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2.
Fixation of CO 2 and N assimilation were studied in synchronous cultures of Scenedesmus obtusiusculus Chod. under saturating and limiting light. Within the photon-flux range studied, the cells maintained C to N assimilation ratios of 7–10 with either NO
3
-
, NO
2
+
or NH
4
+
as the N source. Competitive interactions between C and N assimilation were pronounced under light limitation and were proportional to the oxidation status of the N source. Fixation of CO 2 at saturating light was also slightly reduced by NO
2
-
and NH
4
+
. In the absence of CO 2, NO
3
-
uptake and reduction was light-saturated at a comparatively low photon flux, whereas NO
2
-
uptake and reduction was considerably faster in the absence of CO 2 than in its presence. The pools of reduced pyridine nucleotides (NADPH and NADH) were largely unaffected by the presence or absence of the different N sources. The regulatory influences of CO 2 fixation on N assimilation are discussed in terms of coupling between the rates of CO 2 fixation and NH
4
+
assimilation, as well as the existance of control mechanisms for NO
3
-
uptake and reduction.Abbreviations Chl
chlorophyll
- PF
photon flux 相似文献
3.
Sessile organisms may experience chronic exposure to copper that is released into the marine environment from antifoulants and stormwater runoff. We have identified the site of damage caused by copper to the symbiotic cnidarian, Zoanthus robustus (Anthozoa, Hexacorallia). External changes to the zoanthids were apparent when compared with controls. The normally flexible bodies contracted and became rigid. Histological examination of the zoanthid tissue revealed that copper had caused sub-cellular changes to proteins within the extracellular matrix (ECM) of the tubular body. Collagen in the ECM and the internal septa increased in thickness to five and seven times that of controls respectively. The epithelium, which stained for elastin, was also twice as thick and tough to cut, but exposure to copper did not change the total amount of desmosine which is found only in elastin. We conclude that copper stimulated collagen synthesis in the ECM and also caused cross-linking of existing proteins. However, there was no expulsion of the symbiotic algae ( Symbiodinium sp.) and no effect on algal pigments or respiration (44, 66 and 110 µg Cu L −1). A decrease in net photosynthesis was observed only at the highest copper concentration (156 µg Cu L −1). These results show that cnidarians may be more susceptible to damage by copper than their symbiotic algae. 相似文献
4.
The metabolic pathways in photosynthesis are modelled as an interconnected series of chemical reactions representing the electron transfer system, the carbon reduction cycle and starch and sucrose synthesis according to the model of Laisk and Walker [Proc R Soc Lond 227, 281–302 (1986)]. The model is formulated as a set of non-linear differential equations using mass-action kinetics, and stimulated for transient behaviour using an interactive simulation language. The model responses to switched light demonstrate the existence of oscillatory behaviour, similar to that found experimentally in O 2 evolution and chlorophyll fluorescence, and explain known transient behaviour. The model is also used to investigate the source of oscillatory behaviour in the phosphate translocator, and other transient phenomena associated with the cyclic electron transfer system.Abbreviations PQ
plastoquinone
- PQH 2
plastoquinol
- PC red
reduced plastocyanin
- PC ox
oxidised plastocyanin
- P i
ortho (inorganic) phosphate in chloroplasts
- P io
inorganic orthophosphate in cytosol
- TP
triose phosphate
- Ru5P
ribulose-5-phosphate
- RuBP
ribulose bisphosphate
- PGA
phosphoglyceric acid
- HP
hexose phosphate
- HP o
hexose phosphate-total sugar phosphate in cytoplasm
- S
starch
- SU
sucrose 相似文献
5.
The photosynthetic bacterium Rhodopseudomonas capsulata lacks glutamate dehydrogenase and normally uses the glutamine synthetase/glutamate synthase sequence of reactions for assimilation of N2 and ammonia. The glutamine synthetase in cell-free extracts of the organism is completely sedimented by centrifugation at 140,000 X g for 2 h, is inhibited by L-alanine but not by adenosine 5'-monophosphate, and exhibits two apparent Km values for ammonia (ca. 13 muM and 1 mM). 相似文献
7.
