Allelopathic impact of artificially applied coumarin on <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f.sp. <Emphasis Type="Italic">niveum</Emphasis>

Watermelon production is threatened by fusarium wilt caused by Fusarium oxysporum f.sp. niveum (FON) in continuous cultivation system. Some elements, mainly allelochemicals, released from living roots or decayed plants might be associated with the disease. The purpose of this work was to evaluate the possible impact of coumarin, one kind of watermelon allelochemical, on FON. Furthermore, possible new mechanisms might be investigated during the ecological interactions of plant-microbe. Results showed that coumarin strongly inhibited growth of FON leading to a decrease in its biomass, dry weight of mycelia of FON in a liquid culture. The dry weight was decreased by 62.9% compared with control. The hyphal growth of FON on plates was stopped at high (>400 mg l⁻¹) concentrations of coumarin. At 320 mg l⁻¹, sporulation and enzyme activities of FON were also severely suppressed by coumarin. The yield of conidia, and the activities of proteinase, cellulase, and amylase were reduced by 98.9%, 79.7%, 29.8% and 15.9% respectively. However, conidial germination and mycotoxin (MT) production of FON were greatly stimulated, being increased by 55.7% and 14.9 fold at 320 mg l⁻¹ respectively. We conclude that coumarin acted as an allelochemical substance to inhibit growth and pathogenic enzyme activities of FON but to stimulate mycotoxin production and conidial germination. It was suggested that coumarin acted as a signal transduction element bridging plant and pathogen in the process of plant-microbe interactions.     

Disturbed ammonium assimilation is associated with growth inhibition of roots in rice seedlings caused by NaCl

The effects of NaCl on changes in ammonium level and enzyme activities of ammonium assimilation in roots growth of rice (Oryza sativa L.) seedlings were investigated. NaCl was effective in inhibiting root growth and stimulated the accumulation of ammonium in roots. Accumulation of ammonium in roots preceded inhibition of root growth caused by NaCl. Both effects caused by NaCl are reversible. Exogenous ammonium chloride and methionine sulfoximine (MSO), which caused ammonium accumulation in roots, inhibited root growth of rice seedlings. NaCl decreased glutamine synthetase and glutamate synthase activities in roots, but increased glutamate dehydrogenase activity. The growth inhibition of roots by NaCl or MSO could be reversed by the addition of L-glutamic acid or L-glutamine. The current results suggest that disturbance of ammonium assimilation in roots may be involved in regulating root growth reduction caused by NaCl.Abbreviations GDH glutamate dehydrogenase - GOGAT glutamate synthase - GS glutamine synthetase - MSO methionine sulfoximine     

The effects of dopamine on root growth and enzyme activity in soybean seedlings

In the present study, we investigated the effects of dopamine, an allelochemical exuded from the velvetbean (Mucuna pruriens L DC. var utilis), on the growth and cell viability of soybean (Glycine max L. Merrill) roots. We analyzed the effects of dopamine on superoxide dismutase, phenylalanine ammonia-lyase and cell wall-bound peroxidase activities as well as its effects on lignin contents in the roots. Three-day-old seedlings were cultivated in half-strength Hoagland nutrient solution (pH 6.0), without or with 0.25 to 1.0 mM dopamine, in a growth chamber (25°C, 12L:12D photoperiod, irradiance of 280 μmol m⁻² s⁻¹) for 24 h. In general, the length, fresh weight and dry weight of roots, cell viability, PAL and POD activities decreased, while SOD activities increased after dopamine treatment. The content of lignin was not altered. The data demonstrate the susceptibility of soybean to dopamine and reinforce the role of this catecholamine as a strong allelochemical. The results also suggest that dopamine-induced inhibition in soybean roots is not related to the production of lignin, but may be related to damage caused by reactive oxygen species.     

