Nucleic acid synthesis from blood urea 15N in the sheep

In experiments on 4 sheep fed on a low protein diet [6.2 g N/day] and given a single i.v. dose of 15N-labelled urea [15 mg 15N/kg body mass], the authors found that, from 0.5 to 6 h, mean 15N incorporation rose progressively in the total rumen fluid nitrogen from 0.23 to 0.44 at. % 15N and in the rumen bacterial nitrogen from 0.11 to 0.51 at. % 15N. Up to 3 h, total nitrogen enrichment was greater (0.5 at. % 15N) than enrichment of bacterial nitrogen (0.28 at. % 15N), but from 3 to 6 h there was little difference between them. The mean 15N values in the nucleic acids isolated from rumen fluid bacteria in samples collected 3 and 6 hours after injecting labelled urea into the blood were 0.15 and 0.19 at. % 15N respectively, in nucleic acids isolated from the liver 0.042 and 0.04 at. % 15N, in the total rumen bacterial nitrogen 0.28 and 0.51 at. % 15N and in the total liver nitrogen 0.11 and 0.11 at. % 15N. It is concluded from the results that blood urea nitrogen is utilized for synthesis of the total nitrogenous substances of the sheep's rumen bacteria and liver far more intensively than for synthesis of the nucleic acids isolated from them. At the same time, it is utilized more intensively for nucleic acid synthesis in the rumen bacteria than in the liver.     

Incorporation of N from intravenously administered 15N labelled urea into the bacterial protein in the sheep.

The experiment carried out on two wethers demonstrated that nitrogen of intravenously injected urea, labelled with 15N was incorporated into total and bacterial nitrogen fraction of the digesta flowing through the rumen and duodenum. The amount of 15N in the bacterial fraction flowing throught the rumen and duodenum was relatively low in comparison with the amount of 15N in the total nitrogen (14,8% and 8,1% in the rumen and 6,6% and 7,9% in the duodenum. The ratio of the amount of bacterial-N to total-N in the rumen content (12,7 and 7,5%) was only slightly lower than the ratio of bacterial 15N to total 15N. In the duodenum this ratio was a little higher (8,7 and 10,0%). Blood urea nitrogen was utilized only partly in biosynthesis of bacterial protein. The results showed that only a small amount of blood urea nitrogen retained in the organism was utilized for microbial protein synthesis and the majority in some different way.     

Patterns of soil and plant nitrogen isotope composition and their relationships with climate factors in Xilingol League,Inner Mongolia,China

Background: Plant and soil nitrogen stable isotope (δ¹⁵N) can integrate several fundamental biogeochemical processes in ecosystem nitrogen dynamics, and reflect characteristics of ecosystem nitrogen cycling.

Aims: We investigated how climate change influenced plant-soil nitrogen cycling by relating soil δ¹⁵N, plant δ¹⁵N and Δδ¹⁵N (difference between soil and plant δ¹⁵N) with climatic factors.

Methods: Field investigation was conducted in temperate grasslands in Inner Mongolia during August 2015. Plant δ¹⁵N, soil δ¹⁵N and Δδ¹⁵N were determined, and their relationships with climatic factors were examined by simple regression analyses and general linear models.

Results: Soil δ¹⁵N was significantly higher than plant δ¹⁵N, and there was a positive linear correlation between them. Soil and plant δ¹⁵N were negatively related with mean annual precipitation (MAP) and positively with mean annual temperature (MAT); conversely, Δδ¹⁵N was positively related with MAP and negatively with MAT.

Conclusion: Soil δ¹⁵N was dominantly controlled by MAT, while it was MAP for plant δ¹⁵N. Climate factors influenced plant δ¹⁵N not only through their effects on soil nitrogen dynamics but also strategies of plant nitrogen acquisition. Thus, compared with plant δ¹⁵N, soil δ¹⁵N can more accurately reflect soil nitrogen dynamics, while plant δ¹⁵N may integrate soil nitrogen dynamics and plant nitrogen acquisition.     

