating more lipid peroxidation in the absence of MnSOD. A17 was also more susceptible to photoinhibition by a high light, and it was shown that both PSII and PSI were more severely damaged by the photoinhibitory light in A17, suggesting that the MnSOD plays important roles in protection of both photosystems. Immunoblotting revealed that the MnSOD was present in vegetative cells and heterocysts. Light greatly stimulated nitrogenase activity in the wild type under both aerobic and anaerobic conditions, but stimulated nitrogenase activity in A17 only slightly in air. The results suggest that reactive oxygen species produced in heterocysts under aerobic conditions cause the inactivation of nitrogenase in the absence of MnSOD.

Nitrogenase activity in Alnus incana root nodules. Responses to O(2) and short-term N(2) deprivation.

O(2) and host-microsymbiont interactions are key factors affecting the physiology of N(2)-fixing symbioses. To determine the relationship among nitrogenase activity of Frankia-Alnus incana root nodules, O(2) concentration, and short-term N(2) deprivation, intact nodulated roots were exposed to various O(2) pressures (pO(2)) and Ar:O(2) in a continuous flow-through system. Nitrogenase activity (H(2) production) occurred at a maximal rate at 20% O(2). Exposure to short-term N(2) deprivation in Ar:O(2) carried out at either 17%, 21%, or 25% O(2) caused a decline in the nitrogenase activity at 21% and 25% O(2) by 12% and 25%, respectively. At 21% O(2), nitrogenase activity recovered to initial activity within 60 min. The decline rate was correlated with the degree of inhibition of N(2) fixation. Respiration (net CO(2) evolution) decreased in response to the N(2) deprivation at all pO(2) values and did not recover during the time in Ar:O(2). Increasing the pO(2) from 21% to 25% and decreasing the pO(2) from 21% to 17% during the decline further decreased rather than stimulated nitrogenase activity, showing that the decline was not due to O(2) limitation. The decline was possibly due to a temporary disturbance in the supply of reductant to nitrogenase with a partial O(2) inhibition of nitrogenase at 25% O(2). These results are consistent with a fixed O(2) diffusion barrier in A. incana root nodules, and show that A. incana nodules differ from legume nodules in the response of the nitrogenase activity to O(2) and N(2) deprivation.

Effects of inorganic nitrogen compounds on the activity and synthesis of nitrogenase in Gloeothece (Nägeli) sp. ATCC 27152

Addition of 2 mM nitrite or ammonium to aerobically incubated cultures of Gloeothece rapidly inhibited N2 fixation (measured as acetylene reduction). In contrast, 2 mM nitrate inhibited N2 fixation less rapidly and less extensively, and often temporarily stimulated nitrogenase activity. The inhibitory effects of both nitrate and ammonium could be prevented by addition of 3 mM L‐methionine‐DL‐sulphoximine, suggesting that the true inhibitor of N2 fixation was an assimilatory product of ammonium rather than either ammonium or nitrate itself. The inhibition of N2 fixation by nitrite could not, however, be prevented by addition of L‐methionine‐DL‐ sulphoximine. On the other hand, nitrite (unlike nitrate and ammonium) did not inhibit N2 fixation in cultures incubated under a gas phase lacking oxygen. These findings suggest that the mechanism whereby nitrite inhibits N2 fixation in Gloeothece differs from that of either nitrate or ammonium. The inhibitory effect of nitrite on N2 fixation did not involve reduction of nitrite to nitric oxide, though nitric oxide was a potent inhibitor of nitrogenase activity in Gloeothece. Nitrate and nitrite inhibited the synthesis of nitrogenase in Gloeothece, while ammonium not only inhibited nitrogenase synthesis but also stimulated degradation of the enzyme. In addition, all three compounds favoured the appearance of the Fe‐protein of nitrogenase in its larger, presumed inactive, form.

