Evidence for the role of the oxygen-evolving manganese complex in photoinhibition of Photosystem II

AbstractPhotoinhibition of PSII occurs at the same quantum efficiency from very low to very high light, which raises a question about how important is the rate of photosynthetic electron transfer in photoinhibition. We modulated electron transfer rate and light intensity independently of each other in lincomycin-treated pea leaves and in isolated thylakoids, in order to elucidate the specific effects of light and PSII electron transport on photoinhibition. Major changes in the rate of electron transport caused only small changes in the rate of photoinhibition, suggesting the existence of a significant photoinhibitory pathway that contains an electron-transfer-independent phase. We compared the action spectrum of photoinhibition with absorption spectra of PSII components that could function as photoreceptors of the electron-transfer-independent phase of photoinhibition and found that the absorption spectra of Mn(III) and Mn(IV) compounds resemble the action spectrum of photoinhibition, showing a steep decrease from UV-C to blue light and a low visible-light tail. Our results show that the release of a Mn ion to the thylakoid lumen is the earliest detectable step of both UV- and visible-light-induced photoinhibition. After Mn release from the oxygen-evolving complex, oxidative damage to the PSII reaction center occurs because the Mn-depleted oxygen-evolving complex cannot reduce P680+ normally

Utilization of microalgae in industrial symbiosis, focus on Finland

Levien teollinen ja laajamittainen viljely Suomessa on haastavaa. Vuodenaikojen ja sään vaihtelu suosii sisätiloihin asennettavien suljettujen fotobioreaktorien valintaa levänkasvatusmenetelmäksi. Koska auringon valo ei riitä ympärivuotiseen kasvatukseen Suomessa, tarvitaan myös keinovaloa. Raportissa tarkastellaan kasvatusmenetelmiä, biomassan keruuta ja vedenpoistoa. Leviä voidaan kasvattaa jätevesissä, jolloin ravinteiden kierrätys ja biomassan tuotto yhdistyvät luontevasti jätevesien puhdistukseen. Kaupunkien, teollisuuden ja maatalouden jätevesiä voidaan hyödyntää. Tärkein tekijä jätevesissä on niiden ravinnepitoisuus. Yksi mahdollinen paikka levän kasvatukseen on kiertovesikalankasvatuslaitos, jossa levää mahdollisesti voidaan käyttää ammoniumtypen poistamiseen. Levänkasvatus voidaan myös mahdollisesti yhdistää kasvihuoneviljelyyn, jolloin viljelyrivien väliin kohdistuva ja hukkaan menevä valoenergia saadaan hyötykäyttöön. Muita mahdollisia levänkasvatuksen sovelluskohteita voivat olla kaatopaikkakompleksien yhteydessä sijaitsevat maanalaiset tunnelit, joissa vallitsee tasainen lämpötila ja joissa on jätevettä saatavilla. Tosin tunneleissa keinovalo on ainoa mahdollinen valonlähde. Levänkasvatuksen yhdistäminen olemassa olevaan teollisuuteen tarjoaa mahdollisuuden tuottaa jäteveden puhdistamisen yhteydessä leväbiomassasta biojalostamotyyppisesti useita tuotteita kuten lipidejä, biokaasua ja lannoitteita. Sellu- ja paperiteollisuus tuottaa jätevesiensä lisäksi hiilidioksidia ja lämpöä, joista molempia tarvitaan levien kasvattamiseen. Teollisuusjätevesissä ei tosin ole välttämättä riittävästi ravinteita, minkä vuoksi voi olla tarpeen lisätä osa ravinteista tai yhdistää jätevesiin yhdyskuntajätevesiä. Biodiesel, bioetanoli ja biokaasu ovat yleisimmin esitetyt levistä saatavat energiatuotteet, mutta leväbiomassasta voidaan jalostaa energiatuotteiden lisäksi myös muita orgaanisia molekyylejä. Lipi-dit, pigmentit, proteiinit ja hiilihydraatit ovat mahdollisia korkean lisäarvon tuotteita. Myös kalanrehua ja kasviravinteita sisältäviä lannoitteita voidaan tuottaa. Korkean lisäarvon tuotteiden valmistaminen vaatii yleensä levien geneettistä modifioimista. Yhteenvetona voidaan todeta, että levän teollinen kasvatus Suomessa on teknisesti mahdollista kun se yhdistetään jätevesien hyötykäyttöön ja kun käytetään olemassa olevaa infrastruktuuria. Sisätilat ja keinovalo ovat tarpeelliset kaikissa tarkastelluissa vaihtoehdoissa. Levän kasvatuksen taloudellinen kannattavuus jää ratkaistavaksi.Large-scale cultivation of algae in Finnish conditions is challenging. Seasonal variation in weather conditions in terms of light and temperature invites to select closed photobioreactors for algal culti-vation. Furthermore, the photobioreactors need to be placed indoors. Artificial illumination is need-ed because sunlight is not sufficient for year-round cultivation in Finland. Methods of cultivation, harvesting and dewatering will be discussed. Algae can be cultivated in wastewaters for purification, for uptake and recycling of nutrients, and for production of algal biomass. Wastewaters from municipalities, industries and agriculture can potentially be utilized. The most important factor in wastewaters is their nutrient content. One possible place for growing algae is in recirculating aquaculture, where algae can potentially be used to remove ammonium. Cultivation of algae in greenhouses in combination with plants might allow utilization of light energy that is currently wasted at the corridors. Algae might also be applied in underground tunnels (for example in landfills) that have a constant temperature and in which wastewater is available. However, artificial illumination is the only available light source. Combination of algal cultivation with existing industries offers the possibility to combine wastewater purification with production of lipids, biogas and fertilizers from the algal biomass, fol-lowing the biorefinery concept. Pulp and paper industry produce excess heat, carbon dioxide and wastewater, all of which can be utilized for growing algae. Unfortunately, industrial wastewaters may not have enough nutrients, and therefore it might be necessary to add some of the nutrients or to combine industrial and municipal wastewaters. Biodiesel, bioethanol and biogas are the most commonly discussed energy products but several non-energy products can also be obtained from algal biomass. Microalgal lipids, pigments, proteins and carbohydrates are potential high value non-energy products. Fish feed and fertilizers may also be produced. Production of high-value compounds usually requires genetic modification of the algae. In conclusion, industrial algal cultivation in Finland is technically possible when combined with the use of wastewaters and with the exploitation of existing infrastructure. Use of indoor spaces and artificial light is necessary for all options. The economic feasibility of algal cultivation remains to be elucidated.201

