Hydrogen oxidation and efficiency of nitrogen fixation in succinate-limited chemostat cultures ofRhizobium ORS 571

Rhizobium ORS 571, isolated from stem nodules of the tropical legumeSesbania rostrata is able to grow in the chemostat with molecular nitrogen as sole nitrogen source at a specific growth rate of 0.1 h^-1. Samples from nitrogenfixing cultures showed high acetylene reduction activities: 1,500 nmol ethylene formed per milligram dry weight per hour. Under nitrogen-fixing conditions an uptake hydrogenase is induced. Ammonia-assimilating cultures, without additional hydrogen, did not induce hydrogenase. The addition of hydrogen to succinate-limited nitrogen-fixing cultures resulted in an increase in the molar growth yield on succinate (Y _succinate) from 27 to 35 and a slight decrease in the molar growth yield on oxygen ( ), showing that hydrogen oxidation is less energy-yielding than the oxidation of endogenous substrates. Respiration-driven proton translocation measured with starved cells indicated the functioning of site 1 and 2 of oxidative phosphorylation. Cytochrome spectra showed that cytochromea ₆₀₀, present at high dissolved oxygen tension (d.o.t.) almost completely disappeared at low d.o.t. In flash-photolysis spectra only thea-type cytochrome could be detected as an oxidase in cells both grown at high and low d.o.t. Growth yields in ammonia-assimilating cultures were higher than those measured in nitrogen-fixing cultures. Assuming two sites of oxidative phosphorylation, a molar growth yield on ATP (Y _ATP) of about 3 and 6 was calculated for respecticely nitrogen-fixing and ammonia-assimilating cultures. TheY _ATP under nitrogen-fixing conditions is dependent on the amount of H₂ formed per mol N₂ fixed (H₂/N₂ ratio). A method has been described to calculate the total amount of ATP use by nitrogenase during the fixation of 1 mol N₂ (ATP/N₂ ratio) and H₂/N₂ ratios in aerobic nitrogen fixing organisms. This calculation yielded that nitrogen fixation inRhizobium ORS 571 is a high ATP-consuming process. The calculated ATP/N₂ and H₂/N₂ ratios were respectively 42 and 7.5.Abbreviations d.o.t. dissolved oxygen tension A preliminary account of this work was presented at the 5th International Symposium on Nitrogen Fixation, September 1983, Noordwijkerhout, The Netherlands     

Cyanobacterial H<Subscript>2</Subscript> production — a comparative analysis

Several unicellular and filamentous, nitrogen-fixing and non-nitrogen-fixing cyanobacterial strains have been investigated on the molecular and the physiological level in order to find the most efficient organisms for photobiological hydrogen production. These strains were screened for the presence or absence of hup and hox genes, and it was shown that they have different sets of genes involved in H₂ evolution. The uptake hydrogenase was identified in all N₂-fixing cyanobacteria, and some of these strains also contained the bidirectional hydrogenase, whereas the non-nitrogen fixing strains only possessed the bidirectional enzyme. In N₂-fixing strains, hydrogen was mainly produced by the nitrogenase as a by-product during the reduction of atmospheric nitrogen to ammonia. Therefore, hydrogen production was investigated both under non-nitrogen-fixing conditions and under nitrogen limitation. It was shown that the hydrogen uptake activity is linked to the nitrogenase activity, whereas the hydrogen evolution activity of the bidirectional hydrogenase is not dependent or even related to diazotrophic growth conditions. With regard to large-scale hydrogen evolution by N₂-fixing cyanobacteria, hydrogen uptake-deficient mutants have to be used because of their inability to re-oxidize the hydrogen produced by the nitrogenase. On the other hand, fermentative H₂ production by the bidirectional hydrogenase should also be taken into account in further investigations of biological hydrogen production.Abbreviations Chl chlorophyll - MV methyl viologen     

Determination of the maximum product yield from glucoamylase-producing Aspergillus niger grown in the recycling fermentor

Aspergillus niger has been grown in glucose- and maltose-limited recycling cultures to determine the maximum growth yield, the maximum product yield for glucoamylase production, and the maintenance requirements at very slow specific growth rates. Using the linear equation for substrate utilization, and using the experimental data from both recycling experiments, both the maximum growth yield, Y_xsm, and the maximum product yield, Y_psm, could be determined. The values estimated were 157 g biomass per mol maltose for Y_xsm and 100 g protein per mol maltose for Y_psm. Expressed on a C₁-basis these values are 0.52 and 0.36 C-mole per C-mol for respectively Y_xsm and Y_psm. The found value for Y_psm is half the value found for alkaline serine protease production in Bacillus lichoniformis, and it can be concluded that formation of extracellular protein is more energy consuming in filamentous fungi than in prokaryotic organisms. Maintenance requirements are no significant factor during growth of Aspergillus niger, and reported maintenance requirements are most probably due to differentiation.     

