Suleyman I. Allakhverdiev

Oxidative stress inhibits the repair of photodamage to the photosynthetic machinery

Absorption of excess light energy by the photosynthetic machinery results in the generation of reactive oxygen species (ROS), such as H2O2. We investigated the effects in vivo of ROS to clarify the nature of the damage caused by such excess light energy to the photosynthetic machinery in the cyanobacterium Synechocystis sp. PCC 6803. Treatments of cyanobacterial cells that supposedly increased intracellular concentrations of ROS apparently stimulated the photodamage to photosystem II by inhibiting the repair of the damage to photosystem II and not by accelerating the photodamage directly. This conclusion was confirmed by the effects of the mutation of genes for H2O2‐scavenging enzymes on the recovery of photosystem II. Pulse labeling experiments revealed that ROS inhibited the synthesis of proteins de novo. In particular, ROS inhibited synthesis of the D1 protein, a component of the reaction center of photosystem II. Northern and western blot analyses suggested that ROS might influence the outcome of photodamage primarily via inhibition of translation of the psbA gene, which encodes the precursor to D1 protein.

Salt Stress Inhibits the Repair of Photodamaged Photosystem II by Suppressing the Transcription and Translation of psbA Genes in Synechocystis

Light stress and salt stress are major environmental factors that limit the efficiency of photosynthesis. However, we have found that the effects of light and salt stress on photosystem II (PSII) in the cyanobacterium Synechocystis sp. PCC 6803 are completely different. Strong light induced photodamage to PSII, whereas salt stress inhibited the repair of the photodamaged PSII and did not accelerate damage to PSII directly. The combination of light and salt stress appeared to inactivate PSII very rapidly as a consequence of their synergistic effects. Radioactive labeling of cells revealed that salt stress inhibited the synthesis of proteins de novo and, in particular, the synthesis of the D1 protein. Northern- and western-blotting analyses demonstrated that salt stress inhibited the transcription and the translation of psbA genes, which encode D1 protein. DNA microarray analysis indicated that the light-induced expression of various genes was suppressed by salt stress. Thus, our results suggest that salt stress inhibits the repair of PSII via suppression of the activities of the transcriptional and translational machinery.

Frequently asked questions about in vivo chlorophyll fluorescence: practical issues

The aim of this educational review is to provide practical information on the hardware, methodology, and the hands on application of chlorophyll (Chl) a fluorescence technology. We present the paper in a question and answer format like frequently asked questions. Although nearly all information on the application of Chl a fluorescence can be found in the literature, it is not always easily accessible. This paper is primarily aimed at scientists who have some experience with the application of Chl a fluorescence but are still in the process of discovering what it all means and how it can be used. Topics discussed are (among other things) the kind of information that can be obtained using different fluorescence techniques, the interpretation of Chl a fluorescence signals, specific applications of these techniques, and practical advice on different subjects, such as on the length of dark adaptation before measurement of the Chl a fluorescence transient. The paper also provides the physiological background for some of the applied procedures. It also serves as a source of reference for experienced scientists.

Three types of Photosystem II photoinactivation : I. Damaging processes on the acceptor side.

Oxygen evolving photosystem II particles were exposed to 100 and 250 W m−2 white light at 20°C under aerobic, anaerobic and strongly reducing (presence of dithionite) conditions. Three types of photoinactivation processes with different kinetics could be distinguished: (1) The fast process which occurs under strongly reducing (t1/2≅1–3 min) and anaerobic conditions (t1/2≅4–12 min). (2) The slow process (t1/2≅15–40 min) and (3) the very slow process (t1/2>100 min), both of which occur under all three sets of conditions.The fast process results in a parallel decline of variable fluorescence (Fv) and of Hill reaction rate, accompanied by an antiparallel increase of constant fluorescence (Fo). We assume that trapping of QA in a negatively charged stable state, (QA−)stab, is responsible for the effects observed.The slow process is characterized by a decline of maximal fluorescence (Fm). In presence of oxygen this decline is due to the well known disappearance of Fv which proceeds in parallel with the inhibition of the Hill reaction; Fo remains essentially constant. Under anaerobic and reducing conditions the decline of Fm represents the disappearance of the increment in Fo generated by the fast process. We assume that the slow process consists in neutralization of the negative charge in the domain of QA in a manner that renders QA non-functional. The charge separation in the RC is still possible, but energy of excitation becomes thermally dissipated.The very slow photoinactivation process is linked to loss of charge separation ability of the PS II RC and will be analyzed in a forthcoming paper.

Protein synthesis is the primary target of reactive oxygen species in the photoinhibition of photosystem II

Photoinhibition of photosystem II (PSII) occurs when the rate of photodamage to PSII exceeds the rate of the repair of photodamaged PSII. Recent examination of photoinhibition by separate determinations of photodamage and repair has revealed that the rate of photodamage to PSII is directly proportional to the intensity of incident light and that the repair of PSII is particularly sensitive to the inactivation by reactive oxygen species (ROS). The ROS-induced inactivation of repair is attributable to the suppression of the synthesis de novo of proteins, such as the D1 protein, that are required for the repair of PSII at the level of translational elongation. Furthermore, molecular analysis has revealed that the ROS-induced suppression of protein synthesis is associated with the specific inactivation of elongation factor G via the formation of an intramolecular disulfide bond. Impairment of various mechanisms that protect PSII against photoinhibition, including photorespiration, thermal dissipation of excitation energy, and the cyclic transport of electrons, decreases the rate of repair of PSII via the suppression of protein synthesis. In this review, we present a newly established model of the mechanism and the physiological significance of repair in the regulation of the photoinhibition of PSII.

