Prolonged incubation with low concentrations of mercury alters energy transfer and chlorophyll (Chl) a protein complexes in Synechococcus 6301: changes in Chl a absorption and emission characteristics and loss of the F695 emission band

Synechococcus PCC 6301 cells grown in the presence of low sublethal levels of (about 2 m) mercury induced alterations in chlorophyll (Chl) a absorption without significant alterations in phycocyanin. Chl a fluorescence emission in Hg²⁺ -raised cells showed a large (about 18 nm) blue shift in the peak emission. No major spectral changes in phycobilisome (PBsome) emission characteristic were noticed, indicating major structural alterations in Chl-protein complexes by incubation with Hg²⁺ ions. Low temperature (77 K) emission spectra of cells grown in the presence of Hg²⁺ showed a loss of the characteristic Chl a emission band at 695 nm (F695), which is known to be linked to photosystem II photochemistry and to originate from the Chl a of core antenna polypeptide CP 47 of photosystem II. The SDS-PAGE polypeptide profile of thylakoids indicates a loss of a polypeptide(s) with a molecular mass between 40 and 60 k Da by Hg²⁺ incubation of cells. Our results suggest that prolonged incubation of Synechococcus 6301 cells with low concentrations of Hg²⁺ affects the Chl a spectral properties and the structure of Chl-protein complexes.     

Hg2+ and Cd2+ induced inhibition of light induced proton efflux in the cyanobacteriumAnabaena flos-aquae

Light induced proton efflux in intact cells ofAnabaena flos-aquae is inhibited by the heavy metals Hg²⁺ and Cd²⁺. Furthermore, Hg²⁺ and Cd²⁺ reduced the¹⁴CO₂ fixation, oxygen evolution and carbonic anhydrase activity responsible for H⁺ efflux.     

Prevention by Ce3+ of DNA destruction caused by Hg2+ in fish intestine

The interactions between Hg²⁺, Ce³⁺, and the mixuure of Ce³⁺ and Hg²⁺, and DNA from fish intestine in vitro were investigated by using absorption spectrum and fluorescence emission spectrum. The ultraviolet absorption spectra indicated that the addition of Hg²⁺, Ce³⁺, and the mixture of Ce³⁺ and Hg²⁺ to DNA generated an obviously hypochromic effect. Meanwhile, the peak of DNA at 205.2 nm blue-shifted and at 258.2 nm red-shifted. The size of the hypochromic effect and the peak shift of DNA by metal ion treatments was Hg²⁺>Hg²⁺+Ce³⁺>Ce³⁺. The fluorescence emission spectra showed that with the addition of Hg²⁺, Ce³⁺, and the mixture of Ce³⁺ and Hg²⁺ the emission peak at about 416.2 nm of DNA did not obviously change, but the intensity reduced gradually and the sequence was Hg²⁺>Hg²⁺+Ce²⁺>Ce³⁺. Hg²⁺, Ce³⁺, and the mixture of Ce³⁺ and Hg²⁺ had 1.12, 0.19, and 0.41 binding sites to DNA, respectively; the fluorescence quenching of DNA caused by the metal ions all attributed to static quenching. The binding constants (K _A ) of binding siees were 8.98×10⁴ L/mol and 1.02×10⁴ L/mol, 5.12×10⁴ L/mol and 1.10×10³ L/mol, 6.66×10⁴ L/mol and 2.36×10³ L/mol, respectively. The results showed that Ce³⁺ could relieve the destruction of Hg²⁺ on the DNA structure.     