Short-term immobilisation and crop uptake of fertiliser nitrogen applied to winter wheat: effect of date of application in spring

Previous studies on the fate of fertiliser nitrogen applied to winter wheat in temperate climates have shown that nitrogen (N) applied early, at tillering for wheat, was less efficiently taken up than N applied later in the growth cycle. We examined the extent to which the soil microbial N immobilisation varied during the wheat spring growth cycle and how microbial immobilisation and plant uptake competed for nitrogen. We set up a pulse-15N labelled field experiment in which N was applied at eight development stages from tillering (beginning of March) to anthesis (mid-June). Each application was 50 kg N ha-1 as 15N labelled urea except for the first application which was 25 kg N ha-1. The distribution of fertiliser 15N in shoots, roots, mineral and organic soil N was examined by destructive sampling 7 and 14 days after each 15N pulse. The inorganic 15N pool was almost depleted by day 14. The N uptake efficiency increased with later applications from 45% at tillering to 65% at flowering. N immobilisation was rather constant at 13–16% of N applied, whatever the date of application. The increase in plant 15N uptake resulted in an increase in the total 15N recovery in the plant-soil system (15N in soil +15N in plant), suggesting that gaseous losses were lower at the later application dates.     

Connecting litter quality,microbial community and nitrogen transfer mechanisms in decomposing litter mixtures

Synergistic effects on decomposition in litter mixtures have been suggested to be due to the transfer of nitrogen from N‐rich to N‐poor species. However, the dominant pathway and the underlying mechanisms remain to be elucidated. We conducted an experiment to investigate and quantify the control mechanisms for nitrogen transfer between two litter species of contrasting nitrogen status (¹⁵N labeled and unlabeled Fagus sylvatica and Fraxinus excelsior) in presence and absence of micro‐arthropods. We found that ¹⁵N was predominantly transferred actively aboveground by saprotrophic fungi, rather than belowground or passively by leaching. However, litter decomposition remained unaffected by N‐dynamics and was poorly affected by micro‐arthropods, suggesting that synergistic effects in litter mixtures depend on complex environmental interrelationships. Remarkably, more ¹⁵N was transferred from N‐poor beech than N‐rich ash litter. Moreover, the low transfer of ¹⁵N from ash litter was insensitive to destination species whereas the transfer of ¹⁵N from labeled beech litter to unlabeled beech was significantly greater than the amount of ¹⁵N transferred to unlabeled ash suggesting that processes of nitrogen transfer fundamentally differ between litter species of different nitrogen status. Microbial analyses suggest that nitrogen of N‐rich litter is entirely controlled by bacteria that hamper nitrogen capture of microbes in the environment supporting the source‐theory. In contrast, nitrogen of N‐poor fungal dominated litter is less protected and transferable depending on the nitrogen status and the transfer capacity of the microbial community of the co‐occurring litter species supporting the gradient‐theory. Thus, our results challenge the traditional view regarding the role of N‐rich litter in decomposing litter mixtures. We rather suggest that N‐rich litter is only a poor nitrogen source, whereas N‐poor litter, can act as an important nitrogen source in litter mixtures. Consequently both absolute and relative differences in initial litter C/N ratios of co‐occurring litter species need to be considered for understanding nitrogen dynamics in decomposing litter mixtures.     

Measurement of biological dinitrogen fixation in grassland: Comparison of the enriched 15N dilution and the natural 15N abundance methods at different nitrogen application rates and defoliation frequencies

A plant mixture of white clover (Trifolium repens L.), red clover (Trifolium pratense L.), and ryegrass (Lolium perenne L.) was established in the spring of 1991 under a cover-crop of barley. Treatments were two levels of nitrogen (400 and 20 kg N ha^-1) and two cutting intensities (3 and 6 cuts per season). Fixation of atmospheric derived nitrogen was estimated by two ¹⁵N dilution methods, one based on application of ¹⁵N to the soil, the other utilising small differences in natural abundance of ¹⁵N.Both methods showed that application of 400 kg N ha^-1 significantly reduced dinitrogen fixation, while cutting frequency had no effect. Atmospheric derived nitrogen constituted between 50 and 64% of harvested clover nitrogen in the high-N treatment, while between 73% and 96% of the harvested clover nitrogen was derived from the atmosphere in the low-N treatment. The amounts of fixed dinitrogen varied between 31–72 kg N ha^-1 and 118–161 kg N ha^-1 in the high-N and low-N treatment, respectively. The highest values for biological dinitrogen fixation were estimated by the enriched ¹⁵N dilution method.Estimates of transfer of atmospheric derived nitrogen from clover to grass obtained by the natural ¹⁵N abundance method were consistently higher than those obtained by the enriched ¹⁵N dilution method. Neither mineral nitrogen application nor defoliation frequency affected transfer of atmospheric derived nitrogen from clover to grass.Isotopic fractionation of ¹⁴N and ¹⁵N (B value) was estimated by comparing results for nitrogen fixation obtained by the enriched ¹⁵N dilution and the natural ¹⁵N abundance method, respectively. B was on average +1.20, which was in agreement with a B value determined by growing white clover in a nitrogen free media.     

