Field studies on the ureogenic gulf toadfish, in a subtropical bay. II. Nitrogen excretion physiology

In order to examine the in situ nitrogen excretion physiology of gulf toadfish ( Opsanus beta ) (Fam. Batrachoididae), several biochemical and physiological measurements relating to urea synthesis and excretion were measured in samples taken from freshly collected gulf toadfish from a subtidal population in Biscayne Bay, Florida, U.S.A. This indirect appoach was used, instead of direct measurements of nitrogen excretion, because nitrogen excretion patterns of gulf toadfish are altered markedly during the first 24 h of capture disturbance or laboratory confinement. The values obtained for plasma cortisol levels, and the activities of hepatic ornithine-urea cycle enzymes, including glutamine synthetase (and its partitioning between cytosolic and mitochondrial compartments), suggest that gulf toadfish in Biscayne Bay may excrete a substantial portion of their waste nitrogen as urea. Also conducted were correlation analyses of several biotic variables (plasma [cortisol], enzyme activities, plasma [urea], hepatosomatic index, and plasma [Ca⁺⁺]) with several abiotic variables (temperature, salinity, depth and dissolved oxygen), and with collection site and season. Results of these analyses are discussed in the context of hypotheses to explain ureotely in this teleost fish.     

Understanding carbamoyl phosphate synthetase deficiency: impact of clinical mutations on enzyme functionality

Carbamoyl phosphate synthetase I (CPSI) deficiency, a recessively inherited error of the urea cycle, causes life-threatening hyperammonaemia. CPSI is a multidomain 1500-residue liver mitochondrial matrix protein that is allosterically activated by N-acetyl-l-glutamate, and which synthesises carbamoyl phosphate (CP) in three steps: bicarbonate phosphorylation by ATP, carbamate synthesis from carboxyphosphate and ammonia, and carbamate phosphorylation by ATP. Several missense mutations of CPSI have been reported in patients with CPSI deficiency, but the actual pathogenic potential and effects on the enzyme of these mutations remain non-characterised. Since the structure of Escherichia coli CPS is known and systems for its overexpression and purification are available, we have constructed and purified eight site-directed mutants of E.coli CPS affecting the enzyme large subunit (A126M, R169H, Q262P, N301K, P360L, V640R, R675L, S789P) that are homologous to corresponding missense mutations found in patients with CPSI deficiency, studying their stability and their ability to catalyse the CPS reaction as well as the partial reactions that reflect the different reactional steps, and analysing the substrate kinetics for the overall and partial reactions. The results show that all the mutations significantly decrease CP synthesis without completely inactivating the enzyme (as reflected in the catalysis of at least one partial reaction), that one of these mutations (Q262P) causes marked enzyme instability, and validate the use of E.coli CPS as a pathogenicity testing model for CPSI deficiency. The causality of the reported clinical mutations is supported and the derangements caused by the mutations are identified, revealing the specific roles of the residues that are mutated. In particular, the findings highlight the importance for carbamate phosphorylation and for allosteric activation of a loop that coordinates K(+), stress the key role of intersubunit interactions for CPS stability, and suggest that lid opening at both phosphorylation sites is concerted.     

Lamprey parasitism of sharks and teleosts: high capacity urea excretion in an extant vertebrate relic

