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At least 1,081 records · Page 60Linked to original sources
PubMed1993

Microscale characterization of dissolved organic matter production and uptake in marine microbial mat communities.

Intertidal marine microbial mats exhibited biologically mediated uptake of low molecular weight dissolved organic matter (DOM), including D-glucose, acetate, and an L-amino acid mixture at trace concentrations. Uptake of all compounds occurred in darkness, but was frequently enhanced under natural illumination. The photosystem 2 inhibitor, 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) generally failed to inhibit light-stimulated DOM uptake. Occasionally, light plus DCMU-amended treatments led to uptake rates higher than light-incubated samples, possibly due to phototrophic bacteria present in subsurface anoxic layers. Uptake was similar with either 3H- or 14C-labeled substrates, indicating that recycling of labeled CO2 via photosynthetic fixation was not interfering with measurements of light-stimulated DOM uptake. Microautoradiographs showed a variety of pigmented and nonpigmented bacteria and, to a lesser extent, cyanobacteria and eucaryotic microalgae involved in light-mediated DOM uptake. Light-stimulated DOM uptake was often observed in bacteria associated with sheaths and mucilage surrounding filamentous cyanobacteria, revealing a close association of organisms taking up DOM with photoautotrophic members of the mat community. The capacity for dark- and light-mediated heterotrophy, coupled to efficient retention of fixed carbon in the mat community, may help optimize net production and accretion of mats, even in oligotrophic waters.

Amino Acids↗
PubMed2001

The outward component of photoinduced current in chloroplasts of Peperomia metallica.

The photoinduced currents in whole chloroplasts of Peperomia metallica were studied using suction electrodes and single-turnover flashes. The kinetic profile of the photocurrent contained a minor outward component (rise time, 100 micros). Local application (from the inside of the pipette) of a photosystem 2 inhibitor, DCMU, rapidly suppressed the outward current; conversely, addition of DCMU to the outer medium produced a transient stimulation of the outward component. Permeabilization of the tip-located membrane fragments with Triton X-100 eliminated the outward current, but had no significant influence on the inward current. The data suggest that the outward current originated in the tip-located nonruptured portions of the thylakoid membrane. Different involvement of two photosystems in the generation of the outward current indicates that granal thylakoids enriched with photosystem 2 are less susceptible to the rupture in the pipette tip as compared with stromal thylakoids.

Anti-Bacterial Agents↗
PubMed2001

Photosystem II peripheral accessory chlorophyll mutants in Chlamydomonas reinhardtii. Biochemical characterization and sensitivity to photo-inhibition.

In addition to the four chlorophylls (Chls) involved in primary charge separation, the photosystem II (PSII) reaction center polypeptides, D1 and D2, coordinate a pair of symmetry-related, peripheral accessory Chls. These Chls are axially coordinated by the D1-H118 and D2-H117 residues and are in close association with the proximal Chl antennae proteins, CP43 and CP47. To gain insight into the function(s) of each of the peripheral Chls, we generated site-specific mutations of the amino acid residues that coordinate these Chls and characterized their energy and electron transfer properties. Our results demonstrate that D1-H118 and D2-H117 mutants differ with respect to: (a) their relative numbers of functional PSII complexes, (b) their relative ability to stabilize charge-separated states, (c) light-harvesting efficiency, and (d) their sensitivity to photo-inhibition. The D2-H117N and D2-H117Q mutants had reduced levels of functional PSII complexes and oxygen evolution capacity as well as reduced light-harvesting efficiencies relative to wild-type cells. In contrast, the D1-H118Q mutant was capable of near wild-type rates of oxygen evolution at saturating light intensities. The D1-H118Q mutant also was substantially more resistant to photo-inhibition than wild type. This reduced sensitivity to photo-inhibition is presumably associated with a reduced light-harvesting efficiency in this mutant. Finally, it is noted that the PSII peripheral accessory Chls have similarities to a to a pair of Chls also present in the PSI reaction center complex.