Utilization of [15N]glutamate by cultured astrocytes.

The metabolism of 0.25 mM-[15N]glutamic acid in cultured astrocytes was studied with gas chromatography-mass spectrometry. Almost all 15N was found as [2-15N]glutamine, [2-15N]glutamine, [5-15N]glutamine and [15N]alanine after 210 min of incubation. Some incorporation of 15N into aspartate and the 6-amino position of the adenine nucleotides also was observed, the latter reflecting activity of the purine nucleotide cycle. After the addition of [15N]glutamate the ammonia concentration in the medium declined, but the intracellular ATP concentration was unchanged despite concomitant ATP consumption in the glutamine synthetase reaction. Some potential sources of glutamate nitrogen were identified by incubating the astrocytes for 24 h with [5-15N]glutamine, [2-15N]glutamine or [15N]alanine. Significant labelling of glutamate was noted with addition of glutamine labelled on either the amino or the amide moiety, reflecting both glutaminase activity and reductive amination of 2-oxoglutarate in the glutamate dehydrogenase reaction. Alanine nitrogen also is an important source of glutamate nitrogen in this system.     

内蒙古典型草原羊草群落氮素去向的示踪研究

 在中国科学院内蒙古草原生态系统定位研究站的羊草样地,采用15N同位素示踪技术研究了羊草（Leymus chinensis）群落标记氮素的去向。结果表明：在我国典型草原羊草群落,植物对标记氮素的回收率为31.61%,氮素添加显著影响植物对标记氮素的回收,随着氮素添加量的增加,地上和地下植物器官对标记氮素的回收量均显著提高。标记氮素被凋落物的回收率为2.92%,地下凋落物的回收率显著高于地上凋落物。标记氮素的土壤存留率为36.16%,主要分布在地表0～40 cm的土层范围内;各土层存留的标记氮素量均随着氮素添加量的增加而显著提高。标记氮素的当季损失率为21.77%～43.38%。风险/收益比分析表明,在该试验条件下,添加5.25 g N•m-2与28 g N•m-2的处理风险大于收益,添加17.5 g N•m-2的处理风险最低,收益最高,在草原生态系统的管理中可供参考。     

15N自然丰度法在陆地生态系统氮循环研究中的应用

随着氮沉降的不断增加以及人们对全球变化问题的日益关注, 稳定同位素技术在全球变化研究中得到广泛的应用。因为植物和土壤的氮同位素组成记录了氮循环影响因子的综合作用, 并且具有测量简单以及不受取样时间和空间限制的优点, 所以氮同位素自然丰度法被用于氮循环的研究中。该文从氮循环过程中植物和土壤的氮分馏入手, 总结国内外相关文献, 阐述了植物和土壤氮自然丰度在预测生态系统氮饱和和氮循环长期变化趋势中的应用; 总结了利用树轮δ ¹⁵N法研究氮循环过程中应该注意的事项以及目前尚未解决的问题。     

Validation of lactose[15N,15N]ureide as a tool to study colonic nitrogen metabolism