Regulation by ammonium and glucose of Arthrobacter fluorescens alanine dehydrogenase

Alanine dehydrogenase in Arthrobacter fluorescens exhibited an allosteric behaviour and two K _m values for ammonium were estimated. In batch cultures at different ammonium concentrations and in continuous culture following an NH₄ ⁺ pulse, the level of ADH activity seems to be regulated by the ammonium concentration, high activities being observed when extracellular ammonium was in excess. The response to the growth rate of an ammonium-limited chemostat culture of A. fluorescens seems to indicate that alanine dehydrogenase and glutamine synthetase activities were inversely related. High activities of glutamate oxaloacetate transaminase and glutamate pyruvate transaminase have been found in crude extract of ammonium-limited cultures. From the results obtained in batch cultures grown at different glucose concentrations and in carbon-limited chemostat culture it appeared that the limitation by glucose influenced alanine dehydrogenase activity negatively. No glutamate dehydrogenase activity and no glutamate synthase activity could be detected with either NADH or NADPH as coenzymes.Abbreviations ADH alanine dehydrogenase - GS glutamine synthetase - GDH glutamate dehydrogenase - GOGAT glutamine oxoglutarate aminotransferase - GOT glutamate oxaloacetate transaminase - GPT glutamate pyruvate transaminase     

Influence of Metronidazole, CO, CO(2), and Methanogens on the Fermentative Metabolism of the Anaerobic Fungus Neocallimastix sp. Strain L2

The effects of metronidazole, CO, methanogens, and CO(2) on the fermentation of glucose by the anaerobic fungus Neocallimastix sp. strain L2 were investigated. Both metronidazole and CO caused a shift in the fermentation products from predominantly H(2), acetate, and formate to lactate as the major product and caused a lower glucose consumption rate and cell protein yield. An increased lactate dehydrogenase activity and a decreased hydrogenase activity were observed in cells grown under both culture conditions. In metronidazole-grown cells, the amount of hydrogenase protein was decreased compared with the amount in cells grown in the absence of metronidazole. When Neocallimastix sp. strain L2 was cocultured with the methanogenic bacterium Methanobrevibacter smithii, the fermentation pattern changed in the opposite direction: H(2) and acetate production increased at the expense of the electron sink products lactate, succinate, and ethanol. A concomitant decrease in the enzyme activities leading to these electron sink products was observed, as well as an increase in the glucose consumption rate and cell protein yield, compared with those of pure cultures of the fungus. Low levels of CO(2) in the gas phase resulted in increased H(2) and lactate formation and decreased production of formate, acetate, succinate, and ethanol, a decreased glucose consumption rate and cell protein yield, and a decrease in most of the hydrogenosomal enzyme activities. None of the tested culture conditions resulted in changed quantities of hydrogenosomal proteins. The results indicate that manipulation of the pattern of fermentation in Neocallimastix sp. strain L2 results in changes in enzyme activities but not in the proliferation or disappearance of hydrogenosomes.     

Regulation of ammonia assimilation in an obligate methylotroph Methylobacillus flagellatum under steady-state and transient growth conditions

The obligate methylotroph Methylobacillus flagellatum was grown in the presence of different ammonium concentrations and the regulation of the enzymes associated with ammonium assimilation was investigated in steady-state and transient growth regimes. As the medium changed from C-limitation to dual C/N- and finally to N-limitation, the culture passed through three definite growth phases. The NADP⁺-dependent glutamate dehydrogenase (GDH) was present under ammonium limitation of the culture growth (at 2 mmol l^-1 of ammonium in the growth medium) and increased in response to an increase in nitrogen availability. Glutamine synthetase (GS) and glutamate synthase (GOGAT) activities were negligible during C- and C/N-limitation. In N-limited cells the GOGAT activity increased as the dilution rate increased up to 0.35 h^-1, and then sharply dropped. In the N-sufficient cultures both NAD⁺- and NADP⁺-dependent isocitrate dehydrogenase (NAD-ICDH and NADP-ICDH) activities were up-regulated as dilution rate increased, but in the N-limited culture the NAD-ICDH activity was up-regulated whereas NADP-ICDH one was down-regulated. Pulse additions of ammonium and methanol demonstrated the coordinate regulation of the GDH and ICDHs activities. When pulses were added to the C/N-limited cultures, there was an immediate utilization of the nutrients, resulting in an increase in biomass; at the same time the GDH and ICDH activities increased and the GS and GOGAT activities decreased. When the same ammonium/methanol pulse was added into the N-limited culture, there was a 3-hours delay in the culture response, after which the substrates were utilized at rates close to the ones shown by the C/N-limited culture after the analogous pulse.     