We observed 10 sea lampreys (Petromyzon marinus) parasitizing basking sharks (Cetorhinus maximus), the world's second largest fish, in the Bay of Fundy. Due to the high concentrations of urea in the blood and tissues of ureosmotic elasmobranchs, we hypothesized that sea lampreys would have mechanisms to eliminate co-ingested urea while feeding on basking sharks. Post-removal urea excretion rates (J(Urea)) in two lampreys, removed from separate sharks by divers, were initially 450 ( approximately 9000 micromol N kg-1 h-1) and 75 times ( approximately 1500 micromol N kg-1 h-1) greater than basal (non-feeding) rates ( approximately 20 micromol N kg-1 h-1). In contrast, J(Urea) increased by 15-fold after parasitic lampreys were removed from non-ureosmotic rainbow trout (Oncorhynchus mykiss). Since activities of the ornithine urea cycle (OUC) enzymes, carbamoyl phosphate synthetase III (CPSase III) and ornithine carbamoyl transferase (OCT) were relatively low in liver and below detection in intestine and muscle, it is unlikely that the excreted urea arose from de novo urea synthesis. Measurements of arginase activity suggested that hydrolysis of dietary arginine made a minor contribution to J(Urea.). Post-feeding ammonia excretion rates (J(Amm)) were 15- to 25-fold greater than basal rates in lampreys removed from both basking sharks and rainbow trout, suggesting that parasitic lampreys have a high capacity to deaminate amino acids. We conclude that the sea lamprey's ability to penetrate the dermal denticle armor of sharks, to rapidly excrete large volumes of urea and a high capacity to deaminate amino acids, represent adaptations that have contributed to the evolutionary success of these phylogenetically ancient vertebrates.     

Identification of rare variants causing urea cycle disorders: A clinical,genetic, and biophysical study

Urea cycle disorders (UCDs) are a group of rare metabolic conditions characterized by hyperammonemia and a broad spectrum of phenotypic severity. They are caused by the congenital deficiency in the eight biomolecules involved in urea cycle. In the present study, five cases of UCD were recruited and submitted to a series of clinical, biochemical, and genetic analysis with a combination of high throughput techniques. Moreover, in silico analysis was conducted on the identified missense genetic variants. Various clinical and biochemical indications (including profiles of amino acids and urinary orotic acids) of UCD were manifested by the five probands. Sequence analysis revealed nine diagnostic variants, including three novel ones, which caused Argininosuccinic aciduria (ASA) in one case, Carbamoyl phosphate synthetase 1deficiency (CPS1D) in two cases, Ornithine transcarbamylase deficiency (OTCD) in one case, and Citrin deficiency in 1case. Results of in silico biophysical analysis strongly suggested the pathogenicity of each the five missense variants and provided insight into their intramolecular impacts. In conclusion, this study expanded the genetic variation spectrum of UCD, gave solid evidence for counselling to the affected families, and should facilitate the functional study on the proteins in urea cycle.     

Hemolymph ammonia, urea and uric acid levels and nitrogenous excretion of Marsupenaeus japonicus at different salinity levels

Arginase specific activity, hemolymph ammonia, urea and uric acid levels and nitrogenous excretion were measured in Kuruma shrimp Marsupenaeus japonicus (7.29±1.16 g) acclimated to different salinities of 18‰, 26‰, 34‰ and 42‰. Arginase activity in the gill, midgut, hepatopancreas and muscle were higher and lower for the shrimp in 42‰ and 18‰, respectively. Arginase specific activity of hemolymph was higher at 34‰. Hemolymph ammonia, urea and uric acid increased directly with salinity, and excretions of total nitrogen (total-N), organic nitrogen (organic-N) and urea-N increased directly with salinity. However, ammonia-N excretion and nitrite-N excretion were inversely related to salinity. Ammonia-N accounted for 90.9%, 75.0%, 67.9% and 38.5% of total-N, whereas urea-N accounted for 3.1%, 4.5%, 7.9% and 10.9%, and organic accounted for 4.2%, 19.8%, 23.1% and 50.4% of total-N excreted by the shrimp in 18‰, 26‰, 34‰ and 42‰, respectively. Significantly higher levels of hemolymph urea and uric acid together with an increase in arginase activity indicated that ureogenesis and uricogenesis are activated for M. japonicus in hyperosmotic conditions.     