Animals↗
PubMedDate not supplied

Reaction of freshwater phyto- and zooplankton to pesticides.

The reaction of plankton to the toxic effect of pesticides is considered at four levels: the cell, the organism, the population, and the biocenosis. The basis for the methodological approach to evaluation of response at the cell level was the principle of phase reactions. Depending on concentration, the pesticides may either suppress or stimulate the plankton organisms. Temperature may be decisive intoxic effects (pesticides are effective in a range of 15 to 25 degrees C). Most pesticides having algacidal activity are effective in a range of concentrations of 1-10 mg/liter. Phytoplankton as a whole has a high buffer capacity with respect to pesticides. Changes in ratios of basic components of phytoplankton effected by pesticides most often lead to a change of the dominant. Analogical changes are characteristic also for zooplankton; pesticides lead to the elimination of water fleas from its composition. The stimulating effect of a low concentration of pesticides, changes in the functional activity of the components of bacterial plankton, and changes coupled with this in biogeochemical cycles of nitrogen and phosphorous as well as elimination of water fleas from the plankton promotes the development of "secondary" eutrophication, that is, an increase in the biomass of the phytoplankton.

Animals↗
PubMedDate not supplied

[Mode of action and structure-acitivity-relationships of the aminotriazinone herbicide Metribuzin. Inhibition of photosynthetic electron transport in chloroplasts by Metribuzin (author's transl)].

The influence of the aminotriazinone herbicide Metribuzin on photosynthetic reactions of isolated chloroplasts is investigated. Metribuzin inhibits all Hill-reactions when water is the electron donor, but not photoreductions by photosystem I at the expense of an artifical electron donor. The PI59-value is 6.7. Cyclic photophosphorylation is not affected by Metribuzin. Measurements of the prompt and delayed fluorescence of the photosynthetically active chlorophyll support the notion, that Metribuzin inhibits photosynthetic electron flow between the primary and seondary electron acceptor of photosystem II (Q and plastoquinone). The relationship of inhibitory potency to chemical structure is investigated by comparing a number of related aminotriazinones. The effect of various substituents is discussed.

Chlorella↗
PubMed1976

Isolation of a human plasmin-derived, functionally active, light (B) chain capable of forming with streptokinase an equimolar light (B) chain-streptokinase complex with plasminogen activator activity.

A functionally active human plasmin light (B) chain derivative, stabilized by the streptomyces plasmin inhibitor leupeptin, was isolated from a partially reduced and alkylated enzyme preparation by an affinity chromatography method with a L-lysine-substituted Sepharose column. This light (B) chain derivative was found to be relatively homogeneous by electrophoretic analysis in both an acrylamide gel/dodecyl sulfate system and on cellulose acetate. It possessed approximately 3% of the proteolytic activity (casein substrate) of the original enzyme, and it incorporated 0.09 mol of [3H]diisopropyl phosphorofluoridate per mol of protein. It contained 3.1 +/- 0.3 carboxymethylated cysteines per mol of protein and can be designated as a CmCys5-light (B) chain (CmCys)3. When this isolated light (B) chain derivative was mixed in equal molar amounts with streptokinase, the mixture developed both human and bovine plasminogen activator activities; the bovine activator activity was approximately 66% of the bovine activator activity of the equimolar human plasmin-streptokinase complex. Although this complex now incorporated 0.50 mol of [3H]diisopropyl phosphorofluoridate per mol of protein, its proteolytic activity, on a molar basis, was the same as the proteolytic activity of the isolated light (B) chain derivative. It was shown by electrophoretic analysis in both an acrylamide gel/epsilon-aminocaproic acid system and on cellulose acetate that the light (B) chain derivative and streptokinase forms an equimolar light (B) chain-streptokinase complex, indicating that the binding site for streptokinase is located on the light (B) chain of the enzyme. A functionally active equimolar light (B) chain-streptokinase complex was also isolated from a partially reduced and alkylated equimolar human plasmin-streptokinase complex by the affinity chromatography method. The plasminogen activator activities (human and bovine) of this light (B) chain-streptokinase complex were similar to those of the plasmin-streptokinase complex from which it was derived. Although this complex incorporated 0.70 mol of [3H]diisopropyl phosphorofluoridate per mol of protein, its proteolytic activity, on a molar basis, was only 14% of proteolytic activity of the plasmin-streptokinase complex.