真盐生植物盐角草对不同氮形态的响应

盐角草(Salicornia europaea L.)是一种喜盐植物,其最佳生长的实现需要200—400 mmol/L NaCl。为了解盐渍环境下盐角草氮素吸收利用特点,在水培添加200或400 mmol/L NaCl情况下,从生长指标,光合参数,根系体积和活力,硝酸还原酶与谷氨酰胺合成酶活力,蛋白、总氮、硝态氮及铵态氮含量等方面检测了硝态氮、铵态氮和尿素3种氮形态对盐角草生长的影响。研究发现以氮摩尔浓度(mmol/L)计,在0.1—400 mmol/L浓度范围进行测试,盐角草在0.1 mmol/L低氮条件下仍能维持生长,同时,抑制盐角草生长的氮浓度域值较高,其中铵态氮、尿素和硝态氮分别为50、50及400 mmol/L。研究结果表明盐角草吸收利用氮素的能力强,对氮素的浓度耐受范围宽,3种氮形态都可作为氮源满足其生长需要,但有效促进生长的效果存在差异,总体顺序从高到低依次为硝态氮、铵态氮和尿素。研究为揭示盐生植物氮吸收利用特点提供了基础数据,对提高盐生植物生产力,指导沿海滩涂生态建设具有一定的指导意义。     

Influence of Metronidazole, CO, CO2, and Methanogens on the Fermentative Metabolism of the Anaerobic Fungus Neocallimastix sp. Strain L2

The effects of metronidazole, CO, methanogens, and CO₂ on the fermentation of glucose by the anaerobic fungus Neocallimastix sp. strain L2 were investigated. Both metronidazole and CO caused a shift in the fermentation products from predominantly H₂, acetate, and formate to lactate as the major product and caused a lower glucose consumption rate and cell protein yield. An increased lactate dehydrogenase activity and a decreased hydrogenase activity were observed in cells grown under both culture conditions. In metronidazole-grown cells, the amount of hydrogenase protein was decreased compared with the amount in cells grown in the absence of metronidazole. When Neocallimastix sp. strain L2 was cocultured with the methanogenic bacterium Methanobrevibacter smithii, the fermentation pattern changed in the opposite direction: H₂ and acetate production increased at the expense of the electron sink products lactate, succinate, and ethanol. A concomitant decrease in the enzyme activities leading to these electron sink products was observed, as well as an increase in the glucose consumption rate and cell protein yield, compared with those of pure cultures of the fungus. Low levels of CO₂ in the gas phase resulted in increased H₂ and lactate formation and decreased production of formate, acetate, succinate, and ethanol, a decreased glucose consumption rate and cell protein yield, and a decrease in most of the hydrogenosomal enzyme activities. None of the tested culture conditions resulted in changed quantities of hydrogenosomal proteins. The results indicate that manipulation of the pattern of fermentation in Neocallimastix sp. strain L2 results in changes in enzyme activities but not in the proliferation or disappearance of hydrogenosomes.     

