Nutritional strategy for the preferential uptake of $${{\text{NO}}_{3}}^{ - } {\text{{-}N}}$$ NO 3 - -N by Phaeocystis globosa

Hydrobiologia - Phytoplankton productivity standardized to chlorophyll a and photon flux (mg C mg chl. a−1 mol photons−1) of natural communities from northern Bothnian Sea under dynamic...     

Characteristics of photooxidation of P700 in chloroplast fragments

The characteristics of photooxidation of P700 were investigatedin P700-enriched particles isolated by successive differentialcentrifugations after sonic treatment of chloroplasts. In the presence of ascorbate, the magnitude of photooxidationof P700 was constant at pH values between 5.5 and 9.7. Heat-treatmentat 70°C for 5 min reduced the content of P700 by half; consequently,the magnitude of photooxidation of P700 decreased to half ofthe original value. Intensive sonic treatment for 15 min followedby gel-filtration with Sephadex G-75 did not alter the magnitudeof P700 in the presence of ascorbate. Some reducing agents,i. e. sodium dithionite or sodium borohydride, did not abolishthe photooxidation of P700 under both aerobic and anaerobicconditions. Other treatments such as UV-light irradiation didnot abolish the photooxidation of P700. The nature of the primary electron acceptor of photosystem Iis discussed. (Received July 30, 1971; )     

FTIR study of the primary electron donor of photosystem I (P700) revealing delocalization of the charge in P700(+) and localization of the triplet character in (3)P700.

The effect of global (15)N or (2)H labeling on the light-induced P700(+)/P700 FTIR difference spectra has been investigated in photosystem I samples from Synechocystis at 90 K. The small isotope-induced frequency shifts of the carbonyl modes observed in the P700(+)/P700 spectra are compared to those of isolated chlorophyll a. This comparison shows that bands at 1749 and 1733 cm(-)(1) and at 1697 and 1637 cm(-)(1), which upshift upon formation of P700(+), are candidates for the 10a-ester and 9-keto C=O groups of P700, respectively. A broad and relatively weak band peaking at 3300 cm(-)(1), which does not shift upon global labeling or (1)H-(2)H exchange, is ascribed to an electronic transition of P700(+), indicating that at least two chlorophyll a molecules (denoted P(1) and P(2)) participate in P700(+). Comparisons of the (3)P700/P700 FTIR difference spectrum at 90 K with spectra of triplet formation in isolated chlorophyll a or in RCs from photosystem II or purple bacteria identify the bands at 1733 and 1637 cm(-)(1), which downshift upon formation of (3)P700, as the 10a-ester and 9-keto C=O modes, respectively, of the half of P700 that bears the triplet (P(1)). Thus, while the P(2) carbonyls are free from interaction, both the 10a-ester and the 9-keto C=O of P(1) are hydrogen bonded and the latter group is drastically perturbed compared to chlorophyll a in solution. The Mg atoms of P(1) and P(2) appear to be five-coordinated. No localization of the triplet on the P(2) half of P700 is observed in the temperature range of 90-200 K. Upon P700 photooxidation, the 9-keto C=O bands of P(1) and P(2) upshift by almost the same amount, giving rise to the 1656(+)/1637(-) and 1717(+)/1697(-) cm(-)(1) differential signals, respectively. The relative amplitudes of these differential signals, as well as of those of the 10a-ester C=O modes, appear to be slightly dependent on sample orientation and temperature and on the organism used to generate the P700(+)/P700 spectrum. If it is assumed that the charge density on ring V of chlorophyll a, as measured by the perturbation of the 10a-ester or 9-keto C=O IR vibrations, mainly reflects the spin density on the two halves of the oxidized P700 special pair, a charge distribution ranging from 1:1 to 2:1 (in favor of P(2)) is deduced from the measurements presented here. The extreme downshift of the 9-keto C=O group of P(1), indicative of an unusually strong hydrogen bond, is discussed in relation with the models previously proposed for the PSI special pair.     