The upside-down jellyfish Cassiopea engages in symbiosis with photosynthetic microalgae that facilitate uptake and recycling of inorganic nutrients. By contrast to most other symbiotic cnidarians, algal endosymbionts in Cassiopea are not restricted to the gastroderm but are found in amoebocyte cells within the mesoglea. While symbiont-bearing amoebocytes are highly abundant, their role in nutrient uptake and cycling in Cassiopea remains unknown. By combining isotopic labelling experiments with correlated scanning electron microscopy, and Nano-scale secondary ion mass spectrometry (NanoSIMS) imaging, we quantified the anabolic assimilation of inorganic carbon and nitrogen at the subcellular level in juvenile Cassiopea medusae bell tissue. Amoebocytes were clustered near the sub-umbrella epidermis and facilitated efficient assimilation of inorganic nutrients. Photosynthetically fixed carbon was efficiently translocated between endosymbionts, amoebocytes and host epidermis at rates similar to or exceeding those observed in corals. The Cassiopea holobionts efficiently assimilated ammonium, while no nitrate assimilation was detected, possibly reflecting adaptation to highly dynamic environmental conditions of their natural habitat. The motile amoebocytes allow Cassiopea medusae to distribute their endosymbiont population to optimize access to light and nutrients, and transport nutrition between tissue areas. Amoebocytes thus play a vital role for the assimilation and translocation of nutrients in Cassiopea, providing an interesting new model for studies of metabolic interactions in photosymbiotic marine organisms. 相似文献
8.
Photosynthetic control describes the processes that serve to modify chloroplast membrane reactions in order to co-ordinate the synthesis of ATP and NADPH with the rate at which these metabolites can be used in carbon metabolism. At low irradiance, optimisation of the use of excitation energy is required, while at high irradiance photosynthetic control serves to dissipate excess excitation energy when the potential rate of ATP and NADPH synthesis exceed demand. The balance between pH, ATP synthesis and redox state adjusts supply to demand such that the [ATP]/[ADP] and [NADPH]/[NADP +] ratios are remarkably constant in steady-state conditions and modulation of electron transport occurs without extreme fluctuations in these pools.Abbreviations FBPase
Fructose-1,6-bisphosphatase
- PS I
Photosystem I
- PS II
Photosystem II
- Pi
inorganic phosphate
- PGA
glycerate 3-phosphate
- PQ
plastoquinone
- Q A
the bound quinone electron acceptor of PS II
- q P
Photochemical quenching of chlorophyll fluorescence associated with the oxidation of Q A
- q N
non-photochemical quenching of chlorophyll fluorescence
- q E
non-photochemical quenching associated with the high energy state of the membrane
- RuBP
ribulose-1,5-bisphosphate
- TP
triose phosphate
-
intrinsic quantum yield of PS II
-
quantum yield of electron transport
-
quantum yield of CO 2 assimilation 相似文献
11.
The objectives of this study were to determine the effect of light enhancement and hastened reproductive development on nitrogen and dry matter accumulation by field-grown soybean ( Glycine max [L.] Merr.). The impacts of photosynthate supply and reproductive development on change in the season-long profiles of in vivo leaf nitrate reductase (NR) activity and root nodule acetylene reduction (AR) activity were evaluated. Light enhancement resulted in significant increases in dry matter accumulation, root nodule fresh weight and AR activity. Seed yield was increased in both light enhanced treatments in 1978 and in one in 1979. Hastened flowering and seed development was accomplished through photoperiod manipulation within a single genotype. Seasonal decline in leaf NR activity was most rapid in plants entering reproductive development early. An early increase in root nodule fresh weight and AR activity was also observed in response to this treatment and was followed similarly by early decline. The addition of high levels of soil-applied nitrogen increased leaf NR activity and delayed late season decline in NR activity for both control and early reproductive plants. Nitrate supply was therefore implicated as limiting to leaf NR activity during the decline associated with flowering and early seed development. A limited additional increase in leaf NR activity was observed in response to light enhancement plus soil-applied nitrogen. As no significant increase in leaf NR activity was observed in response to light enhancement alone, leaf nitrate supply was further implicated as more limiting to leaf NR activity than was photosynthate supply during flowering and early seed development. 相似文献
13.