Influence of different levels of ammonium concentrations on cell growth,RNA and protein production byCandida albicans

Candida albicans starved cells were incubated in minimal synthetic liquid media containing different concentrations of ammonium sulphate (0.00, 0.02, 0.05, 0.10, 0.03, 0.50 g/L). Culture growth was monitored by measuring daily the optical density and by evaluating RNA and protein cellular content after 48 and 96 hours from the inoculum. The environmental availability of ammonium ion influenced the biomass production, that was maximum when its concentration was 0.10 and 0.30 g/L. In addition, an effect on cell duplication time was observed, this was particularly evident when the (NH₄)₂SO₄ concentration was 0.10 g/L. The protein content increased in relation to the increase of ammonium ion availability, with a peak in correspondence to 0.30 g/L and a drop when the greatest concentrations were employed. RNA production was inversely proportional in respect to protein production. The optimal range of ammonium sulphate concentration forC. albicans growth was 0.10–0.30 g/L; over these concentrations there was an inhibitory effect. The rate of the protein and RNA syntheses seems to indicate the growth phase and the nitrogen nutritional conditions of the cultures, respectively.     

Influence of ammonium and extracellular pH on the amino and organic acid contents of suspension culture cells of Acer pseudoplatanus

The effects of supplied ammonium and nitrate on the amino and organic acid contents and enzyme activities of cell suspension cultures of Acer pseudoplatanus L. were examined. Regardless of nitrogen source the pH of the culture medium strongly affected the malate and citrate contents of the cells; these organic acid pools declined at pH 5, but increased at pH 7 and 8. Over a period of two days, ammonium had little effect on the responses of the organic acid pool sizes to the pH of the medium. In contrast, ammonium had a strong influence on amino acid pool sizes, and this effect was dependent on the pH of the medium. At pH 5 there was no increase in cell ammonium or amino acid contents, but at higher pH values cellular ammonium content rose, accompanied by accumulation of glutamine, glutamate and asparagine. Over several days, supplied ammonium led to an increase in activity of glutamate dehydrogenase irrespective of any changes in internal ammonium and amino acid contents. If the pH of the medium was allowed to fall below pH 4 in the presence of ammonium, phosphoenolpyruvate (PEP) carboxylase activity declined to a very low value over several days; at higher pH, the activity of this enzyme, and that of NAD malic enzyme and NAD malate dehydrogenase, remained substantial irrespective of whether the nitrogen source was NH⁺₄ or NO-₃.     

Short-term effects of coumarin along the maize primary root axis

The short-term effects of coumarin on three different maize primary root zones, transition zone (TZ, 3 mm) and two non-growing zones (NGZ1 and NGZ2 at 20 and 50 mm, respectively), were studied in order to investigate the effect of the allelochemical on maize root elongation rate (RER). The RER, plasma membrane (pm) H⁺-ATPase activity, quantitative pH changes and cell membrane potentials were evaluated. The results showed that coumarin caused at the TZ (1) an increased RER; (2) an enhancement of pm H⁺-ATPase activity and proton extrusion; and (3) a transient depolarization followed by a hyperpolarization of cell membrane potential. These observations were not evident in the NGZ1 and NGZ2 of the maize root. Coumarin-treatment in the NGZ1 did not change RER, but caused a membrane depolarization, while the NGZ2 was mostly insensitive to the allelochemical. These data suggested that the primary maize root was sensitive to coumarin within a 20 mm section from the root tip, but the more distal NGZ2 was not involved in coumarin-elicited physiological responses.Key words: coumarin, membrane potential, pmH⁺-ATPase, proton efflux, root elongation rate     

NITROGEN ASSIMILATION OF AN OCEANIC DIATOM IN NITROGEN-LIMITED CONTINUOUS CULTURE1

The oceanic diatom Thalassiosira pseudonana Hasle and Heimdal (formerly Cyclotella nana) was grown with 12L:12D illumination cycles in nitrogen-limited continuous culture with a mixture of ammonium and nitrate as the N source. Measurements included, at 3 different growth rates (degrees of N limitation), cell concentration, cell carbon, nitrogen, and chlorophyll a contents, cell volume, photosynthetic carbon assimilation vs. irradiance, short-term uptake of ammonium and nitrate vs. their ambient concentrations, and in vitro activities of the assimilatory enzymes nitrate reductase and glutamic dehydrogenase. The various parameters showed either an increase (pattern a) or a decrease (pattern b) with increasing N limitation. Those following pattern a were nitrate reductase activity and the capacity to assimilate nitrate and ammonium. Those following pattern b were glutamic dehydrogenase activity, photosynthetic rate, nitrogen:carbon and chlorophyll a:carbon composition ratios. Results are discussed in terms of the interpretation such measurement for natural phytoplankton and effects of circadian periodicity.     