Excretory nitrogen metabolism in the Chinese fire-belly newt<Emphasis Type="Italic"> Cynops orientalis</Emphasis> in water,on land,or in high concentrations of environmental ammonia

The Chinese fire-belly newt Cynops orientalis reverts to an aquatic mode of living when sexually mature. Despite living in water, sexually mature C. orientalis maintained high capacity for hepatic urea synthesis. However, it had a lower rate of urea production than other terrestrial amphibians because endogenous ammonia could diffuse out to the external medium as NH₃. This conserves cellular energy because urea synthesis is energetically expensive. Simultaneously, C. orientalis also reduced the rate of urea excretion, and excreted 33% of the total nitrogenous waste as ammonia. Upon exposure to land, C. orientalis increased the rate of urea synthesis from accumulating endogenous ammonia. The increased rate of urea synthesis was within the inherent capacity of the hepatic ornithine–urea cycle; there was no induction of hepatic carbamoyl phosphate synthetase or ornithine transcarbamoylase activities and there was no reduction in ammonia production. When exposed to water containing 75 mmol.l^–1 NH₄Cl, the rates of both urea synthesis and urea excretion increased. Under such experimental conditions, the ornithine–urea cycle may be operating close to its limit; glutamine began to accumulate in the body, and endogenous ammonia production via amino acid catabolism was reduced.Abbreviations CPS carbamoyl phosphate synthetase - FAA free amino acid - OTC ornithine transcarbamoylase - OUC ornithine–urea cycle - TCA trichloroacetic acid Communicated by I.D. Hume     

MTNR1a和MTNR1b在牦牛不同组织中的表达及其在睾丸中的定位

褪黑素对调节季节性繁殖哺乳动物的生殖具有重要调节作用。其受体MTNR1a（Melatonin receptor 1a,褪黑素受体1a）主要参与昼夜节律和生殖调控,MTNR1b（Melatonin receptor 1b,褪黑素受体1b）与多种疾病发生密切相关。为了探讨褪黑素受体基因的生物学功能,本实验对牦牛不同组织中MTNR1a、MTNR1b基因的表达与定位情况进行了研究。采用qRT-PCR (Quantitative Real-Time PCR, qRT-PCR) 检测成年雄性牦牛各组织及不同发育阶段（30日龄,2岁、4岁、6岁和8岁龄）牦牛睾丸组织中MTNR1a、MTNR1b mRNA的表达规律,并运用免疫组化技术对不同年龄牦牛睾丸中MTNR1a、MTNR1b蛋白进行了定位研究。结果发现,MTNR1a mRNA在松果体组织中表达量最高,肺脏、肌肉和睾丸次之;随着年龄增加,MTNR1a mRNA在睾丸中的表达量逐渐升高,到4岁后表达量趋于平稳;MTNR1a蛋白在不同发育阶段牦牛睾丸组织中均有表达,圆形精子呈现较强的免疫阳性,其次为初级精母细胞;MTNR1b mRNA在松果体表达量最高（P<0.05）,肾脏、肝脏和下丘脑次之;在不同年龄牦牛睾丸中MTNR1b mRNA均有表达,且随着年龄的增加表达量逐渐增加,在8岁时表达量最高;MTNR1b蛋白主要定位在圆形精子细胞中。MTNR1a、MTNR1b基因在牦牛不同组织及不同发育阶段睾丸中的广泛表达,揭示了其在雄性牦牛生殖等生理过程中的重要作用。     

Carbamate kinase: New structural machinery for making carbamoyl phosphate, the common precursor of pyrimidines and arginine

The enzymes carbamoyl phosphate synthetase (CPS) and carbamate kinase (CK) make carbamoyl phosphate in the same way: by ATP-phosphorylation of carbamate. The carbamate used by CK is made chemically, whereas CPS itself synthesizes its own carbamate in a process involving the phosphorylation of bicarbonate. Bicarbonate and carbamate are analogs and the phosphorylations are carried out by homologous 40 kDa regions of the 120 kDa CPS polypeptide. CK can also phosphorylate bicarbonate and is a homodimer of a 33 kDa subunit that was believed to resemble the 40 kDa regions of CPS. Such belief is disproven now by the CK structure reported here. The structure does not conform to the biotin carboxylase fold found in the 40 kDa regions of CPS, and presents a new type of fold possibly shared by homologous acylphosphate-making enzymes. A molecular 16-stranded open beta-sheet surrounded by alpha-helices is the hallmark of the CK dimer. Each subunit also contains two smaller sheets and a large crevice found at the location expected for the active center. Intersubunit interactions are very large and involve a central hydrophobic patch and more hydrophilic peripheral contacts. The crevice holds a sulfate that may occupy the site of an ATP phosphate, and is lined by conserved residues. Site-directed mutations tested at two of these residues inactivate the enzyme. These findings support active site location in the crevice. The orientation of the crevices in the dimer precludes their physical cooperation in the catalytic process. Such cooperation is not needed in the CK reaction but is a requirement of the mechanism of CPSs.     

