Effect Of DCMU Research Articles

Combining Nitellopsis obtusa autofluorescence intensity and F680/F750 ratio to discriminate responses to environmental stressors

Detection of autofluorescence parameters is a useful approach to gain insight into the physiological state of plants and algae, but the effect of reabsorption hinders unambiguous interpretation of in vivo data. The exceptional morphological features of Nitellopsis obtusa made it possible to measure autofluorescence spectra along single internodal cells and estimate relative changes in autofluorescence intensity in selected spectral regions at room temperatures, avoiding the problems associated with thick or optically dense samples. The response of algal cells to controlled white light and DCMU herbicide was analyzed by monitoring changes in peak FL intensity at 680 nm and in F680/F750 ratio. Determining the association between the selected spectral FL parameters revealed an exponential relationship, which provides a quantitative description of photoinduced changes. The ability to discern the effect of DCMU not only in the autofluorescence spectra of dark-adapted cells, but also in the case of light-adapted cells, and even after certain doses of excess light, suggests that the proposed autofluorescence analysis of N. obtusa may be useful for detecting external stressors in the field.

Sll1252 Coordinates Electron Transport between Plastoquinone and Cytochrome b6/f Complex in Synechocystis PCC 6803.

A mutant, Δsll1252ins, was generated to functionally characterize Sll1252. Δsll1252ins exhibited a slow-growth phenotype at 70 µmol photons m-2 s-1 and glucose sensitivity. In Δsll1252ins, the rate of PSII activity was not affected, whereas the whole chain electron transport activity was reduced by 45%. The inactivation of sll1252 led to the upregulation of genes, which were earlier reported to be induced in DBMIB-treated wild-type, suggesting that Sll1252 may be involved in electron transfer from the reduced-PQ pool to Cyt b6/f. The inhibitory effect of DCMU on PSII activity was similar in both wild-type and Δsll1252ins. However, the concentration of DBMIB for 50% inhibition of whole chain electron transport activity was 140 nM for Δsll1252ins and 300 nM for wild-type, confirming the site of action of Sll1252. Moreover, the elevated level of the reduced-PQ pool in Δsll1252ins supports that Sll1252 functions between the PQ pool and Cyt b6/f. Interestingly, we noticed that Δsll1252ins reverted to wild-type phenotype by insertion of natural transposon, ISY523, at the disruption site. Δsll1252-Ntrn, expressing only the C-terminal region of Sll1252, exhibited a slow-growth phenotype and disorganized thylakoid structure compared to wild-type and Δsll1252-Ctrn (expressing only the N-terminal region). Collectively, our data suggest that Sll1252 regulates electron transfer between the PQ pool and the Cyt b6/f complex in the linear photosynthetic electron transport chain via coordinated function of both the N- and C-terminal regions of Sll1252.

The DCMU Herbicide Shapes T-cell Functions By Modulating Micro-RNA Expression Profiles.

DCMU [N-(3,4-dichlorophenyl)-N-dimethylurea] or diuron is a widely used herbicide, which can cause adverse effects on human, especially on immune cells, due to their intrinsic properties and wide distribution. These cells are important for fighting not only against virus or bacteria but also against neoplastic cell development. We developed an approach that combines functional studies and miRNA and RNA sequencing data to evaluate the effects of DCMU on the human immune response against cancer, particularly the one carried out by CD8+ T cells. We found that DCMU modulates the expression of miRNA in a dose-dependent manner, leading to a specific pattern of gene expression and consequently to a diminished cytokine and granzyme B secretions. Using mimics or anti-miRs, we identified several miRNA, such as hsa-miR-3135b and hsa-miR-21-5p, that regulate these secretions. All these changes reduce the CD8+ T cells’ cytotoxic activity directed against cancer cells, in vitro and in vivo in a zebrafish model. To conclude, our study suggests that DCMU reduces T-cell abilities, participating thus to the establishment of an environment conducive to cancer development.

Microalgae-Based Fluorimetric Bioassays for Studying Interferences on Photosynthesis Induced by Environmentally Relevant Concentrations of the Herbicide Diuron.