pH对克雷伯氏菌1,3-丙二醇合成关键酶酶活及发酵特性的影响

在5 L发酵罐进行甘油脉冲流加发酵,分析了不同pH值对克雷伯氏肺炎杆菌发酵特性的影响,pH 6.5为菌体最佳生长条件,克雷伯氏肺炎杆菌合成1,3-丙二醇的产量最高。在1,3-丙二醇合成速率较大的对数中前期,进行甘油脉冲流加发酵,提高甘油浓度促进甘油脱水酶、1,3-丙二醇氧化还原酶和甘油脱氢酶活性。不同pH值的脉冲试验表明,甘油脱水酶,2,3-丁二醇脱氢酶比酶活随着pH值的升高而升高,1,3-丙二醇氧化还原酶,乳酸脱氢酶比酶活在pH6.5最高,因此偏酸性的发酵条件和对数期维持一定的甘油浓度能够促进1,3-丙二醇的合成。     

Anaerobic degradation of toluene in denitrifying Pseudomonas sp.: indication for toluene methylhydroxylation and benzoyl-CoA as central aromatic intermediate

The anaerobic degradation of toluene has been studied with whole cells and by measuring enzyme activities. Cultures of Pseudomonas strain K 172 were grown in mineral medium up to a cell density of 0.5 g of dry cells per liter in fed-batch culture with toluene and nitrate as the sole carbon and energy sources. A molar growth yield of 57 g of cell dry matter formed per mol toluene totally consumed was determined. The mean generation time was 24 h. The redox balance between toluene consumed (oxidation and cell material synthesis) and nitrate consumed (reduction to nitrogen gas and assimilation as NH₃) was 77% of expectation if toluene was completely oxidized; this indicated that the major amount of toluene was mineralized to CO₂. It was tested whether the initial reaction in anaerobic toluene degradation was a carboxylation or a dehydrogenation (anaerobic hydroxylation); the hypothetical carboxylated or hydroxylated intermediates were tested with whole cells applying the method of simultanous adaptation: cells pregrown on toluene degraded benzyl alcohol, benzaldehyde, and benzoic acid without lag, 4-hydroxybenzoate and p-cresol with a 90 min lag phase and phenylacetate after a 200 min lag phase. The cells were not at all adapted to degrade 2-methylbenzoate, 4-methylbenzoate, o-cresol, and m-cresol, nor did these compounds support growth within a few days after inoculation with cells grown on toluene. In extracts of cells anaerobically grown on toluene, benzyl alcohol dehydrogenase, benzaldehyde dehydrogenase, and benzoyl-CoA synthetase (AMP forming) activities were present. The data (1) conclusively show anaerobic growth of a pure culture on tolucne; (2) suggest that toluene is anaerobically degraded via benzoyl-CoA; (3) imply that water functions as the source of the hydroxyl group in a toluene methylhydroxylase reaction.     

Citric-acid cycle key enzyme activities during in vitro growth and metacyclogenesis of Leishmania infantum promastigotes.