Calcium depletion alters energy transfer and prevents state changes in intact Anacystis

A time-dependent loss of Photosystem II (PS II) activity seen in Anacystis nidulans grown without Ca²⁺ was paralleled by a loss in chlorophyll (Chl) a fluorescence of variable yield which reflects inhibition of Q reduction and of state changes. Both inhibitions were fully reversed by the addition of Ca²⁺ to the growth medium. The lack of state changes in Ca²⁺-depleted cells was confirmed in 77 K fluorescence difference spectra of light versus dark-adapted cells.Absorption spectra of control and of Ca²⁺-depleted cells were identical whether measured at room temperature or at 77 K. Fluorescence emission spectra measured at 39°C (cell growth temperature) demonstrated higher yields in Ca²⁺-depleted cells compared to controls. Fluorescence emission spectra at 77 K also produced higher yields in Ca²⁺-depleted cells but the increased fluorescence at this temperature occurred principally at 683 nm. The increased relative fluorescence yield in Ca²⁺-depleted samples results from light absorbed by phycocyanin (PC), but not from light absorbed almost exclusively by Chl. The 683 run fluorescence peak probably represents increased allophycocyanin (APC) emission as intact phycobilisomes become energetically disassociated from the photosynthetic apparatus. This inferred disassociation occurred only after PSII activity was mostly inhibited in Ca²⁺-depleted cells, and was not fully reversible.Abbreviations APC Allophycocyanin - Chl chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - EDTA ethylenediaminotetraacetic acid - PC phycocyanin - PS photosystem - Q primary quinone electron acceptor of Photosystem II also a quencher of Chl a fluorescence DPB-CIW Publ. No. 817     

Photosystem 2 Efficiency and Thylakoid Protein Pattern in DCMU-Treated Wheat Seedlings during Senescence

Changes in various components of photosynthetic apparatus during the 6-d dark incubation at 25 °C of detached control and DCMU-treated Triticum aestivum L. leaves were examined. The rate of photosystem 2 (PS2) activity was decreased with increase of the time of dark incubation in control leaves. In contrast to this, DCMU-treated leaves demonstrated high stability by slowing down the inactivation processes. Diphenyl carbazide and NH₂OH restored the PS2 activity more in control leaves than in DCMU-treated leaves. Mn²⁺ failed to restore the PS2 activity in both control and DCMU-treated samples. Similar results were obtained when F_v/F_m was evaluated by chlorophyll fluorescence measurements. The marked loss of PS2 activity in dark incubated control leaves was primarily due to the loss of D1, 33, and 23 kDa extrinsic polypeptides and 28-25 kDa LHCP2 polypeptides.     

Nuclear pea mutants deficient in chlorophyll b and the major polypeptide of the light-harvesting chlorophyll a/b protein of photosystem II

Eight chlorophyll b deficient nuclear mutants of pea (Pisum sativum L.) have been characterized by low temperature fluorescence emission spectra of their leaves and by the ultrastructure, photochemical activities and polypeptide compositions of the thylakoid membranes. The room temperature fluorescence induction kinetics of leaves and isolated thylakoids have also been recorded. In addition, the effects of Mg²⁺ on the fluorescence kinetics of the membranes have been investigated. The mutants are all deficient in the major polypeptide of the light-harvesting chlorophyll a/b protein of photosystem II. The low temperature fluorescence emission spectra of aurea-5106, xantha-5371 and –5820 show little or no fluorescence around 730 nm (photosystem I fluorescence), but possess maxima at 685 and 695 nm (photosystem II fluorescence). These three mutants have low photosystem II activities, but significant photosystem I activities. The long-wavelength fluorescence maximum is reduced for three other mutants. The Mg²⁺ effect on the variable component of the room temperature fluorescence (685 nm) induction kinetics is reduced in all mutants, and completely absent in aurea-5106 and xantha-5820. The thylakoid membranes of these 2 mutants are appressed pairwise in 2-disc grana of large diameter. Chlorotica-1-206A and–130A have significant long-wavelength maxima in the fluorescence spectra and show the largest Mg²⁺ enhancement of the variable part of the fluorescence kinetics. These two mutants have rather normally structured chloroplast membranes, though the stroma regions are reduced. The four remaining mutants are in several respects of an intermediate type.Abbreviations Chl chlorophyll - CPI Chi-protein complex I, F_o, F_v - F_m parameters of room temperature chlorophyll fluorescence induction kinetics - F685, F695 and F-1 components of low temperature chlorophyll emission with maximum at 685, 695 and ca 735 nm, respectively - PSI photosystem I - PSII photosystem II - LHCI and LHCII light-harvesting chlorophyll a/b complexes associated with PSI and PSII, respectively - SDS sodium dodecyl sulfate     