The widespread agricultural use of the phenylurea herbicide Diuron (DCMU) requires the investigation of ecotoxicological risk in freshwater and soil ecosystems in light of potential effects on non-target primary producers and a heavier effect on higher trophic levels. We used microalgae-based fluorimetric bioassays for studying the interferences on the photosynthesis of a freshwater and soil model green microalga (Chlamydomonas reinhardtii) induced by environmentally relevant concentrations of the herbicide DCMU. Measurements of steady-state chlorophyll a (Chl-a) fluorescence emission spectra were performed; as well, the kinetics of the Chl-a fluorescence transient were recorded. Percentage indexes of interference on photosynthesis were calculated after comparison of steady-state and kinetic Chl-a fluorescence measurements of DCMU-exposed and control C. reinhardtii cell suspensions. The results obtained after 30 min exposure to the herbicide DCMU confirmed a significant inhibitory effect of DCMU 2 μg/L, and no significant differences between %ι values for DCMU 0.2 μg/L and 0.02 μg/L exposures. Positive %ε values from kinetic measurements of the Chl-a fluorescence transient confirmed the same interfering effect of 2 μg/L DCMU on PSII photochemistry in the exposed C. reinhardtii cell suspensions. Negative values of %ε observed for 0.2 and 0.02 μg/L DCMU exposures could be attributable to a presumptive ‘stimulatory-like’ effect in the photochemistry of photosynthesis. Short-term exposure to sub-μg/L DCMU concentration (≤0.2 μg/L) affects the photosynthetic process of the model microalga C. reinhardtii. Similar environmental exposures could affect natural communities of unicellular autotrophs, with hardly predictable cascading secondary effects on higher trophic levels.

Hormesis depends upon the life-stage and duration of exposure: Examples for a pesticide and a nanomaterial

Tests to assess toxic effects on the reproduction of adult C. elegans after 72h exposure for two chemicals, (3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)), also known as diuron, and silver nanoparticles (Ag NPs) indicated potential, although not significant hormesis. Follow up toxicity tests comparing the potential hormesis concentrations with controls at high replication confirmed that the stimulatory effect was repeatable and also statistically significant within the test. To understand the relevance of the hormesis effects for overall population fitness, full life-cycle toxicity tests were conducted for each chemical. When nematodes were exposed to DCMU over the full life-span, the hormesis effect for reproduction seen in short-term tests was no longer evident. Further at the putative hormesis concentrations, a negative effect of DCMU on time to maturation was also seen. For the Ag NPs, the EC50 for effects on reproduction in the life-cycle exposure was substantially lower than in the short-term test, the EC50s estimated by a three parameter log logistic model being 2.9mg/L and 0.75mg/L, respectively. This suggests that the level of toxicity for Ag NPs for C. elegans reproduction is dependant on the life stage exposed and possibly the duration of the exposure. Further, in the longer duration exposures, hormesis effects on reproduction seen in the short-term exposures were no longer apparent. Instead, all concentrations reduced both overall brood size and life-span. These results for both chemicals suggest that the hormesis observed for a single endpoint in short-term exposure may be the result of a temporary reallocation of resources between traits that are not sustained over the full life-time. Such reallocation is consistent with energy budget theories for organisms subject to toxic stress.

Pathways of hydrogen photoproduction by immobilized Chlamydomonas reinhardtii cells deprived of sulfur

The green algae Сhlamydomonas reinhardtii entrapped in a thin alginate film have been shown to sustain elevated rates of hydrogen photoproduction under anaerobic incubation in sulfur/phosphorus depleted tris-acetate medium. In the present work we studied mechanisms, underlying hydrogen photoproduction by the immobilized culture, particularly, the roles of PSII and starch accumulation/breakdown. DCMU, a specific inhibitor of electron transport in PSII, is known to suppress hydrogen evolution by circa 80% in suspension cultures of S-deprived C. reinhardtii. In immobilized cells DCMU caused successive stimulatory and inhibitory effects on hydrogen photoproduction, both depending on the deprivation status of the algal cell. The inhibitory effect of DCMU was 25% at 70 h of S deficiency when maximal rates of hydrogen photoproduction were observed. Measurements of the light-induced prompt and delayed chlorophyll fluorescence transients and reflectance at 820 nm (P700 redox transitions) revealed very rapid decline of PSII activity in the entrapped S-deprived cells as compared with the suspension culture, whereas PSI suffered less. The immobilized culture showed a high capacity to accumulate starch during early stages of S deprivation and relatively high rates of anaerobic starch degradation during the following hydrogen evolution period. DCMU partly inhibited starch breakdown. Results of the present work brought us to the conclusion that PSII-independent pathway of hydrogen evolution is elevated in the immobilized S-deprived cells rather due to the rapid inactivation of PSII, efficient starch catabolism and non-photochemical PQ reduction.