Isolation and characterization of hydrogenase-negative mutants of Azorhizobium caulinodans ORS571

Hydrogenase-negative (Hup^-) mutants of Azorhizobium caulinodans ORS571 were isolated by means of Tn5 mutagenesis. The colony test used for screening for Hup^- strains was based on the absence of reduction of triphenyltetrazolium chloride with hydrogen. Suspensions from cultures of the mutant strains grown under derepressing conditions did not use hydrogen with methylene blue or oxygen as the hydrogen acceptor. The mutants were shown to carry single Tn5 insertions at different locations in the A. caulinodans genome. Molar growth yields (corrected for poly--hydroxybutyrate formation) in chemostat cultures of the mutants were similar to those of the wild type. Molar growth yields of the mutants were not increased by passing additional hydrogen through chemostat cultures, which is in agreement with the hydrogenase-negative phenotype of the mutants. H₂/N₂ ratios (mol H₂ formed per mol N₂ fixed) were calculated from the hydrogen content of the effluent gas and the N-content of the bacterial dry weight. Low H₂/N₂ ratios (between 1.2 and 1.9) were found in both energy-limited (oxygen or succinate) cultures and in cultures limited by the supply of an anabolic substrate (Mg²⁺). ATP/2e values (mol ATP used at the transport of 2e to nitrogen or H⁺) were calculated from the H₂/N₂ ratios and the molar growth yields of nitrogen-fixing and ammonia-assimilating cultures. ATP/2e values were between 7 and 11. It was concluded that the calculated ATP/2e values comprise not only 4 mol ATP used at the transport of 2e through nitrogenase but also energy equivalents needed for reversed electron flow from NADH to the low-potential hydrogen donor used by nitrogenase.     

H2 production of Rhodospirillum rubrum during adaptation to anaerobic dark conditions

Rhodospirillum rubrum is able to produce H₂ during fermentation anaerobically in the dark in two ways, namely through formate hydrogen lyase and through the nitrogenase. After chemotrophic preculture aerobically in the dark formate hydrogen lyase was synthesized after a lag phase, whilst after phototrophic preculture a slight activity was present from the beginning of the anaerobic dark culture. During fermentation metabolism its activity increased noticeably. Hydrogen production through the nitrogenase occurred if the nitrogenase had been activated during phototrophic preculture. It ceased during fermentation metabolism after about 3 1/2 h anaerobic dark culture. The CO insensitive H₂ production by the nitrogenase could be partially inhibited by N₂. Potential activity of this system, however, remained and could be increased under conditions of nitrogenase induction. It seems therefore possible that synthesis of nitrogenase under N-deficiency can occur during fermentation metabolism in the same way as the formation of the photosynthetic apparatus in order to prepare for subsequent phototrophic metabolism.Abbreviations CAP chloramphenicol - DSM Deutsche Sammlung von Mikroorganismen, Göttingen - FHL formate hydrogen lyase - O.D optical density - PFL pyruvate formate lyase     

N2-fixation in Erwinia herbicola

Four from 18 strains of Erwinia herbicola tested had nitrogenase activity and grew with N₂ as sole source of nitrogen under strict anaerobic conditions with a doubling time of 20–24 h. Nitrogenase activity started only 96–120 h after transfer to a special medium maintained under anaerobic conditions. A ten fold increase in protein per culture found after the maximum nitrogenase activity of 80–130 nmol C₂H₄. mg protein^-1·min^-1 was accompanied by a fall in pH of the medium (20 mM phosphate buffer and in 125 mM Tris-buffer) from pH 7.2 to 5.4 or less, but only to 6.8 in 100 mM phosphate buffer. In all cases we found a sharp curtailing of nitrogenase activity 48 h after the maximum. The bacteria utilized only 35–50% of the nitrogen fixed for growth. Erwinia herbicola strains differed from two strains of Enterobacter agglomerans in being unable to fix nitrogen on agar surfaces exposed to air. Specific nitrogenase activity in Erwinia herbicola is compared with data reported for other Enterobacteriaceae and is found to be higher than that reported for Klebsiella pneumoniae, Enterobacter cloacae or Citrobacter freundii.     