Photosynthetic sea slugs induce protective changes to the light reactions of the chloroplasts they steal from algae

Sacoglossan sea slugs are able to maintain functional chloroplasts inside their own cells, and mechanisms that allow preservation of the chloroplasts are unknown. We found that the slug Elysia timida induces changes to the photosynthetic light reactions of the chloroplasts it steals from the alga Acetabularia acetabulum. Working with a large continuous laboratory culture of both the slugs (>500 individuals) and their prey algae, we show that the plastoquinone pool of slug chloroplasts remains oxidized, which can suppress reactive oxygen species formation. Slug chloroplasts also rapidly build up a strong proton-motive force upon a dark-to-light transition, which helps them to rapidly switch on photoprotective non-photochemical quenching of excitation energy. Finally, our results suggest that chloroplasts inside E. timida rely on oxygen-dependent electron sinks during rapid changes in light intensity. These photoprotective mechanisms are expected to contribute to the long-term functionality of the chloroplasts inside the slugs. </p

Singlet oxygen production by photosystem II is caused by misses of the oxygen evolving complex

Singlet oxygen (O-1(2)) is a harmful species that functions also as a signaling molecule. In chloroplasts, O-1(2) is produced via charge recombination reactions in photosystem II, but which recombination pathway(s) produce triplet Chl and O-1(2) remains open. Furthermore, the role of O-1(2) in photoinhibition is not clear. We compared temperature dependences of O-1(2) production, photoinhibition, and recombination pathways. O-1(2) production by pumpkin thylakoids increased from -2 to +35 degrees C, ruling out recombination of the primary charge pair as a main contributor. S(2)Q(A)(-) or S(2)Q(B)(-) recombination pathways, in turn, had too steep temperature dependences. Instead, the temperature dependence of O-1(2) production matched that of misses (failures of the oxygen (O-2) evolving complex to advance an S-state). Photoinhibition in vitro and in vivo (also in Synechocystis), and in the presence or absence of O-2, had the same temperature dependence, but ultraviolet (UV)-radiation-caused photoinhibition showed a weaker temperature response. We suggest that the miss-associated recombination of P(680)(+)Q(A)(-) is the main producer of O-1(2). Our results indicate three parallel photoinhibition mechanisms. The manganese mechanism dominates in UV radiation but also functions in white light. Mechanisms that depend on light absorption by Chls, having O-1(2) or long-lived P-680(+) as damaging agents, dominate in red light