Transcriptional regulation of the stress-responsive light harvesting complex genes in Chlamydomonas reinhardtii.

Dissipating excess energy of light is critical for photosynthetic organisms to keep the photosynthetic apparatus functional and less harmful under stressful environmental conditions. In the green alga Chlamydomonas reinhardtii, efficient energy dissipation is achieved by a process called non-photochemical quenching (NPQ), in which a distinct member of light harvesting complex, LHCSR, is known to play a key role. Although it has been known that two very closely related genes (LHCSR3.1 and LHCSR3.2) encoding LHCSR3 protein and another paralogous gene LHCSR1 are present in the C. reinhardtii genome, it is unclear how these isoforms are differentiated in terms of transcriptional regulation and functionalization. Here, we show that transcripts of both of the isoforms, LHCSR3.1 and LHCSR3.2, are accumulated under high light stress. Reexamination of the genomic sequence and gene models along with survey of sequence motifs suggested that these two isoforms shared an almost identical but still distinct promoter sequence and a completely identical polypeptide sequence, with more divergent 3'-untranscribed regions. Transcriptional induction under high light condition of both isoforms was suppressed by treatment with a photosystem II inhibitor, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), and a calmodulin inhibitor W7. Despite a similar response to high light, the inhibitory effects of DCMU and W7 to the LHCSR1 transcript accumulation were limited compared to LHCSR3 genes. These results suggest that the transcription of LHCSR paralogs in C. reinhardtii are regulated by light signal and differentially modulated via photosynthetic electron transfer and calmodulin-mediated calcium signaling pathway(s).

Impairment of symbiont photosynthesis increases host cell proliferation in the epidermis of the sea anemone Aiptasia pallida

Corals exposed to environmental stresses need to engage appropriate physiological strategies to survive. Here, we examined tissue modifications following algal dysfunction. Aiptasia pallida was exposed during 1 week to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), the herbicide called diuron. DCMU treatment produced a drastic loss in photosynthetic efficiency and a subsequent diminution of algae density over the following days. Cell proliferation evaluated by measuring the number of cells labeled with a thymidine analogue (EdU) revealed a significant increase in EdU+ cells in the epidermis after 1 week of DCMU incubation and in the gastrodermis at 4 weeks. TUNEL histology showed that the extent of cell death was, however, similar in the epidermis of control and treated specimens. In addition, we noticed a significant effect of DCMU treatment on the density of epidermal mucocytes after 1, 2 and 4 weeks. These results show that inhibition of Symbiodinium photosynthesis in the absence of any known direct effect of DCMU on host cells can induce an increase in epidermal host cell proliferation in both the epidermis and the gastrodermis. While new host gastrodermal cells are likely to promote tissue regeneration in order to recruit new algae, the new host epidermal cells may contribute to tissue adaptation following a decrease in energy income. Some of these new epidermal cells, such as mucocytes, may contribute to an eventual increase in the host heterotrophic ability until restoration of algal autotrophic contribution.

Photoheterotrophic growth unprecedentedly increases the biosynthesis of mycosporine-like amino acid shinorine in the cyanobacterium Anabaena sp., isolated from hot springs of Rajgir (India)

Cyanobacteria are known to biosynthesize mycosporine-like amino acids (MAAs) as photoprotective compounds against ultraviolet radiation. Anabaena sp., isolated from the hot springs of Rajgir, India, produces a single MAA shinorine (retention time = 2.2 min and absorption maximum at 334 nm) as purified by high-performance liquid chromatography. The MAA biosynthesis was under constitutive control in this cyanobacterium; however, PAR + UV-A + UV-B radiation was found to have highest impact on MAA synthesis. MAA biosynthesis is dependent on photosynthesis for the carbon source since the inhibitory effect of DCMU on MAA synthesis was overcome by externally added fructose. Our results suggest that there is no direct involvement of photosystem II dependent linear electron transport in MAA biosynthesis. However, utilization of energy derived from photosystem I dependent cyclic electron transport in MAA biosynthesis cannot be ruled out. This study also reveals that photoheterotrophic growth can support highest MAA biosynthesis under laboratory conditions in comparison with photoautotrophic and photomixotrophic growth. Thus, photoheterotrophic growth condition can be used for the large-scale production of pharmaceutically important MAAs from cyanobacteria for an industrial application.