Gradual Disassembly of Photosystem 2 In Vivo Induced by Excess Irradiance. A Hypothesis Based on Changes in 77 K Fluorescence Spectra of Chlorophyll a in Barley Leaves

Effects of short-term exposure to different irradiances on the function of photosystem 2 (PS2) were studied for barley grown at low (LI; 50 μmol m⁻² s⁻¹) and high (HI; 1 100 μmol m⁻² s⁻¹) irradiances. HI barley revealed higher ability to down-regulate the light-harvesting within PS2 after exposure to high irradiance as compared to LI plants. This ability was estimated from the light-induced decreases of F685/F742 and E476/E436 in emission and excitation spectra of 77 K chlorophyll (Chl) a fluorescence in vivo which was 65 and 10 % for HI plants as compared to 30 and 2 % for LI plants, respectively. For LI plants this protective down-regulation of the light-harvesting of PS2 was saturated at 430 μmol m⁻² s⁻¹, and progressive PS2 photodamage was induced at higher irradiances. After exposure of LI segments to 2 200 μmol m⁻² s⁻¹ a pronounced maximum at 700 nm appeared in emission spectrum of 77 K Chl a fluorescence. Based on complementary analysis of 77 K excitation spectra measured at the emission wavelength 685 nm we suggest that this emission maximum may be attributed to the formation of aggregates of light-harvesting complexes of PS2 (LHC2) with part of PS2 core during progressive PS2 photodamage. Our results can be explained assuming different contributions of LHC2 and PS2 core to the total nonradiative dissipation of absorbed excitation energy for the LI and HI barley. This revised version was published online in August 2006 with corrections to the Cover Date.     

Regulation of the distribution of excitation energy in Ochromonas danica,an organism containing a chlorophyll-A/C/carotenoid light harvesting antenna

A chlorophyll a, c-fucoxanthin pigment-protein complex8 functions as the major light harvesting antenna in the Chrysophyte Ochromonas danica. The regulated distribution of excitation energy between the two photosystems was investigated in these organisms and was shown to be strongly wavelength dependent. A light state transition was induced by pre-illumination of cells using light 2 (640 nm) and light 1 (700 nm) of equal absorbed intensity, and detected by reversible changes in the 77 K chlorophyll fluorescence emission spectra. Peaks at 690 nm and 720 nm in the low temperature spectra are most likely associated with PS2 and PS1 respectively. A room temperature fluorescence emission at 680 nm induced by modulated light 2 (500 nm) was strongly quenched in the presence of background light 1 (720 nm). Removal of light 1 led to an increase in fluorescence followed by a slow quenching. The room temperature fluorescence changes were directly correlated with changes in the 77 K emission spectra that indicated a change in the distribution of excitation energy between the two photosystems. It was established that DCMU (1 mol) prevented the state 2. The conversion to state 1 followed a simple photochemical dose dependence and had a half-time of 20 s-1.5 min at 6 W m^-2. In contrast, the conversion to state 2 was independent of light intensity. These data indicate that O. danica undergoes a light state transition in response to the preferential excitation of PS2 or PS1.Abbreviations PS2 photosystem 2 - PS1 photosystem 1 - LHC light harvesting chlorophyll a/b protein - fx fucoxanthin - PQ plastoquinone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea     

Alteration of Components of Chlorophyll-Protein Complexes and Distribution of Excitation Energy Between the Two Photosystems in Two New Rice Chlorophyll b-less mutants