Amino Acids↗
PubMedDate not supplied

[Influence of DCMU on the formation of lipids and fatty acids, and on the ultrastructure of Euglena gracilis (author's transl)].

Euglena gracilis Klebs (strain Z) was grown heterotrophically in the dark for 6 days. Subsequently, the cells were grown photoautotrophically under white fluorescent light with varying amounts of 3-(3,4-dichlorophenyl)-1,1 dimethyl urea (DCMU) (1.7X10(-9)--1.0X10(-5)mol/l) added to the nutrient medium. Under non-photosynthetic conditions (addition of 10(-6)--10(-5)mol DCMU/l), the cells still produced chloroplast structures and a considerable percentage of compounds typical for chloroplasts, such as galactolipids (40-50%), chlorophylls (40%), and polyunsaturated C16 and C18 fatty acids (16:2, 16:3, 16:4, 18:2, 18:3) (65%). The results indicate that under these conditions the above compounds are synthesized from carbon sources outside the chloroplasts.

Animals↗
PubMedDate not supplied

[On the possibilities of diagnosis of the state of the phytoplankton photosynthetic apparatus by the method of nonlinear laser fluorimetry].

The possibilities of diagnosis of the state of the phytoplankton photosynthetic apparatus by the method of nonlinear laser fluorimetry (saturation fluorimetry), which can be realized in remote sensing mode, were investigated. A procedure for the determination of the nonsaturated fluorescence parameter phi0, which is proportional to the concentration of chlorophyll a molecules, and the parameter A, which is a product of three photophysical parameters of chlorophyll a molecule in native chloroplast, was elaborated. Laboratory experiments with the axenic culture of eurihaline Thalassiosira weissflogii showed that the parameter A depends on the state of the photosynthetic apparatus of the alga, which was varied by either DCMU treatment or exposure to actinic light.

Chlorophyll↗
PubMed2005

[Spectroscopy research on the origin mechanism for light induced delayed fluorescence of chloroplast].

Research of the mechanism for delayed fluorescence origin play an important role in the application of DF. Based onthe charge recombination theory, the mechanism for delayed fluorescence origin is theoretically analyzed in this paper. The decay kinetics of DF, depending on the backward transport electron and oxidized state P680+, can be well fitted by poly-exponent. The relative fast component of DF originates from the Z+ P680 Q(A)- states, generated during the previous period of the sample illumination. On the other hand, the slow component of DF is ascribed to the state Q(B)= --the next electron carrier of deoxidized side in PS II. Meanwhile, the experimental results confirmed the theoretical conclusion. Furthermore, the authors testified that the third component with lifetime much greater than the time of the registration period (which is presented as a constant), is corresponds to the recombination of P680+ and the backward transport electron from PSI.

Algorithms↗
PubMedDate not supplied

[The application of PS II model for the analysis of fluorescence yield transients induced by actinic single turnover flash in the time range from 100 ns to 10 s].

Changes in flash-induced fluorescence yield in preparations of thermophilic Chlorella pyrenoidosa Chick cells (native and in the presence of DCMU) were investigated in the time range from 100 ns to 10 s using a new measuring system. The results were analyzed by mathematical modeling of processes in photosystem II. It was shown that the detailed description of recombination (including nonradiative) processes in photosystem II is important to simulate the fluorescence yield transients induced by an actinic single turnover flash. The model photosystem II parameters were modified to describe the light-induced effects in the presence of DCMU. By comparing the theoretical fluorescence curves with experimental ones, we obtained the values of relative fluorescence yield and the FM/F0 ratio, which is typical for experimental data. As a result of simulation, the values of the model parameters (rate constants of electron transfers at the donor and acceptor sites of photosystem II, pH in stroma, initial redox state of the plastoquinone pool, the rate of plastoquinol oxidation, and the rate constants of nonradiative recombination processes) were determined.