Assessment of Photosynthesis Tolerance to Herbicides, Heat and High Illumination by Fluorescence Imaging

Fluorescence imaging represents a non-invasive tool for revealing and understanding spatial heterogeneity in leaf performance caused by external factors, such as abiotic stress. Sun (Rosa meillandina and Chrysanthemum morifo- lium) and shade (Spathiphyllum wallisii) plants were used to study their tolerance to heat and high illumination. Fluores- cence yield, effective PSII quantum yield and non-photochemical quenching were analysed in leaves attached to plants by fluorescence imaging. The control plants of all species showed homogeneous images of the fluorescence parameters throughout the leaf. The fluorescence yield (F) was 0.1 or less, the effective PSII quantum yield (Y(II)) around 0.75 and non-photochemical quenching (NPQ) less than 0.3. The two sun plants showed higher tolerance to stress conditions. Few variations were observed in F and Y(II) images after stress photoperiods and some leaf regions showed an increase in NPQ, indicating more thermal energy dissipation in these zones than in other leaf regions. The images of the fluorescence parameters were similar to those of control plants after one recovery photoperiod without stress conditions. Shade plant showed lower tolerance and irreversible damage was observed after the first photoperiod, particularly at the base of the leaf and in the areas adjacent to the ribs. The centre and top of the leaf were less damaged, and effective PSII quantum yield remained high because the leaf curved to reduce the incident radiation. Incubation with the herbicides DCMU and paraquat led to differences in the fluorescence parameter images. The effect of DCMU (0.1 mM) was visible after 30 min incubation, beginning at the ribs and adjacent areas of the leaf. The three species studied showed different degree of sensi- tivity to paraquat (0.2 mM), and the effective quantum yield in each species was affected at different incubation times.

Evaluation of the efficacy of the photosystem II inhibitor DCMU in periphyton and its effects on nontarget microorganisms and extracellular enzymatic reactions

AbstractWe examined the efficacy of the photosystem II inhibitor 3-(3,4-diclorophenyl)-1,1-dimethyl urea (DCMU) for inhibition of algal photosynthesis in periphyton associated with submerged decomposing litter of Typha angustifolia. We also investigated the possible nontarget effects of DCMU exposure on heterotrophic microorganisms (i.e., bacteria and fungi) and extracellular enzyme activity associated with decaying litter. Standing-dead Typha leaf litter was submerged for 34 and 73 d, returned to the laboratory, and used for controlled laboratory experiments that examined the effect of DCMU on algal ([14C]bicarbonate, pulse-amplitude modulated fluorometry), bacterial ([3H]leucine), and fungal ([14C]acetate) production. Simultaneous assays also were conducted to examine the effect of DCMU on the activities of 4 extracellular enzymes (β-glucosidase, β-xylosidase, leucine-aminopeptidase, and phosphatase). DCMU significantly inhibited algal photosynthesis in light-exposed periphyton (p always < 0.0003), with...

H2O2 intensifies CN(-)-induced apoptosis in pea leaves.

H2O2 intensifies CN(-)-induced apoptosis in stoma guard cells and to lesser degree in basic epidermal cells in peels of the lower epidermis isolated from pea leaves. The maximum effect of H2O2 on guard cells was observed at 10(-4) M. By switching on non-cyclic electron transfer in chloroplasts menadione and methyl viologen intensified H2O2 generation in the light, but prevented the CN--induced apoptosis in guard cells. The light stimulation of CN- effect on guard cell apoptosis cannot be caused by disturbance of the ribulose-1,5-bisphosphate carboxylase function and associated OH* generation in chloroplasts with participation of free transition metals in the Fenton or Haber-Weiss type reactions as well as with participation of the FeS clusters of the electron acceptor side of Photosystem I. Menadione and methyl viologen did not suppress the CN(-)-induced apoptosis in epidermal cells that, unlike guard cells, contain mitochondria only, but not chloroplasts. Quinacrine and diphenylene iodonium, inhibitors of NAD(P)H oxidase of cell plasma membrane, had no effect on the respiration and photosynthetic O2 evolution by leaf slices, but prevented the CN(-)-induced guard cell death. The data suggest that NAD(P)H oxidase of guard cell plasma membrane is a source of reactive oxygen species (ROS) needed for execution of CN(-)-induced programmed cell death. Chloroplasts and mitochondria were inefficient as ROS sources in the programmed death of guard cells. When ROS generation is insufficient, exogenous H2O2 exhibits a stimulating effect on programmed cell death. H2O2 decreased the inhibitory effects of DCMU and DNP-INT on the CN(-)-induced apoptosis of guard cells. Quinacrine, DCMU, and DNP-INT had no effect on CN(-)-induced death of epidermal cells.