FTIR difference spectroscopy in combination with isotope labeling for identification of the carbonyl modes of P700 and P700+ in photosystem I

Room temperature, light induced (P700(+)-P700) Fourier transform infrared (FTIR) difference spectra have been obtained using photosystem I (PS I) particles from Synechocystis sp. PCC 6803 that are unlabeled, uniformly (2)H labeled, and uniformly (15)N labeled. Spectra were also obtained for PS I particles that had been extensively washed and incubated in D(2)O. Previously, we have found that extensive washing and incubation of PS I samples in D(2)O does not alter the (P700(+)-P700) FTIR difference spectrum, even with approximately 50% proton exchange. This indicates that the P700 binding site is inaccessible to solvent water. Upon uniform (2)H labeling of PS I, however, the (P700(+)-P700) FTIR difference spectra are considerably altered. From spectra obtained using PS I particles grown in D(2)O and H(2)O, a ((1)H-(2)H) isotope edited double difference spectrum was constructed, and it is shown that all difference bands associated with ester/keto carbonyl modes of the chlorophylls of P700 and P700(+) downshift 4-5/1-3 cm(-1) upon (2)H labeling, respectively. It is also shown that the ester and keto carbonyl modes of the chlorophylls of P700 need not be heterogeneously distributed in frequency. Finally, we find no evidence for the presence of a cysteine mode in our difference spectra. The spectrum obtained using (2)H labeled PS I particles indicates that a negative difference band at 1698 cm(-1) is associated with at least two species. The observed (15)N and (2)H induced band shifts strongly support the idea that the two species are the 13(1) keto carbonyl modes of both chlorophylls of P700. We also show that a negative difference band at approximately 1639 cm(-1) is somewhat modified in intensity, but unaltered in frequency, upon (2)H labeling. This indicates that this band is not associated with a strongly hydrogen bonded keto carbonyl mode of one of the chlorophylls of P700.     

P700氧化还原动力学的测量方法及原理

P700氧化还原动力学技术可快速且无损地检测植物光系统I (PSI)的活性, 是光合研究领域中广泛使用的一种技术。该文系统归纳了P700氧化还原动力学的主要测量方法, 详细阐述其原理并探讨该技术的局限性, 旨在为深入研究光合作用机理提供技术支持。     

苜蓿国际发展态势分析

苜蓿是草食动物的优质饲草, 被誉为“牧草之王”。发展苜蓿产业对提升我国草食畜牧业具有重要意义。该研究采用定性调研与定量分析相结合的方法, 从创新链角度, 研究了全球苜蓿科技产出、代表性国家苜蓿产业格局和全球苜蓿市场贸易等状况及我国苜蓿产业存在的问题, 旨在为我国苜蓿产业发展提供参考。分析发现, 美国是全球最重要的苜蓿生产国, 在苜蓿基础研究、技术开发、品种培育和商业化种植等方面均具有很强的优势, 引领了全球苜蓿产业的发展。欧美等跨国企业掌控着全球苜蓿产业链的各个关键环节, 是苜蓿产品的主要出口市场, 而亚洲苜蓿产品消费缺口最大。近10年来, 我国在苜蓿科技领域表现活跃, 科技成果产出呈快速增长趋势, 但在成果数量和影响力方面与欧美国家差距明显, 且苜蓿育种进程缓慢, 优质苜蓿产品对外依存度仍然较高。综合来看, 我国应持续加大苜蓿的研发力度和科技投入, 推进苜蓿产业化发展, 提升苜蓿产品的自给率, 保障草食畜牧业健康、稳定发展。     

Primary donor photo-oxidation in photosystem I: a re-evaluation of (P700(+) - P700) Fourier transform infrared difference spectra.