The activities of 5 key regulatory enzymes in most energetic systems, namely citrate synthase (EC 4.1.3.7, CS), NADP-specific isocitrate dehydrogenase (EC 1.1.1.42, ICDH), succinate dehydrogenase (EC 1.3.99.1, SDH), L-malate dehydrogenase (EC 1.1.1.37, MDH), and decarboxylating malic enzyme (EC 1.1.1.40, ME), were measured during the growth and metacyclogenesis of a cutaneous (CL) and a visceral (VL) strain of Leishmania infantum. As occurs with other Leishmania species, infective promastigotes were present along all phases of growth, but their percentages were higher at the early stationary phase for VL and the end of the same phase for CL. High CS and SDH activities were detected in both strains, as compared with other trypanosomatids, bringing more evidence for an actively functional citric-acid cycle in L. infantum. Both strains showed higher levels of CS, ICDH, and MDH and lower SDH and ME activities when more metacyclic promastigotes were present, but in VL these changes paralleled an increase in glucose consumption, whereas in CL these changes coincided with an NH3 hyperproduction. This suggests that the energy metabolism during L. infantum growth and metacyclogenesis is affected by regulated enzymes that probably respond to changes in the culture medium in the levels of glucose and amino acids.     

Enhanced K+-stimulated pyrophosphatase activity in NaCl-adapted cells of Acer pseudoplatanus

Cell suspension cultures of Acer pseudoplatanus L. (Bligny cell line) adapted to growth in the presence of NaCl, are a useful tool for investigating mechanisms for cellular salt tolerance. We compared the activities of vanadate-sensitive (plasma membrane) and nitrate-sensitive (tonoplast) ATPases, and tonoplast K⁺-stimulated PPase in microsomal fractions (8000–108 000 g) from unadapted and NaCl-adapted (80 m M ) cells of A. pseudoplatanus . Since NaCl reduces the growth rate of the culture, the two cell lines were harvested and compared at both the same cellular density and at the same growth phase (middle exponential phase or beginning of the stationary phase). The ATPase activity of the plasma membrane (expressed both on the basis of protoplast number and in relation to protein content) was not affected by the adaptation to salinity. The two enzyme activities of the tonoplast (mainly as expressed on a protein basis) were higher in adapted than in unadapted cells. However, a preferential increase in PPase activity took place, although the pH dependence, ionic requirements, and apparent K_m of the PPase activity were the same in the two cell lines. The three enzyme activities showed different sensitivities to detergents such as Triton X-100, Brij 58 and lysophosphatidylcholine (LPC). The stimulation of K⁺-stimulated PPase activity by detergents was higher in adapted than in unadapted cells. This suggests that the salt-induced enhancement of the PPase activity might partially depend on a modification of the lipid component of the tonoplast.     

强壮前沟藻化感物质分析

微藻化感作用是一种极其复杂的生理、生态学现象。选取强壮前沟藻指数生长初期Ⅰ和平台生长初期Ⅱ两个阶段的滤液对中肋骨条藻、海洋原甲藻、锥状斯氏藻及球等鞭金藻生长的影响进行了研究,并萃取了阶段Ⅱ的粗提物,抑藻检测表明其具有"杀藻"效应,通过GC/MS分析该粗提物中具有潜在化感作用的物质种类。研究发现强壮前沟藻两个生长阶段的滤液对中肋骨条藻均产生强烈致死效应(phaseⅠ:F=15.18475,P=0.00298<0.05;phaseⅡ:F=6.24559,P=0.03149<0.05);锥状斯氏藻在强壮前沟藻滤液中生长,实验结束时两个阶段中的细胞密度分别是对照组的79.3%和68.9%;海洋原甲藻在强壮前沟藻生长阶段Ⅱ滤液实验的最后3d,其生长受到显著抑制(F=4.84438,P=0.04925<0.05);而等鞭金藻在强壮前沟藻两个生长阶段滤液中被抑制现象不明显(P>0.05)。强壮前沟藻滤液实验表明,强壮前沟藻能够向微环境中分泌代谢产物来抑制中肋骨条藻和海洋原甲藻的生长,并且这种抑制效应具有种类特殊对应性。上述实验结果还表明,强壮前沟藻生长阶段Ⅱ的滤液具有的生长抑制作用较为明显。采用乙酸乙酯萃取强壮前沟藻生长阶段Ⅱ滤液中的代谢产物,检测发现其代谢粗提物具有溶藻效应,GC/MS分析结果表明粗提物中存在4种可能产生化感抑制作用的物质,其中二丁基羟基甲苯(Butylated Hydroxytoluene BHT)被认为具有抗滤过性病原体和抗微生物活性。     