Two new yellow rice chlorophyll (Chl) b-less (lack) mutants VG28-1 and VG30-5 differ from the other known Chl b-less mutants with larger amounts of soluble protein and ribulose-1,5-bisphosphate carboxylase/oxygenase small sub-unit and smaller amounts of Chl a. We investigated the altered features of Chl-protein complexes and excitation energy distribution in these two mutants, as compared with wild type (WT) rice cv. Zhonghua 11 by using native mild green gel electrophoresis and SDS-PAGE, and 77 K Chl fluorescence in the presence of Mg²⁺. WT rice revealed five pigment-protein bands and fourteen polypeptides in thylakoid membranes. Two Chl b-less mutants showed only CPI and CPa pigment bands, and contained no 25 and 26 kDa polypeptides, reduced amounts of the 21 kDa polypeptide, but increased quantities of 32, 33, 56, 66, and 19 kDa polypeptides. The enhanced absorption of CPI and CPa and the higher Chl fluorescence emission ratio of F685/F720 were also observed in these mutants. This suggested that the reduction or loss of the antenna LHC1 and LHC2 was compensated by an increment in core component and the capacity to harvest photon energy of photosystem (PS) 1 and PS2, as well as in the fraction of excitation energy distributed to PS2 in the two mutants. 77 K Chl fluorescence spectra of thylakoid membranes showed that the PS1 fluorescence emission was shifted from 730 nm in WT rice to 720 nm in the mutants. The regulation of Mg²⁺ to excitation energy distribution between the two photosystems was complicated. 10 mM Mg²⁺ did not affect noticeably the F685/F730 emission ratio of WT thylakoid membranes, but increased the ratio of F685/F720 in the two mutants due to a reduced emission at 685 nm as compared to that at 720 nm.     

Response of Intact Cyanobacterial Cells and their Photosynthetic Apparatus to Cd2+ Ion Treatment

Intact cells of Synechococcus elongatus were treated with different concentrations (0.1 and 1.0 mM = Cd_0.1, Cd_1.0) of CdCl₂ for 24 h. Cd_0.1 treatment stimulated growth of the cell culture and chlorophyll (Chl) a concentration in the culture. Cd_1.0 inhibited both the above mentioned parameters. The oxygen evolving activity of intact cells (H₂O BQ) as well as of isolated thylakoid membranes, TM (H₂O DCPIP; H₂O PBQ + FeCy) decreased after 24 h of Cd_1.0 cultivation to 7 %. Photosystem 1 (PS1) activity was less sensitive to the effect of Cd²⁺ than PS2 activity. CdCl₂ concentration in cultivation media after 24 h of cultivation proved that the cyanobacterium cells take up these ions to a large extent from the cultivation medium. After 24 h of the Cd_1.0 treatment only 12 % of the amount of Cd²⁺ originally added to the cultivation medium was found. The ratio of external-antenna pigments, phycocyanin, and allophycocyanin to Chl increased approximately twofold with growing Cd²⁺ concentration in the cultivation medium. This ratio was found in both TM and dodecylmaltoside extracts.     

Pigment protein complexes and functional properties of tetratype resulting from crosses between CP1 and CP2 less Chlamydomonas mutants

A chlorophyll b-less mutant of Chlamydomonas reinhardtii (Pg ₂₇) was isolated after UV irradiation of the wild type cells. This photosynthetically competent mutant totally lacks chlorophyll b and the CP₂ chlorophyll-protein complex. However, SDS-PAGE, proteolytic digestions and immunodetections demonstrated that the 24–25 Kd apoproteins of the lacking CP₂ complex are still present in thylakoids of the Pg₂₇ mutant. It is concluded that this CP₂-less mutant is affected in the biosynthesis pathway of chlorophyll b.This CP₂-less mutant was crossed with a CP₁-less mutant (Fl₅) Fluorescence emission spectra and fluorescence inductions in the presence of DCMU were analysed in the resulting (cp ₂ ^– , cp ₁ ⁺ ), (cp ₂ ⁺ , cp ₁ ^– ), (cp ₂ ⁺ , cp ₁ ⁺ ), cp ₂ ^– , cp ₁ ^– )tetratype. Differences in PS 2 optical cross section and in the relative amplitude or localisation of fluorescence emission peaks fit well with a quadripartite model where PS1 and PS2 would each correspond to a reaction centre core complex (CP₁ and CP₂ respectively) associated to a light harvesting antenna (LHC1 and LHC2 respectively). The occurrence of energy transfers from PS1 peripheral antenna to PS2 in the Fl ₅ mutant shows that, in absence of CP₁, at least a part of its associated PS1 light harvesting antenna migrates in the PS2 containing appressed thylakoids.Abbreviations Chl Chlorophyll - LHC Light harvesting chl a/b complex - CP₂ Predominant form of LHC or SDS polyacrylamide gels - WT Wild type - DM Double mutant (cp ₁ ^– , cp ₂ ^– ) - SDS-PAGE sodium dodecyl sulfate polyacrylamide gel electrophoresis - DOC-PAGE Deoxycholate polyacrylamide gel electrophoresis     