Effects of carbon, nitrogen, and electron acceptor availability on anaerobic N 2-fixation in a beech forest soil

The effects of different energy sources and inhibitors on the nitrogenase activity of a beech forest soil of north-east Bavaria (Germany) were determined using the acetylene-reduction method. Simple sugars, cellobiose and starch greatly stimulated nitrogenase activity under anaerobic, but not under aerobic, conditions; of the substrates tested, cellobiose yielded the highest anaerobic activity. In contrast, organic acids had no appreciable stimulating effect. Glucose-stimulated anaerobic nitrogenase activity was linked to the formation of butyrate (implicating the involvement of clostridia) and to the apparent growth of anaerobic N 2-fixing microorganisms. The N 2-fixing microorganisms culturable under anaerobic conditions in unamended soil approximated 10 5 cells g −1 dry wt soil and increased three orders of magnitude after 2 d of anaerobic incubation with glucose. In contrast, no N 2-fixing microorganisms were culturable under aerobic conditions. Inhibition of anaerobic nitrogenase activity by supplemental ammonium and nitrate occurred in both unamended and glucose-amended soils. Nitrate was sequentially transformed to N 2O and N 2, with minor amounts also being reduced to ammonium. Thus, nitrate appeared to have an indirect inhibitory effect on N 2-fixation, the reduction of nitrate being more competitive for available reductant than N 2-fixation. This competitive effect was not observed with sulfate. These results suggest that in situ N 2-fixation in the forest soil examined may be regulated in part by the availability of specific reductant sources and the presence of the competing reductant sink nitrate.

Immobilized cyanobacteria as a biofertilizer for rice crops; Intl. Conference on Applied Algology, Knysna, South Africa, April 1996.

N2-fixing cyanobacteria (Anabaena azollae, symbiont strains) were immobilized in polyurethane foam and ammonia production by the cyanobacteria was investigated in the laboratory and rice field. The cyanobacterial symbiont, A. azollae - MPK-SK-AM-24 showed the highest growth rate and biomass production amongst the 5 isolates examined while A. azollae-AS-DS showed the highest nitrogenase activity followed by A. variabilis - SA0 (wild type, non-symbiotic). Treatment of the foam-immobilized cyanobacteria with the systemic fungicide Bavistin stimulated nitrogenase activity while inhibiting glutamine synthetase (GS) activity. Free-living A. azollae-MPK-SK-AF-38, A. azollae - MPK-SK-AM-24 and A. azollae-MPK-SK-AM-27 excreted the highest amounts of ammonia into the growth medium; under foam - immobilized conditions the ammonia production increased further. Treatment of the foam - immobilized cyanobacteria with the fungicides Bavistin and Vitavax resulted in ammonia production at significantly higher rates. Rice seedlings (var. ADT 36) grown in the laboratory in conjunction with foam - immobilized A. azollae showed increased growth. A field experiment with paddy rice and foam - immobilized A. azollae strains indicated that the cyanobacteria excreted significant amounts of ammonia into the flood water in the rice fields resulting in increased chlorophyll content of the plants and increased the rice grain and straw yields. A combination of fertilizer nitrogen and inoculation with foam - immobilized cyanobacteria also significantly increased the rice grain and straw yield. Additionally, both A. azollae and A. variabilis were immobilized in sugarcane waste (bagasse), added to rice paddy and resulted in increased rice grain yield.

Effect of herbicides on nitrogen fixation (C 2H 2 reduction) associated with rice rhizosphere

In a field study nitrogenase activity associated with rice rhizosphere was differently influenced by the applied herbicides. Pretilachlor at two application levels had no effect on nitrogenase activity while butachlor and benthiocarb exerted marginal stimulation. Cinmethylin consistently stimulated nitrogenase activity throughout the plant growth period. Anilofos when applied singly had no substantial effect on nitrogenase activity but in combination with 2,4-D the activity was enhanced. Populations of anaerobic nitrogen-fixing bacteria and Azospirillum sp. and Azotobacter sp. were stimulated in such a combination.

Nitrogenase activity in marine sediments from a temperate saltmarsh lagoon: modulation by complex polysaccharides, ammonium and oxygen