In intact leaves, the maximum fluorescence level ( FM) is independent of the redox state of the plastoquinone pool: A DCMU-inhibition study

The effects of DCMU (3-(3′,4′-dichlorophenyl)-1,1-dimethylurea) on the fluorescence induction transient (OJIP) in higher plants were re-investigated. We found that the initial ( F 0) and maximum ( F M) fluorescence levels of DCMU-treated leaves do not change relative to controls when the treatment is done in complete darkness and DCMU is allowed to diffuse slowly into the leaves either by submersion or by application via the stem. Simultaneous 820 nm transmission measurements (a measure of electron flow through Photosystem I) showed that in the DCMU-treated samples, the plastoquinone pool remained oxidized during the light pulses whereas in uninhibited leaves, the F M level coincided with a fully reduced electron transport chain. The identical F M values with and without DCMU indicate that in intact leaves, the F M value is independent of the redox state of the plastoquinone pool. We also show that (i) the generally observed F 0 increase is probably due to the presence of (even very weak) light during the DCMU treatment, (ii) vacuum infiltration of leaf discs leads to a drastic decrease of the fluorescence yield, and in DCMU-treated samples, the F M decreases to the I-level of their control (leaves vacuum infiltrated with 1% ethanol), (iii) and in thylakoid membranes, the addition of DCMU lowers the F M relative to that of a control sample.

除草剤および無機水銀の藍藻（Ｓｐｉｒｕｌｉｎａ ｐｌａｔｅｎｓｉｓ）への影響評価に対する遅延蛍光時間分解計測技術の応用

We have investigated the influences of herbicides and mercury on the blue-green alga Spirulina platensis by analyzing the long-term behavior of S.platensis delayed fluorescence(DF). S.platensis DF was recorded as a decay curve, and change in the DF intensity with time was analyzed. The decay curve was found to be composed of two components. The first component peaked just after excitation (P1). The second component peaked at approximately 30 seconds after excitation (P2). When the alga was exposed to toxicants, the intensities of P1 (CP1) and P2 (CP2), and the time of P2 appearance (TP2) varied notably. The tested herbicides, diuron (DCMU) and simazine (CAT), increased CP1, and decreased CP2. Mercury increased TP2 and CP1, and decreased CP2. During the measurements at 30 min after the exposure, the effect of DCMU was detectable at a minimum of 0.1μg·l-1, that of CAT at 0.25μg·l-1 and that of mercury at 20μg·l-1.Furthermore, when the alga was exposed to both herbicides and mercury, the influences were combined. This study indicates that the analysis of the long-term behavior of DF of an alga may be useful in the rapid detection of toxicants in contaminated waters.

Effects of starvation, ammonium concentration, and photosynthesis on the UV-dependent accumulation of mycosporine-like amino acids (MAAs) in the coral Stylophora pistillata