Time-course development of the Cd<Superscript>2+</Superscript> hyper-accumulating phenotype in <Emphasis Type="Italic">Euglena gracilis</Emphasis>

To determine the onset of the Cd²⁺-hyperaccumulating phenotype in Euglena gracilis, induced by Hg²⁺ pretreatment (Avilés et al. in Arch Microbiol 180:1–10, 2003), the changes in cellular growth, Cd²⁺ uptake, and intracellular contents of sulfide, cysteine, γ-glutamylcysteine, glutathione and phytochelatins during the progress of the culture were analyzed. In cells exposed to 0.2 mM CdCl₂, the Cd²⁺-hyperaccumulating phenotype was apparent only after 48 h of culture, as indicated by the significant increase in cell growth and higher internal contents of sulfide and thiol-compounds, along with a higher γ-glutamylcysteine synthetase activity. However, the stiochiometry of thiol-compounds/Cd²⁺ accumulated was similar for both control and Hg²⁺-pretreated cells. Moreover, the value for this ratio was 2.1 or lower after 48-h culture, which does not suffice to fully inactivate Cd²⁺. It is concluded that, although the glutathione and phytochelatin synthesis pathway is involved in the development of the Cd²⁺-hyperaccumulating phenotype in E. gracilis, apparently other pathways and sub-cellular mechanisms are also involved. These may be an increase in other Cd²⁺ chelating molecules such as di- and tricarboxylic acids, phosphate and polyphosphates, as well as Cd²⁺ compartmentation into organelles. César Avilés: In memoriam.     

Excitation spectra of chlorophyll fluorescence in spinach and barley chloroplasts at 4 K

Excitation spectra of chlorophyll a fluorescence in chloroplasts from spinach and barley were measured at 4.2 K. The spectra showed about the same resolution as the corresponding absorption spectra. Excitation spectra for long-wave chlorophyll a emission (738 or 733 nm) indicate that the main absorption maximum of the photosystem (PS) I complex is at 680 nm, with minor bands at longer wavelengths. From the corresponding excitation spectra it was concluded that the emission bands at 686 and 695 nm both originate from the PS II complex. The main absorption bands of this complex were at 676 and 684 nm. The PS I and PS II excitation spectra both showed a contribution by the light-harvesting chlorophyll $\text{a}\text{b}$ protein(s), but direct energy transfer from PS II to PS I was not observed at 4 K. Omission of Mg²⁺ from the suspension favored energy transfer from the light-harvesting protein to PS I. Excitation spectra of a chlorophyll b-less mutant of barley showed an average efficiency of 50–60% for energy transfer from β-carotene to chlorophyll a in the PS I and in the PS II complexes.     

A ratiometric fluorescence sensor based on carbon quantum dots realized the quantitative and visual detection of Hg2+

In this paper, based on the fluorescence of carbon quantum dots (CQDs) quenched by mercury ions (Hg²⁺) and the nonresponse of Hg²⁺ to rhodamine B fluorescence, a dual emission ratio fluorescence sensor was constructed to realize the quantitative detection of Hg²⁺. Under excitation at 365 nm, the fluorescence spectrum showed double emission peaks at 437 nm and 590 nm, corresponding to the fluorescence emissions of CQDs and rhodamine B, respectively. This method quantitatively detected Hg²⁺ based on the linear relationship between the ratio of the intensities of the two emission peaks F₄₃₇/F₅₉₀ and the concentration of Hg²⁺. The detection range was 10–70 nM, and the limit of detection (S/N = 3) was 3.3 nM. In addition, this method could also realize the qualitative and semiquantitative detection of Hg²⁺ according to the fluorescence colour change of the probe under ultraviolet light. After various evaluations, the method could be successfully applied to the quantitative and visual detection of Hg²⁺ in tap water, and demonstrated excellent selectivity, anti-interference performance, and repeatability of the method.     