In vitro experiments have shown that fermentation of carbohydrates prevents accumulation of nitrogen in the colon. Variable results have been obtained on modulation of dietary intakes in vivo. Lactose[15N,15N]-labeled ureide has been proposed as a tool to study colonic nitrogen metabolism. However, on oral administration of the marker, different urinary excretion patterns of the 15N label have been found. In this study, 50 mg lactose[15N,15N]ureide was directly instilled in the colon through an orocecal tube to investigate the colonic handling of this molecule in a direct way. In basal conditions, 42% (range, 37-48%) of labeled nitrogen administered as lactose[15N,15N]ureide was retrieved in urine after 72 h. A substantial variability in total urinary excretion of the label was found, but the urinary excretion pattern of the label was similar in all volunteers. When inulin, a fermentable carbohydrate, was administered together with the labeled marker, a significant decrease in urinary excretion of 15N after 72 h was found, to 29% (range, 23-34%). The effect of a smaller dose of inulin (250 mg) on colonic handling of lactose[15N,15N]ureide (50 mg), was investigated in another group of volunteers, and this time, fecal excretion of the marker was also evaluated. The results seem to indicate that fermentation of inulin causes an increased fecal excretion of the marker, thereby reducing urinary excretion but not retention in the human nitrogen pool. This instillation study shows that lactose[15N,15N]ureide is a tool with good properties to investigate the effect of different types of carbohydrates on nitrogen metabolism in the proximal colon in vivo.     

小麦旗叶Rubisco周转与籽粒含氮量的关系

随着旗叶的衰老,Rubisco含量逐渐减少.延缓小麦旗叶的衰老进程(抽穗期施氮肥),可增加旗叶Rubisco的含量,提高籽粒的全氮含量.在小麦旗叶全展后28d内,Rubisco的15N丰度处于较高水平,表明仍有Rubisco的重新合成;而在28d以后,Rubisco的15N丰度处于低水平,表明无Rubisco的重新合成.但这时籽粒的15N丰度却上升.旗叶全展后14d内Rubisco的15N丰度高于旗叶中全氮的丰度,说明此时期Rubisco重新合成的速率高于其它蛋白质;旗叶衰老过程中Rubisco的15N丰度的净转移高于全氮,Rubi-sco净N转移也高于全氮,表明Rubisco向籽粒中转移的氮素多于其它蛋白质,对籽粒含氮量的影响最大.     

[15N]aspartate metabolism in cultured astrocytes. Studies with gas chromatography-mass spectrometry.

The metabolism of 2.5 mM-[15N]aspartate in cultured astrocytes was studied with gas chromatography-mass spectrometry. Three primary metabolic pathways of aspartate nitrogen disposition were identified: transamination with 2-oxoglutarate to form [15N]glutamate, the nitrogen of which subsequently was transferred to glutamine, alanine, serine and ornithine; condensation with IMP in the first step of the purine nucleotide cycle, the aspartate nitrogen appearing as [6-amino-15N]adenine nucleotides; condensation with citrulline to form argininosuccinate, which is cleaved to yield [15N]arginine. Of these three pathways, the formation of arginine was quantitatively the most important, and net nitrogen flux to arginine was greater than flux to other amino acids, including glutamine. Notwithstanding the large amount of [15N]arginine produced, essentially no [15N]urea was measured. Addition of NaH13CO3 to the astrocyte culture medium was associated with the formation of [13C]citrulline, thus confirming that these cells are capable of citrulline synthesis de novo. When astrocytes were incubated with a lower (0.05 mM) concentration of [15N]aspartate, most 15N was recovered in alanine, glutamine and arginine. Formation of [6-amino-15N]adenine nucleotides was diminished markedly compared with results obtained in the presence of 2.5 mM-[15N]aspartate.     

Nitrogen stable isotope ratios in surface sediments, epilithon and macrophytes from upland lakes with differing nutrient status

1. Thirty small upland lakes in Cumbria, Wales, Scotland and Northern Ireland were each visited once during June and July 2000. From each lake, samples of surface sediment epilithon, macrophytes and total dissolved nitrogen (TDN) were collected for nitrogen stable isotope analysis. As part of a wider programme, samples were also collected for chemical analysis and bioassays. 2. Considerable variation was found in δ¹⁵N values in all measured nitrogen compartments. Some regional variation was evident but was generally weak. Sediment and epilithon δ¹⁵N were positively correlated with δ¹⁵N of TDN, suggesting that baseline nitrogen isotope ratios influence those in some organic matter compartments in the lakes. 3. Sediment δ¹⁵N was higher when inorganic nitrogen concentration in the water was low, possibly reflecting reduced isotope fractionation under these conditions. However, this was not the case for epilithon or macrophytes. Sediment δ¹⁵N values were also negatively related to annual nitrogen deposition. 4. Sediment, epilithon and macrophyte δ¹⁵N values all showed significant relations to nutrient limitation in the lakes as determined by algal bioassays. We suggest that sediment δ¹⁵N might be developed as a simple integrating measure of the degree of nitrogen limitation in lakes.     