Light-induced Fourier transform infrared (FTIR) difference spectroscopy has been used to study the photo-oxidation of the primary electron donor (P700) in PS I particles from Chlamydomonas reinhardtii and Synechocystis sp. PCC 6803. To aid in the interpretation of the spectra, PS I particles from a site-directed mutant of C. reinhardtii, in which the axial histidine ligand (HisA676) was changed to serine, were also studied. A high-frequency (3300-2600 cm(-1)) electronic transition is observed for all PS I particles, demonstrating that P700 is dimeric. The electronic band is, however, species-dependent, indicating some differences in the electronic structure of P700 and/or P700(+) in C. reinhardtii and Synechocystis sp. 6803. For PS I particles from C. reinhardtii, substitution of HisA676 with serine has little effect on the ester carbonyl modes of the chlorophylls of P700. However, the keto carbonyl modes are considerably altered. Comparison of (P700(+) - P700) FTIR difference spectra obtained using PS I particles from the wild type (WT) and the HS(A676) mutant of C. reinhardtii indicates that the mutation primarily exerts its influence on the P700 ground state. The 13(1) keto carbonyls of the chlorophylls of P700 of the wild type absorb at similar frequencies, which has previously made these transitions difficult to resolve. However, for the HS(A676) mutant, the 13(1) keto carbonyl of chlorophyll a or chlorophyll a' of P700 on PsaB or PsaA absorbs at 1703.4 or 1694.2 cm(-1), respectively, allowing their unambiguous resolution. Upon P700(+) formation, in both PS I particles from C. reinhardtii, the higher-frequency carbonyl band upshifts by approximately 14 cm(-1) while the lower frequency carbonyl downshifts by approximately 10 cm(-1). The similarity in the spectra for WT PS I particles from C. reinhardtii and Synechocystis sp. 6803 indicates that a similar interpretation is probably valid for PS I particles from both species. The mutant results allow for an interpretation of the behavior of the 13(1) keto carbonyls of P700 that is different from previous work [Breton, J., Nabedryk, E., and Leibl, W. (1999) Biochemistry 38, 11585-11592], in which it was suggested that 13(1) keto carbonyls of P700 absorb at 1697 and 1639 cm(-1), and upshift by 21 cm(-1) upon cation formation. The interpretation of the spectra reported here is more in line with recent results from ENDOR spectroscopy and high-resolution crystallography.     

Heat-induced changes in the EPR signal of tyrosine D ($$ Y^{{OX}}_{D} $$): a possible role of Cytochrome b559

The present study for the first time describes a close relationship between a change in the states of Cyt b559, a damage to Mn complex and a rapid reduction of tyrosine D (Y_D) as a function of temperature in spinach thylakoid membranes. Measurements of the EPR signal of dark stable tyrosine D in heat-treated thylakoid membranes showed a gradual decay of the oxidized state of tyrosine D with the progression of temperature. Simultaneously, it leads to the conversion of high-potential Cytochrome b559 into its low-potential form. We have speculated a possible involvement of Cytochrome b559 in the primary reduction events of tyrosine D in dark at high temperature. However, rapid reduction of tyrosine D may also be due to the disassembly of the Mn clock, which causes exposure of Y_D to the lumen and thereby its reduction by some unknown factor. These conclusions are supported by the measurements of Mn²⁺ release and thermoluminescence curves of various charge pairs in heat-treated thylakoid membranes. The results reveal an important aspect on the role of Cyt b559 in PS II during temperature stress.     

Photoinhibition and P700 in the Marine Diatom Amphora sp

The marine diatom Amphora sp. was grown at a light intensity of 7.0 × 10¹⁵ quanta centimeter⁻² second⁻¹. Light saturation of photosynthesis for these cells was between 6.0 and 7.0 × 10¹⁶ quanta centimeter⁻² second⁻¹. At light intensities greater than saturation, photosynthetic ¹⁴CO₂ fixation was depressed, while P700 unit size (chlorophyll a concentration/P700 activity) increased and number of P700 units per cell decreased. After a 1-hour exposure of Amphora sp. to a photoinhibitory light intensity of 2.45 × 10¹⁷ quanta centimeter⁻² second⁻¹, there was a 45 to 50% decrease in the rate of ¹⁴CO₂ fixation relative to the rate at the culture light intensity. There also was a 25% increase in P700 unit size and a 30% reduction in the number of P700 units per cell but no change in total chlorophyll a concentration. Following this period of photoinhibition, the cells were returned to a light regime similar to that in the original culture conditions. Within 1 hour, both number of P700 units per cell and P700 unit size returned to levels similar to those of cells which were kept at the culture light intensity. The rates of photosynthesis did not recover as rapidly, requiring 2 to 3 hours to return to the rate for the nonphotoinhibited cells. Our results indicate that a decrease in P700 activity (with a resultant increase in P700 unit size) may be partially responsible for the photoinhibition of algal photosynthetic carbon dioxide fixation.     