Metabolic and energetic aspects of biohydrogen production of Clostridium tyrobutyricum: The effects of hydraulic retention time and peptone addition

This study evaluates the microbial metabolism and energy demand in fermentative biohydrogen production using Clostridium tyrobutyricum FYa102 at different hydraulic retention times (HRT) over a period of 1-18 h. The hydrogen yield shows a positive correlation with the butyrate yield, the B/A ratio, and the Y_H2/2(Y_HAc+Y_HBu) ratio, but a negative correlation with the lactate yield. A decrease in HRT, which is accompanied by an increased biomass growth, tends to decrease the B/A ratio, due presumably to a higher energy demand for microbial growth. The production of lactate at a low HRT, however, may involve an unfavorable change in e⁻ equiv distribution to result in a reduced hydrogen production. Finally, the relatively high hydrogen yields observed in the bioreactor with the peptone addition may be ascribed to the utilization of peptone as an additional energy and/or amino-acid source, thus reducing the glucose demand for biomass growth during the hydrogen production process.     

The effect of the dissolved oxygen concentration and anabolic limitations on the behaviour of Rhizobium ORS571 in chemostat cultures

Chemostat cultures of Rhizobium ORS571 limited by the supply of oxygen or an anabolic substrate contained poly--hydroxybutyrate (PHB). Low amounts of PHB (about 10%) were present in ammonia- or nitrate-limited cultures; higher amounts were found in Mg⁺⁺-limited cultures (about 20%) and in oxygen-limited nitrogen-fixing cultures (37%). A method is described to calculate Y_ATP values (g PHB-free biomass · mol^-1 ATP) from the Y_succ values (g dry wt·mol^-1 succinate) measured. Y_succ and Y_ATP values in cultures limited by the supply of an anabolic substrate and in the oxygen-limited ammonia-assimilating culture were much lower than the values found in the PHB-free succinate-limited cultures. This shows that uncoupling of growth and energy production occurred. Therefore, H₂/N₂ ratio (mol hydrogen formed per mol nitrogen fixed) in nitrogen-fixing cultures could not be calculated from the comparison of the Y_ATP value found in the nitrogen-fixing culture and the value found in the corresponding ammonia-assimilating culture.Although the optimal dissolved oxygen concentration (d.o.c.) for nitrogen-fixing cultures of Rhizobium ORS571 is 5 or 10 M, nitrogen-fixing cultures could be obtained up to a d.o.c. of 40 M. Not only nitrogenase but also hydrogenase was active at this d.o.c. However, accumulation of PHB (10%) may indicate that cultures grown at unfavourable oxygen concentrations (15–40 M O₂) were N-limited rather than energy-limited, which may be the result of partial inactivation or repression of nitrogenase at a higher d.o.c.     

Effects of photoperiodicity and light irradiance on phosphate-limited Oscillatoria agardhii in chemostat cultures

Oscillatoria sp. strain 23 is a filamentous, non-heterocystous cyanobacterium that fixes nitrogen aerobically. Although, in this organism nitrogenase is inactivated by oxygen a high tolerance is observed. Up to a pO₂ of 0.15 atm, oxygen does not have any measurable effects on acetylene reduction. Higher concentrations of oxygen inhibited the activity to a relatively high degree. Evidence for two mechanisms of oxygen protection of nitrogenase in this cyanobacterium was obtained. A high rate of synthesis of nitrogenase may allow the organism to maintain a certain amount of active enzyme under aerobic conditions. Secondly, a switch off/on mechanism may reversibly convert the active enzyme into a non-active form which is insensitive to oxygen inactivation after a sudden and short-term exposure to high oxygen concentrations. It is conceived that these mechanisms in addition to a temporal separation of nitrogen fixation from oxygenic photosynthesis sufficiently explain the regulation process of aerobic nitrogen fixation in this organism.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - CAP chloramphenicol     