Light-induced damage to photosystem II at a very low temperature (195 K) depends on singlet oxygen

Photosynthetic organisms, like evergreen plants, may encounter strong light at low temperatures. Light, despite being the energy source of photosynthesis, irreversibly damages photosystem II (PSII). We illuminated plant thylakoid membranes and intact cyanobacterial cells at -78.5? and assayed PSII activity with oxygen evolution or chlorophyll fluorescence, after thawing the sample. Both UV radiation and visible light damaged PSII of pumpkin (Cucurbita maxima) thylakoids at -78.5?, but visible-light-induced photoinhibition at -78.5?, unlike at +20?, proceeded only in the presence of oxygen. A strong magnetic field that would decrease triplet chlorophyll formation by recombination of the primary radical pair slowed down photoinhibition at -78.5?, suggesting that singlet oxygen produced via recombination of the primary pair is a major contributor to photoinhibition at -78.5?. However, a magnetic field did not affect singlet oxygen production at +25?. Thylakoids of winter leaves of an evergreen plant, Bergenia, were less susceptible to photoinhibition both at -78.5? and +20?, contained high amounts of carotenoids and produced little singlet oxygen (measured at +20?), compared to thylakoids of summer leaves. In contrast, high carotenoid amount and low singlet oxygen yield did not protect a Synechocystis mutant from photoinhibition at -78.5?. Thylakoids isolated from Arabidopsis thaliana grown under high light, which reduces PSII antenna size, were more resistant than control plants against photoinhibition at -78.5? but not at +20?, although carotenoid amounts were similar. The results indicate that visible-light-induced photoinhibition at -78.5? depends on singlet oxygen, whereas photoinhibition at +20? is largely independent of oxygen

Plastoquinone pool redox state and control of state transitions in Chlamydomonas reinhardtii in darkness and under illumination

Movement of LHCII between two photosystems has been assumed to be similarly controlled by the redox state of the plastoquinone pool (PQ-pool) in plants and green algae. Here we show that the redox state of the PQ-pool of Chlamydomonas reinhardtii can be determined with HPLC and use this method to compare the light state in C. reinhardtii with the PQ-pool redox state in a number of conditions. The PQ-pool was at least moderately reduced under illumination with all tested types of visible light and oxidation was achieved only with aerobic dark treatment or with far-red light. Although dark incubations and white light forms with spectral distribution favoring one photosystem affected the redox state of PQ-pool differently, they induced similar Stt7-dependent state transitions. Thus, under illumination the dynamics of the PQ-pool and its connection with light state appears more complicated in C. reinhardtii than in plants. We suggest this to stem from the larger number of LHC-units and from less different absorption profiles of the photosystems in C. reinhardtii than in plants. The data demonstrate that the two different control mechanisms required to fulfill the dual function of state transitions in C. reinhardtii in photoprotection and in balancing light utilization are activated via different means

Testing the Potential of Regulatory Sigma Factor Mutants for Wastewater Purification or Bioreactor Run in High Light

It is shown that a freshly inoculated culture of the model cyanobacterium Synechocystis sp. PCC 6803 consumed almost all phosphate and 50% of nitrate within 6 days from the nutrient-rich BG-11 growth medium, indicating potential of cyanobacteria to purify wastewaters. Synechocystis sp. PCC 6803 control strain also collected nutrients efficiently from a landfill leachate wastewater KA2 (5.9-6.9 mM ammonium and 0.073-0.077 mM phosphate). Wastewaters might induce oxidative stress to microalgae, which prompted us to test growth of sigma factor inactivation strains, as Delta sigBCE and Delta sigCDE strains show superior growth in chemically induced oxidative stress. All cyanobacterial strains, including a stress-sensitive strain Delta sigBCDE, grew well in KA2 for four days, indicating that KA2 did not cause immediate oxidative stress. Completely arrested growth and bleaching of Delta sigBCDE cells after one week in KA2 wastewater point to the importance of group 2 sigma factor-mediated changes in gene expression during wastewater treatment. The growth of Delta sigBCD was arrested early in un-buffered and Hepes buffered (pH 7.5) KA2. In Delta sigBCD, all phosphate transporter genes are upregulated in standard conditions, and Delta sigBCD cells showed growth defects in low-phosphate BG-11 medium. Delta sigBCD cells removed phosphate slower from KA2 than the control strain, but phosphate supplementation of KA2 did not improve growth of Delta sigBCD. The Delta sigBCE strain showed superior growth in a laboratory-scale bioreactor in bright light and removed phosphate even slightly more efficiently than the control strain if KA2 was Hepes buffered although Delta sigBCE grew slowly in un-buffered KA2 and in low-phosphate BG-11 medium. The results indicate that engineering expression of regulatory group 2 sigma factor(s) might be useful for practical applications