Arginine synthesis from enteral glutamine in healthy adults in the fed state

Recent studies have documented transfer of labeled nitrogen from [2-(15)N]glutamine to citrulline and arginine in fasting human adults. Conversely, in neonates and piglets we have shown no synthesis of arginine from [2-(15)N]glutamate, and others have shown in mice that glutamine is a nitrogen, but not a carbon donor, for arginine synthesis. Therefore, we performed a multitracer study to determine whether glutamine is a nitrogen and/or carbon donor for arginine in healthy adult men. Two glutamine tracers, 2-(15)N and 1-(13)C, were given enterally to five healthy men fed a standardized milkshake diet. There was no difference in plasma enrichments between the two glutamine tracers. 1-(13)C isotopomers of citrulline and arginine were synthesized from [1-(13)C]glutamine. Three isotopomers each of citrulline and arginine were synthesized from the [2-(15)N]glutamine tracer: 2-(15)N, 5-(15)N, and 2,5-(15)N(2). Significantly greater enrichment was found of both [5-(15)N]arginine (0.75%) and citrulline (3.98%) compared with [2-(15)N]arginine (0.44%) and [2-(15)N]citrulline (2.62%), indicating the amino NH(2) from glutamine is mostly transferred to arginine and citrulline by transamination. Similarly, the enrichment of the 1-(13)C isotopomers was significantly less than the 2-(15)N isotopomers, suggesting rapid formation of α-ketoglutarate and recycling of the nitrogen label. Our results show that the carbon for 50% of newly synthesized arginine comes from dietary glutamine but that glutamine acts primarily as a nitrogen donor for arginine synthesis. Hence, studies using [2-(15)N]glutamine will overestimate arginine synthesis rates.     

Origin of the ammonia used for mitochondrial citrulline synthesis as revealed by 13C-15N spin coupling patterns observed by 13C NMR.

The sources of ammonia used by isolated, intact rat liver mitochondria in the production of citrulline have been investigated in situ using a novel methodology based on the analysis of 13C-15N heteronuclear couplings observed by 13C NMR. Isolated mitochondria from rat liver were incubated with ornithine, 13CO3H- and 15NH4Cl, using unlabeled glutamate or glutamine as alternative, intramitochondrial nitrogen donors. The production of (7-13C, 8-15N) or (7-13C, 8-14N) citrulline was determined in situ by 13C NMR and the relative proportions of 15N- and 14N-citrullines confirmed by high resolution 13C NMR analysis of the C-7 citrulline resonance observed in perchloric acid extracts prepared at the end of the incubations. The 15N fractional enrichment of the intramitochondrial NH3 pool was manipulated either by modifying the 15N enrichment of added 15NH4Cl, or by altering the concentration of the unlabeled nitrogen donors in the incubation medium. Fractional 15N enrichments measured in the N-8 nitrogen of the resulting (7-13C) citrulline closely paralleled those of the external 15NH4Cl with minor dilutions derived from the unlabeled nitrogen contribution from the alternative substrates. In the presence of 10 mM 15NH4Cl, 10 mM glutamate contributed 4% of the citrulline N-8 nitrogen. Under similar conditions, the contribution of nitrogen from 10 mM glutamine to N-8 citrulline was 6%. These results indicate that the primary source of ammonia used for citrulline synthesis by isolated, intact rat liver mitochondria is extramitochondrial, providing also an illustration of the use of 13C-15N spin coupling patterns observed by 13C NMR, as a new tool in the study of ammonia metabolism.     

A break in the nitrogen cycle in aridlands? Evidence from δp15N of soils

We examined the content and isotopic composition of nitrogen within soils of a juniper woodland and found that a cryptobiotic crust composed of cyanobacteria, lichens, and mosses was the predominant source of nitrogen for this ecosystem. Disturbance of the crust has resulted in considerable spatial variability in soil nitrogen content and isotopic composition; intercanopy soils were significantly depleted in nitrogen and had greater abundance of ¹⁵N compared to intra-canopy soils. Variations in the ¹⁵N/¹⁴N ratio for inter- and intra-canopy locations followed similar Rayleigh distillation curves, indicating that the greater ¹⁵N/¹⁴N ratios for inter-canopy soils were due to relatively greater net nitrogen loss. Coverage of cryptobiotic crusts has been reduced by anthropogenic activities during the past century, and our results suggest that destruction of the cryptobiotic crust may ultimately result in ecosystem degradation through elimination of the predominant source of nitrogen input.     