TheParasponia parviflora — Rhizobium symbiosis. Isotopic nitrogen fixation,hydrogen evolution and nitrogen-fixation efficiency,and oxygen relations

Summary Isotopic¹⁵N₂ experiments confirmed nitrogen fixation inParasponia parviflora. The conversion ratio C₂H₄/N₂ was 6.7 under the experimental conditions employed. Measurements of the δ¹⁵N in leaves of Parasponia and Trema showed on the basis of these determinations thatParasponia parviflora possesses N₂-fixing capacity and can be distinguished in this respect from the non-nitrogen-fixingTrema cannabina tested by the same method. Therefore, δ¹⁵N can be used to monitor N₂ fixation in natural ecosystems. Hydrogen evolution and the relative efficiency of N₂ fixation in this relation have been determined. DetachedParasponia parviflora root nodules grown in soil and tested in an argon/oxygen atmosphere produced appr. 4 μmol H₂.h⁻¹.g⁻¹ fresh weight root nodules. The relative efficiency of hydrogen utilization as measured in argon, air, and in the presence of C₂H₂ 10% (v/v) was for both equations used for to express this efficiency 0.96 and 0.97, respectively. This indicates that Parasponia like the root nodules of some actinorhizal symbioses (Alnus, Myrica, Elaeagnus) and some tropical legumes (Vigna sinensis) has evolved mechanisms of minimizing net hydrogen production in air, thus increasing the efficiency of electron transfer to nitrogen. The oxygen relation of nitrogen fixation (C₂H₂) inParasponia parviflora root nodules was determined. The nitrogenase activity of Parasponia root nodules increased at increasing oxygen concentrations up till c. 40% O₂. At oxygen levels above 40% O₂, the nitrogenase activity of the root nodules was nil or very erratic suggesting that at these oxygen levels the nitrogenase is not longer protected against the harmful effect of oxygen. In this respect Parasponia root nodules differ from actinorhizal root nodules in other nonlegumes, where optimal nitrogenase activity was observed in the range of 12–25% oxygen. Respiration experiments with Parasponia root nodules showed that in the range 10, 20, and 40% oxygen, the respiration rate (CO₂ evolution) increased concomitantly with an increase of the acetylene reduction rate. The CO₂/C₂H₄ values obtained varied between 8.1 and 19.2, being therefore 2–3 times higher than similar estimations in some actinorhizal and legume root nodules. The respiratory quotient (RQ) of detachedParasponia parviflora root nodules was in air initially approximately 2.0, but this value dropped to about 1.0 in a 3-hours period.     

Dependency of growth yield,maintenance and K s-values on the dissolved oxygen concentration in continuous cultures of Azotobacter vinelandii

Azotobacter vinelandii was grown diazotrophically in sucrose-limited chemostat cultures at either 12, 48, 108, 144 or 192 M dissolved oxygen. Steady state protein levels and growth yield coefficients (Y) on sucrose increased with increasing dilution rate (D). Specific rate of sucrose consumption (q) increased in direct proportion to D. Maintenance coefficients (m) extrapolated from plots of q versus D, as well as from plots of 1/Y versus 1/D exhibited a nonlinear relationship to the dissolved oxygen concentration. Constant maximal theoretical growth yield coefficients (Y ^G) of 77.7 g cells per mol of sucrose consumed were extrapolated irrespective of differences in ambient oxygen concentration. For comparison, glucose-, as well as acetate-limited cultures were grown at 108 M oxygen. Fairly identical m- and Y ^G-values, when based on mol of substrate-carbon with glucose and sucrose grown cells, indicated that both substrates were used with the same efficiency. However, acetate-limited cultures showed significantly lower m- and, at comparable, D, higher Y-values than cultures limited by either sucrose or glucose. Substrate concentrations (K _s) required for half-maximal growth rates on sucrose were not constant, they increased when the ambient oxygen concentration was raised and, at a given oxygen concentration, when D was decreased. Since biomass levels varied in linear proportion to K _s these results are interpreted in terms of variable substrate uptake activity of the culture.Abbreviations D dilution rate - K _s substrate concentration required for half maximal growth rate - m maintenance coefficient - q specific rate of substrate consumption - Y growth yield coefficient - Y ^G maximum theoretical growth yield coefficient     