Model for the Fluorescence Induction Curve of Photoinhibited Thylakoids

AbstractThe fluorescence induction curve of photoinhibited thylakoids measured in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea was modeled using an extension of the model of Lavergne and Trissl (Biophys. J. 68:2474–2492), which takes into account the reversible exciton trapping by photosystem II (PSII) reaction centers and exciton exchange between PSII units. The model of Trissl and Lavergne was modified by assuming that PSII consists of photosynthetically active and photoinhibited (inactive in oxygen evolution) units and that the inactive PSII units can efficiently dissipate energy even if they still retain the capacity for the charge separation reaction. Comparison of theoretical and experimental fluorescence induction curves of thylakoids, which had been subjected to strong light in the presence of the uncoupler nigericin, suggests connectivity between the photoinhibited and active PSII units. The model predicts that photoinhibition lowers the yield of radical pair formation in the remaining active PSII centers. However, the kinetics of PSII inactivation in nigericin-treated thylakoids upon exposure to photoinhibitory light ranging from 185 to 2650μmol photons m−2 s−1 was strictly exponential. This may suggest that photoinhibition occurs independently of the primary electron transfer reactions of PSII or that increased production of harmful substances by photoinhibited PSII units compensates for the protection afforded by the quenching of excitation energy in photoinhibited centers

Group 2 Sigma Factors Are Central Regulators of Oxidative Stress Acclimation in Cyanobacteria

Regulatory σ factors of the RNA polymerase (RNAP) adjust gene expression according to environmental cues when the cyanobacterium Synechocystis sp. PCC 6803 acclimates to suboptimal conditions. Here we show central roles of the non-essential group 2 σ factors in oxidative stress responses. Cells missing all group 2 σ factors fail to acclimate to chemically induced singlet oxygen, superoxide or H2O2 stresses and lose pigments in high light. The SigB and SigD are the major σ factors in oxidative stress whereas SigC and SigE play only minor roles. The SigD factor is upregulated in high light, singlet oxygen and H2O2 stresses, and overproduction of the SigD factor in the ΔsigBCE strain leads to superior growth of ΔsigBCE cells in those stress conditions. Superoxide does not induce the production of the SigD factor but instead SigB and SigC factors are moderately induced. The SigB factor alone in ΔsigCDE can support almost as fast growth in superoxide stress as full complement of σ factors in the control strain but an overdose of the stationary-phase-related SigC factor causes growth arrest of ΔsigBDE in superoxide stress. Drastic decrease of the functional RNA polymerase limits the transcription capacity of the cells in H2O2 stress, which explains why cyanobacteria are sensitive to H2O2. Formation of RNAP-SigB and RNAP-SigD holoenzymes is highly enhanced in H2O2 stress and cells containing only SigB (ΔsigCDE) or SigD (ΔsigBCE) show superior growth in H2O2 stress

Inactivation of group 2 sigma factors upregulates production of transcription and translation machineries in the cyanobacterium Synechocystis sp PCC 6803

We show that the formation of the RNAP holoenzyme with the primary sigma factor SigA increases in the Delta sigBCDE strain of the cyanobacterium Synechocystis sp. PCC 6803 lacking all group 2 s factors. The high RNAP-SigA holoenzyme content directly induces transcription of a particular set of housekeeping genes, including ones encoding transcription and translation machineries. In accordance with upregulated transcripts, Delta sigBCDE contain more RNAPs and ribosomal subunits than the control strain. Extra RNAPs are fully active, and the RNA content of Delta sigBCDE cells is almost tripled compared to that in the control strain. Although Delta sigBCDE cells produce extra rRNAs and ribosomal proteins, functional extra ribosomes are not formed, and translation activity and protein content remained similar in Delta sigBCDE as in the control strain. The arrangement of the RNA polymerase core genes together with the ribosomal protein genes might play a role in the co-regulation of transcription and translation machineries. Sequence logos were constructed to compare promoters of those housekeeping genes that directly react to the RNAP-SigA holoenzyme content and those ones that do not. Cyanobacterial strains with engineered transcription and translation machineries might provide solutions for construction of highly efficient production platforms for biotechnical applications in the future