Interaction of Anions and Cations in Regulating Energy Distribution between the Two Photosystems

Cations such as Mg²⁺ regulate spillover of absorbed excitation energy mainly in favour of photosystem (PS) 2. Effect of low concentration (<10 mM) of the monovalent cation Na⁺ on chlorophyll (Chl) a fluorescence was completely overridden by divalent cation Mg²⁺ (5 mM). Based on Chl a fluorescence yield and 77 K emission measurements, we revealed the role and effectiveness of anions (Cl^-, SO₄ ^2-, PO₄ ^3-) in lowering the Mg²⁺-induced PS2 fluorescence. The higher the valency of the anion, the lesser was the expression of Mg²⁺ effect. Anions may thus overcome Mg²⁺ effects up to certain extent in a valency dependent manner, thereby diverting more energy to PS1 even in the presence of MgCl₂. They may do so by reversing Mg²⁺-induced changes.     

The development of antenna complexes of barley (Hordeum vulgare cv. Akcent) under different light conditions as judged from the analysis of 77 K chlorophyll a fluorescence spectra

Using 77 K chlorophyll a (Chl a) fluorescence spectra in vivo, the development was studied of Photosystems II (PS II) and I (PS I) during greening of barley under intermittent light followed by continuous light at low (LI, 50 μmol m⁻² s⁻¹) and high (HI, 1000 μmol m⁻² s⁻¹) irradiances. The greening at HI intermittent light was accompanied with significantly reduced fluorescence intensity from Chl b excitation for both PS II (F685) and PS I (F743), in comparison with LI plants, indicating that assembly of light-harvesting complexes (LHC) of both photosystems was affected to a similar degree. During greening at continuous HI, a slower increase of emission from Chl b excitation in PS II as compared with PS I was observed, indicating a preferred reduction in the accumulation of LHC II. The following characteristics of 77 K Chl a fluorescence spectra documented the photoprotective function of an elevated content of carotenoids in HI leaves: (1) a pronounced suppression of Soret region of excitation spectra (410–450 nm) in comparison with the red region (670–690 nm) during the early stage of greening indicated a strongly reduced excitation energy transfer from carotenoids to the Chl a fluorescing forms within PS I and PS II; (2) changes in the shape of the excitation band of Chl b and carotenoids (460–490 nm) during greening under continuous light confirmed that the energy transfer from carotenoids to Chl a within PS II remained lower as compared with the LI plants. This revised version was published online in June 2006 with corrections to the Cover Date.     

Cr3+和Cd2+对普通小球藻生长及抗氧化酶活性的影响

[目的] 为探究重金属对淡水绿藻生长的影响。[方法] 选取对水质检测具有明显指示作用的普通小球藻（Chlorella vulgaris）为实验材料,CdCl₂·2H₂O和CrCl₃·7H₂O提供重金属离子,探究不同浓度Cr³⁺和Cd²⁺在单一和复合胁迫下对藻细胞浓度、叶绿素a及相关抗氧化酶活性的影响。[结果] 随着Cr³⁺和Cd²⁺浓度不断增加,藻细胞浓度呈先增长后下降趋势;叶绿素a含量呈现先下降后升高再下降的现象,浓度为1 mg/L的单一和复合胁迫下有最大值,且毒性作用表现为Cr³⁺ < Cd²⁺ < Cr³⁺+Cd²⁺;与藻细胞膜相关的丙二醛（MDA）和过氧化氢（H₂O₂）含量随着重金属离子浓度的增大而增长;重金属离子浓度低于10 mg/L时对藻细胞内抗氧化酶系统中的超氧化物歧化酶（SOD）、过氧化氢酶（CAT）和过氧化物酶（POD）表现为促进作用,而大于10 mg/L时具有抑制作用。[结论] 结果表明在单一或复合重金属胁迫下,普通小球藻会充分调动与抗逆性相关的酶来维持自身的正常生长。     