The P700-chlorophyll a-protein of a blue-green alga

The previously isolated chlorophyll a-protein of blue-green algae has been shown to contain P700 in a ratio of 1 reaction center molecule per 100 light-harvesting chlorophyll molecules. One-fifth of the molecules in the preparation contain P700 together with some 20 light-harvesting molecules, whereas the other molecules contain bulk chlorophyll only. Both pigment-protein entities are considered to be essentially the same and cannot be fractionated. An aggregate containing both types probably makes up the photochemical portion of the algal Photosystem I in vivo. The absorption and emission spectra of the pigment-protein are reported, as well as the spectral changes associated with the photochemical reaction. In addition to chlorophyll, carotenoid and protein the complex contains a quinone, which is not a plastoquinone. This unidentified quinone appears to participate in secondary electron transfer reactions occurring in the complex. Horse cytochrome c can be bound to the complex and will donate electrons to P⁺700 upon illumination. Current hypotheses for the identity of the primary electron acceptor were tested. It appears unlikely that flavins, pteridines or iron fill this role.     

Equilibration between cytochrome f and P700 in intact leaves

Electron transport between the two photosynthetic reaction centres of high plants is mediated by plastoquinone, a rieske iron-sulfur centre, cytochrome f and plastocyanin. Measurements of redox equilibration amongst these have produced confusing results, with apparent equilibrium constants being estimated that are inconsistent with in vitro measurements of redox midpoint potentials of the components concerned. We have critically reexamined methods for deconvoluting cytochrome f absorbance signals in intact leaves. We have determined the decay of cytochrome f+ following light to dark transitions from steady state and compared this with the decay of the oxidised photosystem I primary donor, P700+. Measurements across a wide range of different irradiances and CO2 concentrations were all consistent with cyt f and P700 existing in redox equilibrium, with a potential difference of around 117 mV. These results are discussed in relation to our understanding of the organisation of the photosynthetic electron transport. They also have implications for measurements of PSI electron flux--provided more than about 20% of P700+ is oxidised in the light, then the initial decay in the concentration of P700+ following a light to dark transition provides a good estimate of electron flux through PSI. Where P700 is largely reduced in the light, net reduction of cyt f+ might need to be corrected for.     

Evaluation of two-center Coulomb and hybrid integrals over complete orthonormal sets of $$ {\Psi^{\rm{\alpha }}} - {\hbox{ETO}} $$ using auxiliary functions

Journal of Molecular Modeling - By the use of ellipsoidal coordinates, the two-center Coulomb and hybrid integrals over complete orthonormal sets of $$ {\Psi^{\rm{\alpha }}} - {\hbox{exponential}}...     

P700: the primary electron donor of photosystem I.

The primary electron donor of photosystem I, P700, is a chlorophyll species that in its excited state has a potential of approximately -1.2 V. The precise chemical composition and electronic structure of P700 is still unknown. Recent evidence indicates that P700 is a dimer of one chlorophyll (Chl) a and one Chl a'. The Chl a' and Chl a are axially coordinated by His residues provided by protein subunits PsaA and PsaB, respectively. The Chl a', but not the Chl a, is also H-bonded to the protein. The H-bonding is likely responsible for selective insertion of Chl a' into the reaction center. EPR studies of P700(+*) in frozen solution and single crystals indicate a large asymmetry in the electron spin and charge distribution towards one Chl of the dimer. Molecular orbital calculations indicate that H-bonding will specifically stabilize the Chl a'-side of the dimer, suggesting that the unpaired electron would predominantly reside on the Chl a. This is supported by results of specific mutagenesis of the PsaA and PsaB axial His residues, which show that only mutations of the PsaB subunit significantly alter the hyperfine coupling constants associated with a single Chl molecule. The PsaB mutants also alter the microwave induced triplet-minus-singlet spectrum indicating that the triplet state is localized on the same Chl. Excitonic coupling between the two Chl a of P700 is weak due to the distance and overlap of the porphyrin planes. Evidence of excitonic coupling is found in PsaB mutants which show a new bleaching band at 665 nm that likely represents an increased intensity of the upper exciton band of P700. Additional properties of P700 that may give rise to its unusually low potential are discussed.     

Multi-Phasic Kinetics of P700+ Dark Re-Reduction in Nicotiana Tabacum

Reduction kinetics of P700⁺ after far-red radiation (FR)-induced oxidation in intact tobacco leaves was examined by analysing the post-irradiation relaxation of 810–830 nm absorbance difference. The reduction curve could be de-convoluted distinctively into two or three exponential decaying components, depending on the FR irradiance, the treating and measuring temperatures, and the extent of dark adaptation. The multi-phasic kinetics of P700⁺ re-reduction upon the turning off of FR irradiation is related to the heterogeneity of electron transport around photosystem 1 in thylakoid membranes.