Nitrogenase activity (acetylene reduction) was examined in marine sediments associated with seagrass beds and thalassinidean prawn burrows, in a temperate saltmarsh lagoon. Nitrogenase activity in unamended sediments from different microhabitats ranged from undetectable values to ≈0.75 nmol C 2H 4 (g dry sediment) −1·hour −1. The bulk of organic material available to heterotrophic bacteria in saltmarsh lagoons is probably derived from primary production. Amendment of sediments with structural plant polysaccharides showed that xylan and alginate stimulated nitrogenase activity on average by 5 to 18-fold relative to unamended sediments, whereas cellulose and carrageenan were less effective. Amendment of sediments with storage plant polysaccharides produced the greatest stimulation of nitrogenase activity. Addition of laminarin and glycogen (amylopectin) significantly ( p <0.05) stimulated nitrogenase activity by 19 to 92-fold. In contrast to these polysaccharides of plant origin, chitin (a polymer of prawn exoskeletons) did not significantly enhance nitrogenase activity in these sediments. Aerobic conditions stimulated nitrogenase activity by 2 to 20-fold in surface sediments, but not in sub-surface sediments, indicating that aerobic or microaerophilic respiration was a significant source of energy for nitrogenase activity in surface sediments. Oxygen-stimulation of nitrogenase activity was less marked in aerobic sediments around the rim of prawn burrow openings where subsurface sediments had been displaced to the surface by bioturbation. Ammonium did not appear to play a major role in the regulation of nitrogenase activity in these sediments as in situ concentrations were ≈20-fold lower than the concentration (50 μM) which resulted in a slight (0–15%) reduction in nitrogenase activity.

The Role of Oxygen in the Regulation of Nitrogenase Activity in Drought-Stressed Soybean Nodules.

The aim of this study was to investigate the mechanism of nitrogenase inhibition in drought-stressed soybean (Glycine max L.) nodules to determine whether this stress was similar to other inhibitory treatments (e.g. detopping) known to cause an O2 limitation of nodule metabolism. Nodulated soybean plants were either detopped or subjected to mild, moderate, or severe drought stress by growth in different media and by withholding water for different periods. All treatments caused a decline in nitrogenase activity, and in the drought-stressed nodules, the decline was correlated with more negative nodule water potentials. Increases in rhizosphere O2 concentration stimulated nitrogenase activity much more in detopped plants than in drought-stressed plants, reflecting a greater degree of O2 limitation with the detopped treatment than with the drought-stressed treatment. These results indicated that drought stress differs from many other inhibitory treatments, such as detopping, in that its primary cause is not a decrease in nodule permeability and a greater O2 limitation of nodule metabolism. Rather, drought stress seems to cause a decrease in the maximum O2-sufficient rate of nodule respiration or nitrogenase activity, and the changes in nodule permeability reported to occur in drought-stressed nodules may be a response to elevated O2 concentrations in the infected cell that may occur as nodule respiration declines.

Inoculation of wheat straw to enhance lignocellulose breakdown and associated nitrogenase activity

A range of lignocellulolytic fungi isolated from decaying straw was examined for ability to increase the rate of straw breakdown (as measured by CO 2 production), to cross-feed two selected diazotrophs and hence to increase the rate of nitrogen fixation (measured as C 2H 2 reduction) under both axenic conditions and in native soil. Cyathus stercoreus was the only isolate obtained from decaying straw which was capable of degrading lignocellulose. Three out of 20 fungi which were isolated on their ability to degrade cellulose, showed varying degrees of ability to cross-feed the two diazotrophs. With straw as the sole source of carbon and nitrogen, co-inoculation of Beijerinckia indica B15 with Cyathus stercoreus stimulated nitrogenase activity in both axenic culture (1.7–12.22-fold increase according to the cellulolytic co-inoculant) and a native soil (pH 6.0; 2.2-fold increase). Azospirillum sp. DN64 in axenic culture was not stimulated when co-inoculated with Cyathus stercoreus alone but did respond to co-inoculation with a mixed group of cellulolytic fungi (22-fold increase in nitrogenase activity). The proportion of carbon released from the decaying straw as CO 2 was as high as 33% over an 8-week period when Cyathus stercoreus was present, but was only 16% in the absence of C. stercoreus, when only the cellulolytic fungi were present. The results indicate that, with the selection of suitable lignocellulolytic, cellulolytic and N 2-fixing microorganisms, it may be possible to improve the rate of straw breakdown and to fix enough atmospheric N 2 to supply the requirements of the cellulolytic organisms and to contribute positively to the N 2 balance of the following crop.