Glyoxylate decreases the oxygen sensitivity of nitrogenase activity and photosynthesis in the cyanobacterium Anabaena cylindrica

Raising the pO₂ reduced nitrogenase activity (C₂H₂ reduction) of Anabaena cylindrica for both glyoxylate-treated (5 mM) and untreated cells. The stimulation caused by glyoxylate, however, increased with increases of pO₂ from 2 to 99 kPa. As the pO₂ increased the net CO₂ fixation was lowered (Warburg effect) while the CO₂ compensation point increased. Glyoxylate partly relieved this sensitivity of net photosynthesis to oxygen and reduced the compensation point considerably. The cells used were preincubated in the dark to exhaust photosynthetic pools. A more pronounced reduction in sensitivity of nitrogenase to oxygen for glyoxylate-treated cells was evident when a preincubation in air with reduced pCO₂ (13 l l^-1) was used. This was, however, not evident until after a 10-h incubation in air. Before this point 2 kPa O₂ sustained the highest nitrogenase activity. Addition of 0.5 and 5 mM of HCO ₃ ^- to Anabaena cultures preincubated at low CO₂ levels (29 l l^-1) abolished the stimulatory effect of glyoxylate on the nitrogenase. Thus, the results sustain the suggestion that glyoxylate may act as an inhibitor of photorespiratory activities in cyanobacteria and can be used as a means of increasing their nitrogen and CO₂ fixation capacities.Abbreviation RuBP ribulose 1,5-bisphosphate     

Influence of nitrogen source on growth and nitrogenase activity inAzotobacter vinelandii

Growth and nitrogenase activity were studied in cultures ofAzotobacter vinelandii growing with dinitrogen, ammonium sulfate, aspartic acid or yeast extract. Nitrogenase activity was measured by means of the C₂H₂ reduction test.In the presence of ammonium sulfate nitrogenase is completely repressed. After exhaustion of ammonia its activity is restored following a diauxic lag period of 30 min. With aspartic acid nitrogenase activity is partially repressed, and growth yield is higher than in the culture growing with N₂ only. This is due to simultaneous use of dinitrogen and aspartate. Fluctuations of nitrogenase activity occurring during exponential growth and the mechanism of their regulation are discussed.Abbreviations NA nitrogenase activity - BNF Burk's nitrogen free medium     

Kinetic studies on surfactant production byPseudomonas aeruginosa 44T1

Summary Biosurfactant accumulation occurred in the exponential and stationary phases. Production started when the nitrogen level was very low. Surfactant was produced with a diauxic pattern. Rhamnolipid concentration increased as nitrogen levels increased. Maximum product yield (Y _p/x) 2.9 was detected when C/N ratio was 6.6 and specific rate of product formation (p _q) was calculated. The examination of these kinetics parameters such as product yield and specific rate of product formation should be taken into account to develop a high efficient production process.     

Growth yields and growth rates of Desulfovibrio vulgaris (Marburg) growing on hydrogen plus sulfate and hydrogen plus thiosulfate as the sole energy sources

Desulfovibrio vulgaris (Marburg) was grown on H₂ plus sulfate and H₂ plus thiosulfate as the sole energy sources and acetate plus CO₂ as the sole carbon sources. Conditions are described under which the bacteria grew exponentially. Specific growth rates () and molar growth yields (Y) at different pH were determined. and Y were found to be strongly dependent on the pH. Highest growth rates and molar growth yields were observed for growth on H₂ plus sulfate at pH 6.5 (=0.15h^-1; Y _{SO 4} ^2- =8.3g·mol^-1) and for growth on H₂ plus thiosulfate at pH 6.8 (=0.21h^-1; Y _{S 2}O ₃ ² =16.9g·mol^-1).The growth yields were found to increase with increasing growth rates: plots of 1/Y versus 1/ were linear. Via extrapolation to infinite growth rates a Y _SO4 ^2- /max of 12.2g·mol^-1 and a YS₂O ₃ ^2- /max of 33.5g·mol^-1 was obtained.The growth yield data are interpred to indicate that dissimilatory sulfate reduction to sulfide is associated with a net synthesis of 1 mol of ATP and that near to 3 mol of ATP are formed during dissimilatory sulfite reduction to sulfide.     