Oxygen consumption and ammonia excretion of juvenile pink shrimp (Farfantepenaeus paulensis) in culture: temperature effects

We evaluated oxygen consumption and ammonia excretion by juveniles of the pink shrimp Farfantepenaeus paulensis at three different temperatures (15, 20 and 25 °C). The shrimp were collected in the coastal region of Cananéia, São Paulo State, Brazil. The selected temperatures are the limits recorded in aquaculture tanks in the coastal region of Cananéia. We measured oxygen consumption and ammonia excretion as proxies for metabolic activity. Oxygen consumption and ammonia excretion increased with increasing temperature, but no change was observed at 15 and 20 °C. It is possible that within this temperature range, there is thermal independence in juvenile F. paulensis do not need to allocate additional energy to compensate for temperature changes because they are physiologically adapted for this range.     

The effect of salinity on respiratory metabolism,survival and moulting in the first zoea of Macrobrachium amazonicum (Heller) (Crustacea,Palaemonidae)

Survival, duration of intermoult cycle and respiratory metabolism were evaluated as a function of salinity (0–35‰; 25° C) in early zoeae of the cinnamon shrimp, Macrobrachium amazonicum. Zoeae are extremely resistant to salinity, mortality occurring only in fresh and sea-water after several days. Moulting occurs in all salinities, longer cycles being recorded in 0 and 35‰ S. The metabolism-salinity curve is broadly U-shaped between 0 and 28‰ S but declines sharply in sea-water. Such physiological responses characterise the early zoeae as strongly euryhaline and typically estuarine. Data are discussed in relation to the degree of adaptation of the organism to the freshwater biotope and the position of the species within the generic pattern of adaptive radiation.     

Field studies of ammonia excretion in Aphanius iberus (Pisces; Cyprinodontidae): body size and habitat effects

The effects of body size and habitat variability on ammonia excretion rates (R_AMs) of Aphanius iberus were analyzed in situ for the first time. At hourly intervals during a 5‐h field experiment, ammonia excretion was measured in 75 mature specimens from three sampling sites (small creek, marine salt‐mine, and salt‐marsh) established in a gradient of water salinity (0–5; 35–40; 65–70‰). Our results showed a specific size dependence pattern of R_AMs in the reproduction period, which might reflect an effect of the reproductive effort. In addition, the results point to a significant decrease in mean R_AM values of each population from freshwater aquatic systems (3.81 ± 0.58 μmol g⁻¹ h⁻¹ in fish of 2.8 ± 0.3 mm total length, TL) to salt aquatic systems with significantly higher alkalinity (2.52 ± 0.35 μmol g⁻¹ h⁻¹ in fish of 3.1 ± 0.5 mm TL in marine salt‐mine; 1.98 ± 0.55 μmol g⁻¹ h⁻¹ in fish of 3.1 ± 0.4 mm TL in salt‐marsh). Due to the size‐dependent pattern, R_AM in different habitats cannot be compared directly; ancova , followed by residual compared analysis (regression‐related techniques), is seen as a valid method for this purpose. This work presents the first field data on ammonia excretion in the Aphanius genus and the flexible physiologic response characteristic of Cyprinodontids has been demonstrated.     

Glutaminase in neurons and astrocytes cultured from mouse brain: Kinetic properties and effects of phosphate,glutamate, and ammonia

Phosphate activated glutaminase comprises two kinetically distinguishable enzyme forms in cultures of cerebellar granule cells, of cortical neurons and of astrocytes. Specific activity of glutaminase is higher in cultured neurons compared with astrocytes. Glutaminase is activated by phosphate in all cell types investigated, however, glutaminase in astrocytes reguires a much higher concentration of phosphate for half maximal activation. One of the products, glutamate, inhibits the enzyme strongly, whereas the other product ammonia has only a slight inhibitory action on the enzyme.     