Effects of some inhibitors and carbon sources on acetylene reduction and hydrogen production of isolated heterocysts ofAnabaenasp. (strain CA)

Metabolically active heterocysts isolated from wild-type Anabaena sp. strain CA showed high rates of light dependent acetylene reduction and H2 evolution. Fructose and erythrose significantly stimulated nitrogenase activity but not H2 evolution. DCMU and cyanide were not effective. DBMIB significantly inhibited both nitrogenase and nitrogenase-catalysed H2 evolution. This inhibition was overcome by a catalytic amount of TMPD. These data suggest that in the isolated heterocysts all electrons, irrespective of source, must pass through the plastoquinone pool before reducing ferredoxin, which in turn can reduce dinitrogen to ammonia.

Evidence for the involvement of azospirillum brasi lense and helicomyces roseus in the aerobic nitrogen-fixing/cellulolytic system from sugarcane litter

A crude N 2-fixing-celluloytic culture from sugracane litter was enriched by inoculation onto filter paper then maintained on sterilized wheat straw under aerobic conditions. Attempts to reconstitute this system from assemblages of N 2 fixers, fungi and heterotrophic, non-N 2-fixing bacteria isolated from sugarcane culture were unseccesful, however, a variety of evidence indicates the ivolvement of the microerophilic N 2 fixer, Azospirillum brasilense, and dematiaceous fungus, Helicomyces roseus. While both Azotobacter and Azospirillum were isolated from field material, A. brasilense was the sole aerobic N 2 fixer to be isolated from culture maintained on straw and was obtained in high numbers (to 8 × 10 6 c.f.u. g −1 straw). Addition of a fungal inhibitor, cycloheximide, to sugarcane culture greatly reduced nitrogenase activity under air, except when malate or xylan (substrates used by A. brasilense) were added. Sucrose and glucose did not stimulate nitrogenase activity of cylclohexamide-treated culture under air. Onset of nitrogenase activity in straw or filter paper inoculated with sugarcane culture was invariably accompanied by darkening of this substrate and proliferaction of dematiaceous hyphae. H. roseaus was the only dimetiaceous fungus consistently isolated and its helical conidia were abundant shortly after straw was inoculated with sugarcane culture.

Effect of potassium on nitrogen fixation, nitrogen transport, and nitrogen harvest index of bean

Abstract Four cultivars of Phaseolus vulgaris L., Carioca, Venezuela‐ 350, Rio Tibagi and Negro Argel, were grown to maturity in a glass house with a low (1 mol m‐3) and a high (4.5 mol m‐3) potassium supply. Potassium stimulated nitrogenase activity (acetylene reduction) and the accumulation of ureides in pod walls of Carioca and Negro Argel, cultivars which had the highest rates of acetylene reduction. In three of the four cultivars the higher level of potassium nutrition resulted in a greater partitioning of above ground N to the seeds. The proportion of xylem sap N in the form of the ureides, allantoln and allantoic acid (allantoate), was only slightly increased by the higher rate of potassium supply. Seed yields were increased by high potassium in the cultivars Venezuela‐350 and Negro Argel. The results suggest that potassium stimulates the transport of nitrogenous compounds to developing fruits.

Macronutrient controls on nitrogen fixation in planktonic cyanobacterial populations

Abstract Experiments described in the literature have demonstrated that additions of N, Fe, and occasionally P, influence planktonic cyanobacterial nitrogen fixation in lakes and estuaries throughout the world. Increase in abundance of cyanobacteria which can fix nitrogen did not necessarily indicate that N2 fixation had occurred. In natural plankton assemblages, N2 fixation was normally stimulated by low total inorganic nitrogen (TIN) and depressed by additions of TIN. Nitrogenase was often stimulated by addition of Fe but soluble reactive phosphate (SRP) alone only stimulated nitrogenase activity occasionally. Luxury consumption and storage of P, but not N, explains the lack of P stimulation in nature. Nitrogenase activity was usually repressed at TIN oncentrations of >50–100 μg l‐1. Additions of N + P had variable effects which may depend on the balance between nitrogenase inhibition by N and general growth stimulation by N + P (which reduces ambient N).