Physiology of nitrogen fixation in Azospirillum lipoferum Br 17 (ATCC 29 709)

Changes of cellular activities during batch cultures with Azospirillum lipoferum strain Br 17 (ATCC 29 709) were observed within the growth cycle, at optimal pO₂ (0.002–0.003 atm). The relative growth rate for cells growing with N₂ as sole nitrogen source during log phase was =0.13 h^-1 and the doubling time was 5.3 h. Nitrogenase activity was not accompanied by hydrogen evolution at any growth stage, and a very active uptake hydrogenase was demonstrated. The hydrogenase activity increased towards the end of the growth period when glucose became limiting and N₂ fixation reached its maximal specific activity. Oxygen consumption and oxygen tolerance at the various growth stages, increased simultaneously with the uptake hydrogenase activity indicating a possible role of this enzyme in an oxygen protection mechanism of A. lipoferum nitrogenase. The efficiency of nitrogen fixation expressed as mg total nitrogen fixed in cells and supernatant per g glucose consumed, was 20 at the early log phase and increased to 48 at the late log phase. About 25% of the total fixed nitrogen was recovered in the culture supernatant.Abbreviations DOT Dissolved oxygen tension - PHB Poly--hydroxybutyric acid - O.D. Optical density (560 nm) - A.T.C.C. American type culture collection - NTA Nitrilotriacetic acid Graduate student of the Universidade Federal Rural do Rio de Janeiro, Brazil     

Inhibitory effect of molecular hydrogen on C2H2-reducing activity in Anabaena 7120 preilluminated in red or blue light

Molecular hydrogen inhibits nitrogenase activity in Anabaena pre-illuminated with red or blue light. The inhibitory effect of molecular hydrogen decreased in the presence of oxygen and several electron acceptors. When NH₄Cl and urea were added simultaneously with molecular hydrogen, marked synergistic inhibitory effects took place. The inhibitory effect of molecular hydrogen disappeared or was weakened after the suppression of hydrogenase activity. The addition of O₂ and electron acceptors to systems showed no enhancing effect on the C₂H₂-reducing activity.     

Long term field study shows increased biomass production in tree legumes inoculated with Rhizobium

Nodulation potential, nitrogen fixation efficiency (nitrogenase activity) and biomass yield response of Leucaena leucocephala and Acacia nilotica to inoculation with 6 selected fast growing Rhizobium strains was explored in long-term (5 year) field trials. All the strains formed nodules and fixed nitrogen in L. leucocephala and A. nilotica. Seasonal effects on nitrogenase activity was observed and in winter (ambient temperature about 20 °C), nitrogenase activity could not be detected. However, with the onset of spring and a rise in temperature, fresh nodulation (renodulation) by all the inoculant rhizobial strains was observed in both the tree legumes. In L. leucocephala, maximum renodulation was exhibited by strain A1 while in A. nilotica, strain AB3 formed the maximum renodulation 24 months after transplantation. Dry matter yield of all the inoculated plants demonstrated a significant increase over that of the uninoculated plants at the end of five years after transplanting. In L. leucocephala, strain NGR8 gave the maximum response (45% more dry matter yield) in dry matter production while in A. nilotica, strain USDA 3325 showed a 25% increase in total dry matter yield five years after transplantation.     

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 (_max), 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.Abbreviations GS glutamine synthetase - R Rhodopseudomonas - Rs. Rhodospirillum - CTAB cetyltrimethylammonium bromide Dedicated to Prof. Dr. H. G. Schlegel on the occasion of his 60th birthday     

Impact on growth and aflatoxin B1 accumulation by Kluyveromyces isolates at different water activity conditions

This study showed the impact on germination, mycelial growth and aflatoxin B₁ accumulation when interacting Aspergillus aflatoxigenic strains with Kluyveromyces isolates and the effect of water activity on this relationship. Isolates Y₁₄ and Y₁₆ reduced the percentage of germination of all Aspergillus strains and decrease germ tube elongation rate at majority of water activity assayed. Similarly they produced an increase of germination lag phase and lag phase of growth beside decreased growth rate of all Aspergillus strains. At water activities 0.994, 0.982, 0.955 and 0.937, no aflatoxins were produced in paired cultures with isolates Y_25, Y₂₂, Y₁₆, and Y₁₄, and Kluyveromyces isolates Y₁₄ and Y₁₆ impact both growth and aflatoxin accumulation at wide range of water activity.