Effects of different excitation and detection spectral regions on room temperature chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence parameters

The effects of different spectral region of excitation and detection of chlorophyll (Chl) a fluorescence at room temperature on the estimation of excitation energy utilization within photosystem (PS) 2 were studied in wild-type barley (Hordeum vulgare L. cv. Bonus) and its Chl b-less mutant chlorina f2 grown under low and high irradiances [100 and 1 000 μmol(photon) m⁻² s⁻¹]. Three measuring spectral regimes were applied using a PAM 101 fluorometer: (1) excitation in the red region (maximum at the wavelength of 649 nm) and detection in the far-red region beyond 710 nm, (2) excitation in the blue region (maximum at the wavelength of 461 nm) and detection beyond 710 nm, and (3) excitation in the blue region and detection in the red region (660– 710 nm). Non-photochemical quenching of maximal (NPQ) and minimal fluorescence (SV₀), determined by detecting Chl a fluorescence beyond 710 nm, were significantly higher for blue excitation as compared to red excitation. We suggest that this results from higher non-radiative dissipation of absorbed excitation energy within light-harvesting complexes of PS2 (LHC2) due to preferential excitation of LHC2 by blue radiation and from the lower contribution of PS1 emission to the detected fluorescence in the case of blue excitation. Detection of Chl a fluorescence originating preferentially from PS2 (i.e. in the range of 660–710 nm) led to pronounced increase of NPQ, SV₀, and the PS2 photochemical efficiencies (F_V/F_M and F_V′/F_M′), indicating considerable underestimation of these parameters using the standard set-up of PAM 101. Hence PS1 contribution to the minimal fluorescence level in the irradiance-adapted state may reach up to about 80 %.     

Functional comparison of metallothioneins MTT1 and MTT2 from Tetrahymena thermophila

Metallothioneins MTT1 and MTT2 from Tetrahymena thermophila have been characterized. The MTT1 contains mainly characteristic Cys-Cys-Cys and Cys-Cys clusters, but MTT2 contains mainly Cys-X-Cys cluster. Cd₁₆-MTT1 mainly consists of α-helix and β-turns, in contrast, Cd₁₁-MTT2 mainly consists of random coils. Reaction of Cd₁₆-MTT1 and Cd₁₁-MTT2 with nitric oxide leads to intramolecular disulfide bond formation, respectively. Binding stabilities of Cd²⁺, Hg²⁺ and Zn²⁺ to MTT1 are stronger than those to MTT2. Cu²⁺ can not replace Cd²⁺ from Cd₁₆-MTT1 complex, but can replace Cd²⁺ from Cd₁₁-MTT2 complex. The analysis of qRT-PCR revealed MTT2 mRNA levels were 31-fold higher than those of MTT1 under basal conditions. These results further suggest MTT1 possibly play a role in the detoxification of heavy metal ions, and MTT2 may be involved in the homeostasis of copper ions.     

Photoinhibition and its wavelength dependence in the cyanobacterium Anabaena variabilis

In the cyanobacterium Anabaena variabilis the dependence of photoinhibition on fluence rate, duration and wavelength of irradiation were studied by measurements of oxygen production and fluorescence emission spectra. The analysis of the photosynthetic activity revealed that photoinhibition affects exclusively photosystem II (PS II), whereas photosystem I (PS I) remained largely unimpaired. Furthermore, PS II fluorescence emission decreased much faster in bleached than in unbleached controls.Studying the wavelength dependence of photoinhibition it was found that only radiation between 520 and 680 nm causes photoinhibition. This is about the same range of wavelengths which causes photobleaching. Fluorescence emission spectra of samples exposed to high fluence rates of 582 and 662 nm, respectively, essentially agree with those samples exposed to high fluence rates of white light, whereas the fluorescence emission spectra of samples exposed to blue light resemble those exposed to dim white light.NaN₃, a substance which prevents photobleaching, inhibits the photosynthetic O₂ production of Anabaena and, hence, enhances the photoinhibitory effect.