Optimization of ammonium acquisition and metabolism by potassium in rice (Oryza sativa L. cv. IR-72)

We present the first characterization of K⁺ optimization of N uptake and metabolism in an NH₄⁺‐tolerant species, tropical lowland rice (cv. IR‐72). ¹³N radiotracing showed that increased K⁺ supply reduces futile NH₄⁺ cycling at the plasma membrane, diminishing the excessive rates of both unidirectional influx and efflux. Pharmacological testing showed that low‐affinity NH₄⁺ influx may be mediated by both K⁺ and non‐selective cation channels. Suppression of NH₄⁺ influx by K⁺ occurred within minutes of increasing K⁺ supply. Increased K⁺ reduced free [NH₄⁺] in roots and shoots by 50–75%. Plant biomass was maximized on 10 mm NH₄⁺ and 5 mm K⁺, with growth 160% higher than 10 mm NO₃^‐‐grown plants, and 220% higher than plants grown at 10 mm NH₄⁺ and 0.1 mm K⁺. Unlike in NH₄⁺‐sensitive barley, growth optimization was not attributed to a reduced energy cost of futile NH₄⁺ cycling at the plasma membrane. Activities of the key enzymes glutamine synthetase and phosphoenolpyruvate carboxylase (PEPC) were strongly stimulated by elevated K⁺, mirroring plant growth and protein content. Improved plant performance through optimization of K⁺ and NH₄⁺ is likely to be of substantial agronomic significance in the world's foremost crop species.     

The labyrinth of nutrient cycles and buffers in wetlands: results based on research in the Camargue (southern France)