EVIDENCE FOR CALCIUM‐MEDIATED REGULATION OF HETEROCYST FREQUENCY AND NITROGENASE ACTIVITY IN NOSTOC6720

SummaryThe proportion of heterocysts (heterocyst frequency) in cultures of Nostoc 6720 varied with the Ca2+ concentration present in the growth medium, the proportion increasing from 5 to 9 % as the Ca2+ concentration was increased from 10−5 to 10−4 M. The total nitrogenase activity of the culture declined by 20% whereas the activity per heterocyst decreased by 30%. In the 10−4 to 10−3 M range of Ca2+ concentrations, which includes those employed in most standard media, the heterocyst frequency and nitrogenase activity did not change. At Ca2+ concentrations above 2 × 10−3 M the heterocyst frequency decreased and this decrease was associated with a lower growth rate.The presence of the calcium ionophore A23187 increased the heterocyst content of the culture and total nitrogenase activity, but still decreased the nitrogenase activity per heterocyst. The effects of A23187 and Ca2+ concentration were additive and demonstrated a positive interaction between Ca2+ and the ionophore. In contrast, the Ca2+ antagonist, lanthanum, decreased the heterocyst frequency and, at concentrations below 10−8 M, stimulated nitrogenase activity. At higher concentrations of lanthanum, growth, heterocyst frequency and nitrogenase activity were markedly diminished; cell degradation occurred at concentrations in excess of 10−4 M.An analysis of the intra‐ and extracellular binding of 45Ca2+ showed that decreasing the Ca2+ concentrations in the medium from 10−4 to 10−5 M lowered the intracellular Ca2+ content. In the presence of A23187, intracellular Ca2+ increased. The lanthanum treatment was exceptional in that the intracellular Ca2+ was increased about four‐fold.These results are interpreted as favouring the hypothesis that a Ca2+‐mediated regulatory process modulates heterocyst frequency and nitrogenase activity in Nostoc 6720.

A distinct ferredoxin for nitrogen fixation isolated from heterocysts of the cyanobacterium Anabaena variabilis

Ferredoxin from heterocysts of Anabaena variabilis has been isolated and its biological activity compared to ferredoxin obtained from vegetative cells of the same organism. Both ferredoxins catalyze equally effective NADP + photoreduction by heterocyst thylakoids. When photoreduced, both ferredoxins transfer electrons to nitrogenase from A. variabilis, with heterocyst ferredoxin being twice as active as vegetative cell ferredoxin. In the dark, however, only heterocyst ferredoxin is able to link reducing power, generated by soluble systems, such as H 2/hydrogenase (from Clostridium pasteurianum) and NADPH/ferredoxin:NADP + oxidoreductase (from A. variabilis), to the cyanobacterial nitrogenase. Using heterocyst homogenates with glucose 6-phosphate as electron donor, only ferredoxin from heterocysts is able to stimulate nitrogenase activity further.

Ammonia and light effect on nitrogenase activity in nitrogen-limited continuous cultures of Rhodopseudomonas capsulata. Role of glutamine synthetase

Nitrogen-limited continuous cultures of Rhodopseudomonas capsulata were used to investigate some aspects of the regulation of nitrogenase activity. The role of glutamine synthetase (GS) in this regulation was examined by measuring changes of its adenylylation state when the light intensity and the nitrogen source were varied. Maximal nitrogenase activity was observed at a dilution rate corresponding to about one third of the maximum specific growth rate (μmax), both in ammonia- and in glutamate-limited cultures. At higher dilution rates, both GS and nitrogenase were inactivated by ammonia. Determination of the kinetics of inhibition of both enzymes indicated that the degree of inactivation of nitrogenase and the adenylylation state of GS were not closely related. Increase of light intensity stimulated nitrogenase activity dramatically. Conversely, a shift-down in light intensity to a limiting value resulted in a decrease of nitrogenase activity suggesting that synthesis was inhibited. On the other hand, the adenylylation state of glutamine synthetase appeared to be unaffected by changes in light intensity, indicating that GS is probably not involved in the regulation of nitrogenase expression by light.

Sarcosine: a possible regulatory compound in thePeltigera praetextata-Nostocsymbiosis

Abstract In analyses of free amino acids in the lichen Peltigera praetextata high levels of sarcosine ( N -methylglycine) were found. Sarcosine was found to stimulate nitrogenase activity of the cyanobiont Nostoc , isolated from Peltigera canina . This stimulation was optimal at 1 mM sarcosine, a concentration which is close to that calculated as being present in a water-saturated lichen thallus. Sarcosine also was found to inhibit the activity of glutamine synthetase in Nostoc . This inhibitory effect may account for the stimulation of nitrogenase activity and may be important in the symbiotic regulation of cyanophilic lichens.