Manganese Compounds as Water-Oxidizing Catalysts: From the Natural Water-Oxidizing Complex to Nanosized Manganese Oxide Structures

All cyanobacteria, algae, and plants use a similar water-oxidizing catalyst for water oxidation. This catalyst is housed in Photosystem II, a membrane-protein complex that functions as a light-driven water oxidase in oxygenic photosynthesis. Water oxidation is also an important reaction in artificial photosynthesis because it has the potential to provide cheap electrons from water for hydrogen production or for the reduction of carbon dioxide on an industrial scale. The water-oxidizing complex of Photosystem II is a Mn-Ca cluster that oxidizes water with a low overpotential and high turnover frequency number of up to 25-90 molecules of O2 released per second. In this Review, we discuss the atomic structure of the Mn-Ca cluster of the Photosystem II water-oxidizing complex from the viewpoint that the underlying mechanism can be informative when designing artificial water-oxidizing catalysts. This is followed by consideration of functional Mn-based model complexes for water oxidation and the issue of Mn complexes decomposing to Mn oxide. We then provide a detailed assessment of the chemistry of Mn oxides by considering how their bulk and nanoscale properties contribute to their effectiveness as water-oxidizing catalysts.

Hierarchical electrospun nanofibers for energy harvesting, production and environmental remediation

As the demand for energy is rapidly growing worldwide ahead of energy supply, there is an impulse need to develop alternative energy-harvesting technologies to sustain economic growth. Due to their unique optical and electrical properties, one-dimensional (1D) electrospun nanostructured materials are attractive for the construction of active energy harvesting devices such as photovoltaics, photocatalysts, hydrogen energy generators, and fuel cells. 1D nanostructures produced from electrospinning possess high chemical reactivity, high surface area, low density, as well as improved light absorption and dye adsorption compared to their bulk counterparts. So, research has been focused on the synthesis of 1D nanostructured fibers made from metal oxides, composites, dopants and surface modification. Furthermore, fine tuning these NFs has facilitated fast charge transfer and efficient charge separation for improved light absorption in photocatalytic and photovoltaic properties. The recent trend in exploring these electrospun nanostructures has been promising in-terms of reducing costs and enhancing the efficiency compared to conventional materials. This review article presents the synthesis of 1D nanostructured fibers made via electrospinning and their applications in photovoltaics, photocatalysis, hydrogen energy harvesting and fuel cells. The current challenges and future perspectives for electrospun nanomaterials are also reviewed.

Inhibition of the repair of Photosystem II by oxidative stress in cyanobacteria

The activity of Photosystem II (PS II) is severely restricted by a variety of environmental factors and, under environmental stress, is determined by the balance between the rate of damage to PS II and the rate of the repair of damaged PS II. The effects of oxidative stress on damage and repair can be examined separately, and it appears that, while light can damage PS II directly, oxidative stress acts primarily by inhibiting the repair of PS II. Studies in cyanobacteria have demonstrated that oxidative stress suppresses the de novo synthesis of proteins, in particular, the D1 protein, which is required for the repair of PS II.

Nano-sized manganese oxides as biomimetic catalysts for water oxidation in artificial photosynthesis: a review

There has been a tremendous surge in research on the synthesis of various metal compounds aimed at simulating the water-oxidizing complex (WOC) of photosystem II (PSII). This is crucial because the water oxidation half reaction is overwhelmingly rate-limiting and needs high over-voltage (approx. 1 V), which results in low conversion efficiencies when working at current densities required for hydrogen production via water splitting. Particular attention has been given to the manganese compounds not only because manganese has been used by nature to oxidize water but also because manganese is cheap and environmentally friendly. The manganese–calcium cluster in PSII has a dimension of about approximately 0.5 nm. Thus, nano-sized manganese compounds might be good structural and functional models for the cluster. As in the nanometre-size of the synthetic models, most of the active sites are at the surface, these compounds could be more efficient catalysts than micrometre (or bigger) particles. In this paper, we focus on nano-sized manganese oxides as functional and structural models of the WOC of PSII for hydrogen production via water splitting and review nano-sized manganese oxides used in water oxidation by some research groups.

Reactive oxygen species: Re-evaluation of generation, monitoring and role in stress-signaling in phototrophic organisms

This review provides an overview about recent developments and current knowledge about monitoring, generation and the functional role of reactive oxygen species (ROS) - H2O2, HO2, HO, OH(-), (1)O2 and O2(-) - in both oxidative degradation and signal transduction in photosynthetic organisms including microscopic techniques for ROS detection and controlled generation. Reaction schemes elucidating formation, decay and signaling of ROS in cyanobacteria as well as from chloroplasts to the nuclear genome in eukaryotes during exposure of oxygen-evolving photosynthetic organisms to oxidative stress are discussed that target the rapidly growing field of regulatory effects of ROS on nuclear gene expression.