Wetlands, especially in the Mediterranean area, are subject to severe eutrophication. This may upset the equilibrium between phytoplankton production in undesirable quantities and a quantitatively desirable macrophyte production. In order to manage this equilibrium, a quantitative knowledge of nutrient input and fluxes is essential and the role of sediments in these processes must be understood. This knowledge can be useful even for agriculture, e.g. rice cultivation, where optimal utilization of fertilizers can lead to an economic benefit.In this article different aspects of nutrient cycles are discussed in view of approaching a sufficiently precise quantification. The nutrient input balance of the Camargue was therefore measured which showed that the input of nutrients with the irrigation water, taken from the river Rhone, roughly equals the quantity of fertilizers added.Phytoplankton growth can be approached reasonably with the Monod model, although there are still many practical problems, such as the influence of the pH on P uptake and the problem of measuring P uptake in the field. The situation is worse for macrophyte growth; quantitative data are scarce and studies have often been carried out with unrealistic nutrient concentrations or without addressing the influence of the sediment. This influence can also include negative factors, such as high concentrations of Fe²⁺, H₂S or FeS, but cannot yet be quantified.The nitrogen cycle in wetlands is dominated by denitrification. Most wetlands have sediments with high concentrations of organic matter, therefore with a large reducing capacity. Besides this process, we have shown that denitrification can also be controlled by FeS. In the Camargue sediments this denitrification is mediated by bacteria from the sulfur cycle; this appeared to be the major pathway. It was shown that a stoicheiometric relation exists between nitrate reduced and sulphate produced. The influence of the temperature was quantified and appeared to be stronger at high organic matter concentrations than at lower ones. Denitrification with FeS means that the bacteria use nitrate also for their N demands, while this is not necessarily the case during denitrification with organic matter.Mineralization of macrophytes is a much slower process than that of phytoplankton, probably because of their high C/N ratio. We could, however, not confirm the general assumption that the addition of nitrogen stimulates this mineralization. On the contrary, we found that two amino acids both with a C/N ratio of 6 had different mineralization rates. The amino acid composition of dead macrophytes and the C/N ratio may be of equal importance.Unlike nitrogen, phosphate is always strongly adsorbed onto sediments. The two mechanisms of the adsorption of inorganic phosphate onto sediments, i.e. the adsorption onto Fe(OOH) and the precipitation of apatite, have been quantified. The adsorption of phosphate onto Fe(OOH) can be satisfactory described with the Freundlich adsorption isotherm: P_ads = A^* (o-P)^B. The adsorption coefficient A depends on the pH of the system and the Ca²⁺ concentration of the overlying water and can be quantified preliminarily by A = a.10(^–0.416*pH).(2.86 – (1.86.e^–Ca2+)). B can be approached by 0.333, which means the cube root of the phosphate concentration. The second mechanism is the solubility of apatite. We found a solubility product of 10^–50 for hard waters. The two mechanisms are combined in solubility diagrams which describe equilibrium situations for specific lakes.The conversion of Fe(OOH) to FeS has a strong influence on phosphate adsorption, although the partial reduction of Fe(OOH) P by H₂S does not release significant quantities of phosphate. Even after complete conversion to FeS only a small part of the bound phosphate was released.Besides the two inorganic phosphate compounds, we established the existence of two organic pools, one soluble after extraction with strong acid (ASOP), the other one with strong alkali. The first pool is probably humic bound phosphate, while the larger part of the second pool was phytate. The ASOP was remineralized during the desiccation of a Camargue marsh; this drying up oxidized FeS, thus improving the phosphate adsorption and decreasing the denitrification capacity. It can, therefore, be an important tool for management. The phytate was strongly adsorbed onto Fe(OOH), which explains the non-bioavailability towards bacteria.The fact that the sediment phosphate concentration can be approached by multiplying the relevant sediment adsorption constant with % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaOqaaeaaca% WGVbGaeyOeI0IaamiuaaWcbaGaaG4maaaaaaa!3B8D!\[\sqrt[3]{{o - P}}\] concentration has the consequence that much larger quantities of phosphate accumulate in the sediments than in the overlying water. This means that even if the phosphate input is stopped, the eutrophication will only be reversed very slowly, and not at all, if the shallow waters in wetlands have no through flow — as is often the case in many marshes in Mediterranean wetlands.Abbreviations used o-P = dissolved ortho phosphate (or its concentration) - N_part, P_part = particulate N or P - Tot-N_inorg = Total inorganic nitrogen (= NH₃ + NO ₂ ^– + NO ₃ ^– ) This paper, giving an overview of the research in the sediments of the Camargue, was read during the symposium Nutrient Cycles — A Joy Forever, on the occasion of my retirement, 19th of May 1993 at the I.H.E. in Delft (Netherlands).     

Detection and basic properties of carbamoyl phosphate synthetase III during teleost ontogeny: a case study in the Atlantic halibut (Hippoglossus hippoglossus L.)

The presence of carbamoyl phosphate synthetase III (CPSase III), catalyzing the first step of the urea cycle in fish, in Atlantic halibut (Hippoglossus hippoglossus L.) yolk-sac larvae and adult white muscle has been established using gel filtration chromatography to separate the CPSase III from the pyrimidine-pathway related CPSase II. The results are consistent with the hypothesis that teleostean fish express urea cycle enzymes during early development and with recent observations of low levels of CPSase III in muscle tissue. The presence of CPSase III in crude extracts could not be established using sensitive assay conditions to discriminate between CPSase III and CPSase II. However, kinetic characterization after chromatographic separation identified each as typical CPSase II and CPSase III activities, respectively. The CPSase III was less sensitive to activation by N-acetyl- -glutamate and had a higher K_m for ammonia than CPSase III found in other species. These results suggest that precise quantitation of low levels of CPSase III in the presence of CPSase II by assaying crude extracts may be difficult unless the enzymes are first separated and the kinetic properties of CPSase III are determined; the results indicate that assaying larval extracts of Atlantic halibut in the presence of uridine triphosphate results in CPSase activity that reflects mostly CPSase III and can, therefore, be used to measure changes in CPSase III activity.     