A reappraisal of protein turnover values in neonates fed human milk or formula

We investigated the effect of human milk feeding on the nitrogen metabolism of appropriate-for-gestational age infants of birth weight 1.5-2.0 kg. Eight infants received pooled mature human milk. The remaining 20 were divided into two equal groups, who received one of two low-protein, milk-based formulae. The formulae were identical in composition except for the protein source, which was either casein- or whey-predominant. The three diet groups received similar total nitrogen (390 mg N.kg-1.d-1) and energy (500 kJ.kg-1.d-1) intakes. The human-milk-fed group, however, received a significantly higher intake of nonprotein and urea nitrogen and a significantly lower true protein nitrogen. Nitrogen metabolism was studied using a modified constant infusion of [15N]glycine, mixed with the feeding every 2-3 h. Urine was collected in approximately 3-h aliquots and analysed for total ammonia and urea nitrogen. Excretion of the 15N label was measured in urinary urea and ammonia. No differences were seen between the three diet groups in total [15N]urea or [15N]ammonia urinary excretion. However, the concentration of 15N in urinary urea in the human-milk-fed group was lower than in the two formula-fed groups. This reduction in concentration appeared due to a higher dietary intake of urea among the human-milk-fed group, and the consequent dilution of the label in the urine. As a result, protein turnover rates calculated from the [15N]urea end product were artificially raised in the milk-fed group, and were significantly higher than those in the formula groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Capture and allocation of nitrogen byQuercus douglasii seedlings in competition with annual and perennial grasses

Summary The spatial overlap of woody plant root systems and that of annual or perennial grasses promotes competition for soil-derived resources. In this study we examined competition for soil nitrogen between blue oak seedlings and either the annual grassBromus mollis or the perennial grassStipa pulchra under controlled outdoor conditions. Short-term nitrogen competition was quantified by injecting¹⁵N at 30 cm depth in a plane horizontal to oak seedling roots and that of their neighbors, and calculating¹⁵N uptake rates, pool sizes and¹⁵N allocation patterns 24 h after labelling. Simultaneously, integrative nitrogen competition was quantified by examining total nitrogen capture, total nitrogen pools and total nitrogen allocation.Stipa neighbors reduced inorganic soil nitrogen content to a greater extent than didBromus plants. Blue oak seedlings responded to lower soil nitrogen content by allocating lower amounts of nitrogen per unit of biomass producing higher root length densities and reducing the nitrogen content of root tissue. In addition, blue oak seedlings growing with the perennial grass exhibited greater rates of¹⁵N uptake, on a root mass basis, compensating for higher soil nitrogen competition inStipa neighborhoods. Our findings suggest that while oak seedlings have lower rates of nitrogen capture than herbaceous neighbors, oak seedlings exhibit significant changes in nitrogen allocation and nitrogen uptake rates which may offset the competitive effect annual or perennial grasses have on soil nitrogen content.     

Free amino acid turnover in methanogens measured by 15N NMR spectroscopy

Turnover of the nitrogen moiety from free amino acid pools in two thermophilic methanogens, Methanobacterium thermautotrophicum delta H and Methanococcus thermolithotrophicus SN1, has been monitored with 15N NMR spectroscopy. In cells growing exponentially on 15NH4Cl, glutamate was the major soluble 15N-labeled species in both organisms. When the Mb. thermoautotrophicum cells were harvested, washed, and resuspended into medium containing 14NH4Cl, the resonance for [15N]glutamate decreased with a half-life of 0.5 h. This is considerably faster than the turnover rate for the carbon side chain of glutamate (7 h) obtained when a 13CO2 pulse followed by a 12CO2 chase was incorporated into the 15N/14N-labeling experiment. Such behavior is consistent with recycling of the glutamate carbon skeleton via alpha-ketoglutarate after transamination reactions remove the 15N for biosynthesis of other amino acids, nucleic acids, etc. When the cells were in stationary phase, 15N turnover was considerably slower indicating that transaminase activity had also decreased. Mc. thermolithotrophicus has a much more fragile cell wall and easily lyses. To avoid cell loss in the 15N/14N experiment, 15NH+4 growth followed by 14NH4+ dilution was used. In this organism the glutamate-labeled nitrogen turns over quite rapidly (t1/2 approximately 9 min), at a rate comparable to that for the carbon skeleton (t1/2 approximately 10 min). Beta-Glutamate, the second major carbon and nitrogen pool in this organism, turns over its 15N label very slowly. Therefore, this beta-amino acid does not appear to serve as a nitrogen donor in Mc. thermolithotrophicus.     