Chlorella↗
PubMed1990

A novel metabolic form of the 32 kDa-D1 protein in the grana-localized reaction center of photosystem II.

Two forms of the 32 kDa-D1 reaction center protein of photosystem II (PSII), having slightly different mobilities on denaturing polyacrylamide gels, have been resolved in Spirodela oligorrhiza, Glycine max L., Gossypium hirsutum L., Triticum aestivum L., and Zea mays L. The protein band with faster mobility is identified as the 32 kDa-D1 protein, and the less mobile band as a novel form, designated 32*. The two forms are structurally similar based on immunological and partial proteolytic tests. 32* is associated exclusively with the grana and is present in the PSII reaction center. Temporally, 32* appears several hours after the translocation of newly synthesized and processed 32 kDa-D1 protein from the stroma lamellae to the grana. Formation of the 32* is strictly light-dependent under physiological light intensities and correlates with a reciprocal loss of the 32-kDa form. Light induced formation of 32* is inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea but is not coupled to linear electron transport.

2,4-Dinitrophenol↗
PubMed1988

Role of the cytochrome b6.f complex in the redox-controlled activity of Acetabularia thylakoid protein kinase.

The regulation of the protein kinase activity responsible for the phosphorylation of the light-harvesting complex of photosystem II (LHCII) 27-kDa polypeptide involved in the State I-State II transitions in Acetabularia thylakoids was investigated. The LHCII kinase of isolated thylakoids retains its activity in absence of light-driven electron flow or reductants added in the dark. However, the kinase is reversibly inactivated by addition of oxidants in vitro or by far red (710 nm) light in vivo. Inhibitors of the quinol oxidase site of the cytochrome b6.f complex inactivate the LHCII kinase in the dark, and also in the light, or in presence of duroquinol when the plastoquinone pool is reduced. Inhibitors of the quinone reductase site of the b6.f complex have practically no effect in the dark and stimulate the kinase activity in the light. Based on these data and on our previous report, showing specific loss of LHCII kinase activity in a Lemna mutant lacking the cytochrome b6.f complex (Gal, A., Shahak, Y., Schuster, G., and Ohad, I. (1987) FEBS Lett. 221, 205-210), we propose that the activity of the LHCII kinase is regulated by the redox state of a cytochrome b6.f complex component(s) which responds to the balance of electron flow from photosystem II via the plastoquinone pool to photosystem I.

Acetabularia↗
PubMedDate not supplied

[Methylviologen photoreduction by chloroplasts].

The condition of methylviologen photoreduction by chloroplasts was investigated. Argon bubbling through the suspension of chloroplasts or degasing in vacuum caused inhibition of methylviologen reduction probably due to the denaturation of chloroplast membranes at the water/air boundary. Adding glycerol or bovine serum albumine or removing oxygen from chloroplast suspension with the aid of the oxygen absorbing-systems preserved the activity of chloroplasts. Methylviologen photoreduction is inhibited by DCMU (10(-7) M) and Tris-buffer treatment and is activated by uncouples. The pH-dependence is similar to that of the Hill reaction. Triton X-100 (0.007%), ethyl ether (2%) and heating up to 42 degrees activated the Hill reaction but inhibited methylviologen reduction. Water molecule probably acts as an initial electron donor in this reaction. It is proposed that the steady level of methylviologen photoreduction is determined by a relationship between the rate of methylviologen electron acceptance and cyclic electron flow short-circuiting photosystem I.

Chloroplasts↗