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 295:135-156 (2005) - doi:10.3354/meps295135 Effects of starvation, ammonium concentration, and photosynthesis on the UV-dependent accumulation of mycosporine-like amino acids (MAAs) in the coral Stylophora pistillata J. Malcolm Shick1,*, Christine Ferrier-Pagès2, Renaud Grover2, Denis Allemand2,3 1School of Marine Sciences, University of Maine, 5751 Murray Hall, Orono, Maine 04469-5751, USA2Centre Scientifique de Monaco, Avenue Saint-Martin, 98000 Monaco, Principality of Monaco3UMR 1112 INRA-UNSA, University of Nice-Sophia Antipolis, Parc Valrose, BP 71, 06108 Nice Cedex 2, France *Email: shick@maine.edu ABSTRACT: This study addresses several unresolved questions regarding the biosynthesis, metabolism, regulation, and diversity of MAAs in zooxanthellate scleractinian corals. Starved colonies of Stylophora pistillata accumulated the same concentrations of mycosporine-like amino acids (MAAs) as fed corals after 28 d of exposure to photosynthetically available radiation (PAR) and ultraviolet radiation (UVR), suggesting that dietary MAAs are of little quantitative importance in this phototrophic symbiosis. Starved corals continued to accumulate MAAs and conserved them disproportionally compared with declining protein and chlorophyll a, indicating the priority placed on maintaining this UV-sunscreen defense. In 2 different colonies (SP1 and SP2) exposed to enriched (10 µM) ammonium and PAR + UVR during starvation, the final concentrations of MAAs and chlorophyll a were identical. However, under ambient ammonium (<0.4 µM), SP2 produced MAAs at the expense of chlorophyll a, whereas SP1 maintained chlorophyll a levels but synthesized less MAA. Ammonium consistently affected only the accumulation of primary, Symbiodinium-MAAs (mycosporine glycine, shinorine, porphyra-334, and mycosporine-2 glycine) and not secondary MAAs derived from the former, probably in the host’s tissues. Mycosporine-2 glycine and palythine (a secondary MAA) were synthesized by SP1 but not SP2, suggesting (1) genotypic differences between the zooxanthellae in SP1 and SP2, and (2) a biosynthetic relationship between these 2 MAAs that we proposed previously. Exposure to UVR alone did not support large-scale biosynthesis of MAAs in S. pistillata, and accumulation of the full suite of MAAs required PAR + UVR; together with an inhibitory effect of DCMU, this indicates that photosynthesis is required for the UV-stimulated, de novo biosynthesis of MAAs. Cultures of zooxanthellae isolated from S. pistillata and exposed to PAR + UVR showed increased levels of shinorine, and some production of mycosporine-glycine. Host extract had no qualitative effect on the MAAs produced by these zooxanthellae. KEY WORDS: Mycosporine-like amino acids (MAAs) · Coral · Zooxanthellae · Ultraviolet radiation (UVR) · Dissolved ammonium · Photosynthesis Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 295. Online publication date: June 23, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Identification and regulation of high light-induced genes in Chlamydomonas reinhardtii.

We have used restriction fragment differential display for isolating genes of the unicellular green alga Chlamydomonas reinhardtii that exhibit elevated expression on exposure of cells to high light. Some of the high light-activated genes were also controlled by CO2 concentration. Genes requiring both elevated light and low CO2 levels for activation encoded both novel polypeptides and those that function in concentrating inorganic carbon (extracellular carbonic anhydrase, low CO2-induced protein, ABC transporter of the MRP subfamily). All the genes in this category were shown to be under the control of Cia5, a protein that regulates the responses of C. reinhardtii to low-CO2 conditions. Genes specifically activated by high light, even under high-CO2 conditions, encoded a 30 kDa chloroplast membrane protein, a serine hydroxymethyltransferase, a nuclease, and two proteins of unknown function. Experiments using DCMU, an inhibitor of photosynthetic electron transport, and mutants devoid of either photosystem I or photosystem II activity, showed aberrant expression of all the genes regulated by both CO2 and high light, suggesting that redox plays a role in controlling their expression. In contrast, there was little effect of DCMU or lesions that block photosynthetic electron transport on the activity of genes that were specifically controlled by high light.

Effects of salinity stress on photosystem II function in cyanobacterial Spirulina platensis cells.

The changes in PSII photochemistry in Spirulina platensis cells exposed to salinity stress (0-0.8 M NaCl) for 12 h were studied. Salinity stress induced a decrease in oxygen evolution activity, which correlated with the decrease in the quantum yield of PSII electron transport (PhiPSII). Phycocyanin content decreased significantly while chlorophyll content remained unchanged in salt-stressed cells. Salinity stress induced an increase in non-photochemical quenching (qN) and a decrease in photochemical quenching (qP). Analyses of the polyphasic fluorescence transients (OJIP) showed that with the increase in salt concentration, the fluorescence yield at the phases J, I and P declined sharply and the transient almost levelled off at salt concentration of 0.8 M NaCl. The effects of DCMU on the polyphasic rise of fluorescence transients decreased significantly. Salinity stress resulted in a decrease in the efficiency of electron transfer from QA- to QB. The slope at the origin of the relative variable fluorescence curves (dV/dto) and the relative variable fluorescence at phase J (VJ) increased in the absence of DCMU, but decreased in the presence of DCMU. The shape of the relative variable fluorescence transients in salt-stressed cells was comparable to that of the control cells incubated with DCMU. The results in this study suggest that salt stress inhibited the electron transport at both donor and acceptor sides of PSII, resulted in damage to phycobilisome and shifted the distribution of excitation energy in favour of PSI.