Oxygen consumption and ammonia excretion rates in Octopus maya,loligo forbesi and Lolliguncula brevis (Mollusca: Cephalopoda)

Oxygen consumption and ammonia excretion rates were investigated in young Octopus maya (hatching to 139 days old; 0.11–81.23 g wet body weight, BW; 22.5–23.9°C), young squids of Loligo forbesi (hatching to 45 days old; 9.4–115.3 mg BW; 12.3–13.1°C) and young squids of Lolliguncula brevis (2.00–39.98 g BW; 23.8–24.7°C). Except at hatching, oxygen consumption and ammonia excretion rates on an individual basis (M) of these three cephalopods increased linearly with increasing body weight (BW) expressed as M = aBW^b . Values of b for oxygen consumption were 0.900, 0.910 and 0.848 and for ammonia excretion were 0.744, 0.809 and 0.751 for O. maya, L. forbesi and L. brevis, respectively. Among the three species the value a varied widely, while b was similar for both oxygen consumption and ammonia excretion rates. Based upon these data, metabolism for hatchlings of O. maya and L. forbesi was estimated to be relatively lower than that of older juveniles. The O/N ratios for hatchlings of O. maya and L. forbesi were relatively high and indicate an apparent dependence upon lipids in the immediate post‐hatching period, followed by standard protein energy utilization thereafter.     

Aerobic glucose metabolism of Saccharomyces kluyveri: growth, metabolite production, and quantification of metabolic fluxes.

The growth and product formation of Saccharomyces kluyveri was characterized in aerobic batch cultivation on glucose. At these conditions it was found that ethyl acetate was a major overflow metabolite in S. kluyveri. During the exponential-growth phase on glucose ethyl acetate was produced at a constant specific rate of 0.12 g ethyl acetate per g dry weight per hour. The aerobic glucose metabolism in S. kluyveri was found to be less fermentative than in S. cerevisiae, as illustrated by the comparably low yield of ethanol on glucose (0.08 +/- 0.02 g/g), and high yield of biomass on glucose (0.29 +/- 0.01 g/g). The glucose metabolism of S. kluyveri was further characterized by the new and powerful techniques of metabolic network analysis. Flux distributions in the central carbon metabolism were estimated for respiro-fermentative growth in aerobic batch cultivation on glucose and respiratory growth in aerobic glucose-limited continuous cultivation. It was found that in S. kluyveri the flux into the pentose phosphate pathway was 18.8 mmole per 100 mmole glucose consumed during respiratory growth in aerobic glucose-limited continuous cultivation. Such a low flux into the pentose phosphate pathway cannot provide the cell with enough NADPH for biomass formation which is why the remaining NADPH will have to be provided by another pathway. During batch cultivation of S. kluyveri the tricarboxylic acid cycle was working as a cycle with a considerable flux, that is in sharp contrast to what has previously been observed in S. cerevisiae at the same growth conditions, where the tricarboxylic acid cycle operates as two branches. This indicates that the respiratory system was not significantly repressed in S. kluyveri during batch cultivation on glucose.     

温度、盐度和pH对马氏珠母贝稚贝清滤率的联合效应

清滤率(Clearance rate, CR)与滤食性贝类生长发育密切相关,采用Box-Behnken设计(BBD)和响应曲面法,在实验室条件下研究了温度(18-34℃)、盐度(20-40)和pH(6.5-9.5)对马氏珠母贝(Pinctada martensii)稚贝清滤率(CR)的联合效应,旨在建立温度、盐度和pH对马氏珠母贝稚贝清滤率的定量关系模型,并通过统计优化方法得出温度、盐度和pH的最佳组合。结果表明:温度的一次效应、温度和pH的互作效应、盐度和pH的互作效应以及温度、盐度和pH的二次效应对马氏珠母贝稚贝清滤率的影响均极显著(P<0.01);盐度的一次效应、pH的一次效应以及温度和盐度的互作效应对清滤率无显著影响 (P>0.05)。实验得出的清滤率模型决定系数为0.9950,预测决定系数为0.9284,表明该模型建立有效并可用于预测马氏珠母贝稚贝的清滤率。通过采用优化方法得出,在温度26.95℃,盐度29.69,pH8.09时,稚贝清滤率达到最大,最大值为1.4894×10^-3L/h,满意度为0.9886。研究结果可为马氏珠母贝滤食生理研究及稚贝培育提供理论依据。