重组CHO-GS细胞降低氨毒副作用的代谢研究

在重组CHOGS细胞无血清批培养过程中,由于GS系统的引入,使氨对细胞的毒副作用显著降低,从而引起细胞生长和代谢途径发生变化。当起始氨浓度为1.42mmolL时,细胞最高密度可达到15.6×105cellsmL,随着氨浓度的增加,尽管细胞生长受到一定的抑制,但在氨浓度为12.65mmolL时,细胞密度仍可达到8.9×105cellsmL。当起始氨浓度从0.36mmolL增加到12.65mmolL时,细胞对葡萄糖的得率系数和乳酸对葡萄糖的得率系数降低,己糖激酶(HK)、丙酮酸激酶(PK)和乳酸脱氢酶(LDH)酶活分别提高了43%、140%和25%,表明细胞对葡萄糖的利用增加,糖代谢更倾向于高能量生成途径。在谷氨酰胺代谢途径中,氨促进了谷丙转氨酶(GPT)酶活,谷氨酸到α酮戊二酸的转化逐渐倾向于谷丙转氨途径,谷氨酸脱氢酶(GDH)酶活降低,脱氨途径相应受到抑制。此外,氨浓度的增加使细胞群体处于G0G1期的比例逐渐升高,当氨浓度为12.65mmolL时,重组蛋白比生产速率比氨浓度为0.36mmolL时提高了2.1倍。     

Effect of carbon and nitrogen growth limitation upon nutrient uptake and metabolism in batch cultures of Catharanthus roseus (L) G. Don

Cell suspension cultures of the Madagascan Periwinkle, Catharanthus roseus (L). G. Don were grown as batch cultures in two different types of media; in one medium the limiting nutrient was inorganic nitrogen, and in the other it was carbon. The response of the cells to these growth-limiting conditions was monitored by measuring cellular fresh weight, dry weight and protein accumulation, cell viability, medium sugar and nitrate levels, and the activities of certain intracellular enzymes throughout growth in batch culture. The enzymes investigated were glucose-6-phosphate dehydrogenase (EC 1.1.1.49), 6-phosphogluconate dehydrogenase (EC 1.1.1.44), hexokinase (EC 2.7.1.40), phosphofructokinase (EC 2.7.1.11), nitrate reductase (EC 1.6.6.1), glutamate dehydrogenase (EC 1.4.1.2) and glutamine synthetase (EC 6.3.1.2). The effect of culturing the cells under different nutritional regimes was apparent in all aspects of growth; only some enzyme activities were unaffected. Cell viability remained at a high level for several days after growth limitation in both types of culture. The possibility that protein degradation in nitrogen-limited batch cultures is under very stringent control is discussed.     

镉对满江红（Azolla imbricata）-鱼腥藻（Anabaena azollae）共生体氮代谢的影响

在实验室条件下,研究了不同浓度(0、0.01、0.05、0.1、0.5mg/L)的Cd对满江红-鱼腥藻共生体异型胞频率,固氮酶、谷氨酰氨合成酶活性以及铵态氮、游离氨基酸、可溶性蛋白、总氮含量的影响.结果表明,在整个实验期间,0.01mg/L Cd处理对上述指标均没产生显著影响,说明满江红-鱼腥藻共生体对Cd具有较强的耐性.当培养液中Cd浓度≥0.05mg/L时,随溶液中Cd浓度的增加和处理时间的推移,异型胞频率、固氮酶活性、谷氨酰氨合成酶活性、可溶性蛋白含量和总氮含量逐渐下降,而铵态氮含量在处理初期显著降低,随后迅速增加,游离氨基酸含量则逐渐增加.研究结果表明高浓度的Cd处理导致满江红-鱼腥藻共生体氮代谢的紊乱,最终造成氮素积累量的下降.