PHOTOAUTOTROPHIC GROWTH OF A RECENTLY ISOLATED N2‐FIXING MARINE NON‐HETEROCYSTOUS FILAMENTOUS CYANOBACTERIUM, SYMPLOCA SP. (CYANOBACTERIA)

Photoautotrophic growth of a marine non‐heterocystous filamentous cyanobacterium, Symploca sp. strain S84, was examined under nitrate‐assimilating and N2‐fixing conditions. Under continuous light, photon flux density of 55 μmol photons·m−2 ·s−1 was at a saturating level for growth, and light did not inhibit the growth rate under N2‐fixing conditions even when the photon flux density was doubled (110 μmol photons·m−2 ·s−1). Doubling times of the N2‐fixing cultures under 55 and 110 μmol photons·m−2 ·s−1 were about 30 and 31 h, respectively. Under 110 μmol photons·m−2 ·s−1 during the light phase of an alternating 12:12‐h light:dark (L:D) cycle, the doubling time of the N2‐fixing culture was also about 30 h. When grown diazotrophically under a 12:12‐h L:D regime, C2H2 reduction activity was observed mainly during darkness. In continuous light, relatively large cyclic fluctuations in C2H2 reduction were observed during growth. The short‐term (&lt;4 h) effect of 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU; 5 μM) indicated that C2H2 reduction activity was not influenced by photosynthetic O2 evolution. Long‐term (24 h) effects of DCMU indicated that photosynthesis and C2H2 reduction activity occur simultaneously. These results indicate that strain S84 grows well under diazotrophic conditions when saturating light is supplied either continuously or under a 12:12‐h L:D diel light regime.

Microscopic image instrumentation for chlorophyll a fluorescence from in situ Closterium navicula

We have developed a new image instrumentation system of chlorophyll a fluorescence with an optical microscope, capable of in situ analysis at the cellular level. With this system we analyzed the effects of DCMU on photosynthetic activity of green alga Closterium moniliferium. The fluorescence parameters qP, qN and NPQ, and Yield, which represent photochemical quenching, non-photochemical quenching, and photosystem II photochemical efficiency, were calculated from images taken under actinic and saturating illumination. Chlorophyll fluorescence (F and F?m) intensity of alga whose photosystem inhibited by DCMU was higher than healthy alga. The qN and NPQ was reduced greatly throughout injured alga. These results indicated the inhibition by DCMU eliminated non-photochemical quenching. In contrast, qP and Yield were not very different from healthy alga. These results showed that photochemical quenching occurred ordinarily, but Yield could not be applied as an indicator of CO2 fixation rate in such a case. It is necessary to choose the experimental condition to determine the validity of Yield.

Restoration of the high-potential form of cytochrome b559 of photosystem II occurs via a two-step mechanism under illumination in the presence of manganese ions

Spinach photosystem II membranes that had been depleted of the Mn cluster contained four forms of cytochrome (Cyt) b559, namely, high-potential (HP), HP′, intermediate-potential (IP) and low-potential (LP) forms that exhibited the redox potentials of +400, +310, +170 and +35 mV, respectively, in potentiometric titration. When the membranes were illuminated with flashing light in the presence of 0.1 mM Mn 2+, the IP form was converted to the HP′ form by two flashes and then the HP′ form was converted to the HP form by an additional flash. The quantum efficiency of the first conversion appeared to be quite high since the conversion was almost complete after two flashes. By contrast, the second conversion proceeded with low quantum efficiency and 40 flashes were required for completion. The effects of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) suggested that the first conversion did not require electron transfer from Q A to Q B while the second conversion had an absolute requirement for it. It was also suggested that the first conversion involved the reduction of the heme of Cyt b559, probably by Q A −, and we propose that direct reduction by Q A − induces a shift in the redox potential of the heme. The second conversion was also accompanied by the reduction of heme but it appeared that this conversion did not necessarily involve the reduction. The effects of DCMU on the reduction of heme suggested that the heme became reducible by Q B − after the first conversion had been completed. This observation implies that the efficiency of electron transfer from Q A to Q B increased upon the conversion of the IP form to the HP′ form, and we propose that restoration of the high-potential forms of Cyt b559 itself acts to make the acceptor side of photosystem II functional.