不同碳氮比对生物絮团形成及对日本沼虾生长、抗氧化酶和消化酶的影响

为了研究不同碳氮比对生物絮团形成及对日本沼虾(Macrobrachium nipponense)生长、抗氧化酶和消化酶的影响, 设置5个不同实验组[对照组(不做任何添加), 碳氮比10组(C/N10)、碳氮比15组(C/N15)、碳氮比20组(C/N20)和碳氮比25组(C/N25)], 每组设三重复; 将初始体重(0.25±0.03) g的日本沼虾置于不同碳氮比的玻璃缸(30 cm×40 cm×100 cm)中, 进行50d的饲养实验。研究结果表明, 随着碳氮比升高, 生物絮团含量有上升趋势, 生物絮团含量C/N20>C/N25>C/N15>C/N10, C/N20生物絮团最多, 生物絮团体积和总固体悬浮物分别为(328.67±7.09) mL/L和(40.33±1.53) mg/L而C/N10几乎没有生物絮团产生, C/N15和C/N25有少量生物絮团, 但含量显著低于C/N20。对照组的氨氮和亚硝酸氮浓度持续升高, C/N10和C/N25的氨氮和亚硝酸氮浓度有较大波动, 先升高后降低, 随后又有升高的趋势, 氨氮和亚硝酸氮浓度分别高于2和1.5 mg/L, C/N15和C/N20的氨氮和亚硝酸氮浓度在整个养殖期间都维持在较低水平, 氨氮和亚硝酸氮浓度分别低于0.35和0.6 mg/L, 且无剧烈波动。增重率依次是C/N20>C/N25>C/N15>C/N10>Control, C/N10、C/N15、C/N20和C/N25分别比对照组高出29.69%、50.22%、89.52%和75.98%(P<0.05); 特定生长率依次是C/N20>C/N25>C/N15>C/N10>Control, C/N10和C/N15、C/N20和C/N25分别比对照组高出18.41%、24.69%、42.26%和33.89%(P<0.05); 在抗氧化酶方面, 谷胱甘肽过氧化物酶(GPX)活性C/N20>C/N25>C/N15>C/N10>Control, 与对照组相比, C/N10、C/N15、C/N20和C/N25分别高出1.70%、21.42%、43.19%和31.49%(P<0.05); 超氧化物歧化酶(SOD)活性C/N20>C/N25>C/N15>C/N10>Control, 与对照组相比, C/N10、C/N15、C/N20和C/N25分别高了19.34%、35.26%、73.35%和47.12%。在消化酶方面, 淀粉酶活性依次是C/N20>C/N15>C/N25>Control>C/N10, 与对照组相比, C/N15、C/N20和C/N25分别高出68.09%、231.91%和42.55%(P<0.05); 脂肪酶活性依次是C/N20>C/N25>C/N15>Control>C/N10, 与对照组相比, C/N15、C/N20和C/N25分别高出2.86%、25.45%和23.12%(P<0.05); 胰蛋白酶活性依次是C/N20>C/N25>C/N15>C/N10>Control, 与对照组相比, C/N10、C/N15、C/N20和C/N25组分别高出12.98%、14.52%、36.45%和24.63%(P<0.05)。研究结果提示, 日本沼虾在生物絮团养殖模式下, 当碳氮比达到20时能有效产生生物絮团, 降低水体氨氮和亚硝酸氮浓度并显著提高日本沼虾的生长性能、肠道消化酶活性和肝胰腺抗氧化酶活性。