The aim of this work was to examine the effect upon photosynthetic capacity of short-term exposure (up to 10 h) to low temperatures
(5° C) of darkened leaves of barley ( Hordeum vulgare L.) plants. The carbohydrate content, metabolite status and the photosynthetic rate of leaves were measured at low temperature,
high light and higher than ambient CO 2. Under these conditions we could detect whether previous exposure of leaves to low temperature overcame the limitation by
phosphate which occurs in leaves of plants not previously exposed to low temperatures. The rates of CO 2 assimilation measured at 8° C differed by as much as twofold, depending upon the pretreatment. (i) Leaves from plants which
had previously been darkened for 24 h had a low content of carbohydrate, had the lowest CO 2-assimilation rates at low temperature, and photosynthesis was limited by carbohydrate, as shown by a large stimulation of
photosynthesis by feeding glucose, (ii) Leaves from plants which had previously been illuminated for 24 h and which contained
large carbohydrate reserves showed an accumulation of phosphorylated intermediates and higher CO 2-assimilation rates at low temperature, but nevertheless remained limited by phosphate, (iii) Maximum rates of CO 2 assimilation at low temperature were observed in leaves which had intermediate reserves of carbohydrate or in leaves which
were rich in carbohydrate and which were also fed phosphate. It is suggested that carbohydrate reserves potentiate the system
for the achievement of high rates of photosynthesis at low temperatures by accumulation of photosynthetic intermediates such
as hexose phosphates, but that this potential cannot be realised if, at the same time, carbohydrate accumulation is itself
leading to feedback inhibition of photosynthesis.
This work was supported by the Agricultural and Food Research Council, UK (Research grant PG50/67) and by the Science and
Engineering Reserach Council, UK. C.A.L. was supported by the British Council, by an Overseas Research Student Award and by
the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq), Brazil. 相似文献
15.
The relationship between the gas-exchange characteristics of attached leaves of Amaranthus edulis L. and the contents of photosynthetic intermediates was examined in response to changing irradiance and intercellular partial pressure of CO 2. After determination of the rate of CO 2 assimilation at known intercellular CO 2 pressure and irradiance, the leaf was freeze-clamped and the contents of ribulose-1,5-bisphosphate, glycerate-3-phosphate, fructose-1,6-bisphosphate, glucose-6-phosphate, fructose-6-phosphate, triose phosphates, phosphoenolpyruvate, pyruvate, oxaloacetate, aspartate, alanine, malate and glutamate were measured. A comparison between the sizes of metabolite pools and theoretical calculations of metabolite gradients required for transport between the mesophyll and the bundle-sheath cells showed that aspartate, alanine, glycerate-3-phosphate and triose phosphates were present in sufficient quantities to support transport by diffusion, whereas pyruvate and oxaloacetate were not likely to contribute appreciably to the flux of carbon between the two cell types. The amounts of ribulose-1,5-bisphosphate were high at low intercellular partial pressures of CO 2, and fell rapidly as the CO 2-assimilation rate increased with increasing intercellular partial pressures of CO 2, indicating that bundle-sheath CO 2 concentrations fell at low intercellular partial pressures of CO 2. In contrast, the amount of phosphoenolpyruvate and of C 4-cycle intermediates declined at low intercellular partial pressures of CO 2. This behaviour is discussed in relation to the co-ordination of carbon assimilation between the Calvin and C 4 cycles.Abbreviations PEP
phosphoenolpyruvate
- PGA
glycerate-3-phosphate
-
p
i
intercellular CO 2 pressure
- RuBP
ribulose-1,5-bisphosphate
- triose-P
triose phosphates 相似文献
16.
The aim of this work was to examine the effect of temperature in the range 5 to 30 ° C upon the regulation of photosynthetic carbon assimilation in leaves of the C 4 plant maize ( Zea mays L.) and the C 3 plant barley ( Hordeum vulgare L.). Measurements of the CO 2-assimilation rate in relation to the temperature were made at high (735 bar) and low (143 bar) intercellular CO 2 pressure in barley and in air in maize. The results show that, as the temperature was decreased, (i) in barley, pools of phosphorylated metabolites, particularly hexose-phosphate, ribulose 1,5-bisphosphate and fructose 1,6-bisphosphate, increased in high and low CO 2; (ii) in maize, pools of glycerate 3-phosphate, triose-phosphate, pyruvate and phosphoenolpyruvate decreased, reflecting their role in, and dependence on, intercellular transport processes, while pools of hexose-phosphate, ribulose 1,5-bis phosphate and fructose 1,6-bisphosphate remained approximately constant; (iii) the redox state of the primary electron acceptor of photosystem II (Q A) increased slightly in barley, but rose abruptly below 12° C in maize. Non-photochemical quenching of chlorophyll fluorescence increased slightly in barley and increased to high values below 20 ° C in maize. The data from barley are consistent with the development of a limitation by phosphate status at low temperatures in high CO 2, and indicate an increasing regulatory importance for regeneration of ribulose 1,5-bisphosphate within the Calvin cycle at low temperatures in low CO 2. The data from maize do not show that any steps of the C 4 cycle are particularly cold-sensitive, but do indicate that a restriction in electron transport occurs at low temperature. In both plants the data indicate that regulation of product synthesis results in the maintenance of pools of Calvin-cycle intermediates at low temperatures.Abbreviations Glc6P
glucose-6-phosphate
- Fru6P
fructase-6-phosphate
- Frul,6bisP
fructose-1,6-bisphosphate
- PGA
glycerate-3-phosphate
-
p
i
intercellular partial pressure of CO 2
- RuBP
ribulose-1,5-bisphosphate
- triose-P
sum of glyceraldehyde-3-phosphate and dihydroxyacetone phosphate
We thank the Agricultural and Food Research Council, UK (Research grant PG50/67) and the Science and Engineering Research Council, UK for financial support. C.A.L. was supported by the British Council, by the Conselho Nacional de Desenvolvimento Cientiflco e Tecnologico (CNPq), Brazil and by an Overseas Research Student Award. We also thank Mark Stitt (Bayreuth, FRG) and Debbie Rees for helpful discussions. 相似文献
17.
The relationship between the gas-exchange characteristics of attached leaves of Zea mays L. and the contents of photosynthetic intermediates was examined at different intercellular partial pressure of CO 2 and at different irradiances at a constant intercellular partial pressure of CO 2. (i) The behaviour of the pools of the C 4-cycle intermediates, phosphoenolpyruvate and pyruvate, provides evidence for light regulation of their consumption. However, light regulation of phosphoenolpyruvate carboxylase does not influence the assimilation rate at limiting intercellular partial pressures of CO 2. (ii) A close correlation between the pools of phosphoenolpyruvate and glycerate-3-phosphate exists under many different flux conditions, consistent with the notion that the pools of C 4 and C 3 cycles are connected via the interconversion of glycerate-3-phosphate and phosphoenolpyruvate. (iii) The ratio of triose-phosphate to glycerate-3-phosphate is used as an indicator of the availability of ATP and NADPH. Changes of this ratio with CO 2 and with irradiance are compared with results obtained in C 3 leaves and indicate that the mechanism of regulation of carbon assimilation by light in leaves of C 4 plants may differ from that in C 3 plants. (iv) The behaviour of the ribulose-1,5-bisphosphate pool with CO 2 and irradiance is contrasted with the behaviour of these pools measured in leaves of C 3 plants.Abbreviations
P
i
intercellular CO 2 pressure
- RuBP
ribulose-1,5-bisphosphate
- PEP
phosphoenolpyruvate
- triose-P
triose phosphates
- PGA
glycerate-3-phosphate 相似文献
18.
The CO 2 compensation points of Coccochloris peniocystis, a blue-green alga and Chlamydomonas reinhardtii, a green alga, were determined at pH 8.0 in a closed system by a gas chromatographic technique. The compensation point of Chlamydomonas increased markedly with temperature, rising from 0.79 microliter per liter CO 2 at 15 C to 2.5 microliters per liter CO 2 at 35 C. In contrast, the compensation point of Coccochloris at 20 C was 0.71 microliter per liter CO 2 and rose to only 0.95 microliter per liter CO 2 at 40 C. 相似文献
19.
Cyanobacteria are photosynthetic bacteria that populate widely different habitats. Accordingly, cyanobacteria exhibit a wide spectrum of lifestyles, physiologies, and morphologies and possess genome sizes and gene numbers which may vary by up to a factor of ten within the phylum. Consequently, large differences exist between individual species in the size and complexity of their regulatory networks. Several non-coding RNAs have been identified that play crucial roles in the acclimation responses of cyanobacteria to changes in the environment. Some of these regulatory RNAs are conserved throughout the cyanobacterial phylum, while others exist only in a few taxa. Here we give an overview on characterized regulatory RNAs in cyanobacteria, with a focus on regulators of photosynthesis, carbon and nitrogen metabolism. However, chances are high that these regulators represent just the tip of the iceberg. 相似文献
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