The cyanobacterium Synechococcus resists UV-B by exchanging photosystem II reaction-center D1 proteins

Integrated Physiologic and Proteomic Analyses Reveal the Molecular Mechanism of Navicula sp. in Response to Ultraviolet Irradiation Stress

With the continuous development of space station construction, space ecosystem research has attracted increasing attention. However, the complicated responses of different candidate plants and algae to radiation stress remain unclear. The present study, using integrated physiologic and proteomic analyses, was carried out to reveal the molecular mechanism of Navicula sp. in response to ultraviolet (UV) irradiation stress. Under 12~24 h of high-dose UV irradiation conditions, the contents of chlorophyll and soluble proteins in Navicula sp. cells were significantly higher than those in the control and 4~8 h of low-dose UV irradiation groups. The activity of catalase (CAT) increased with the extension of irradiation time, and the activity of superoxide dismutase (SOD) decreased first and then increased. Furthermore, differential volcano plot analysis of the proteomic data of Navicula sp. samples found only one protein with a significant difference. Differential protein GO analysis unveiled that UV irradiation can activate the antioxidant system of Navicula sp. and further impact photosynthesis by affecting the photoreaction and chlorophyll synthesis of Navicula sp. The most significant differences in KEGG pathway analysis were also associated with photosynthesis. The above results indicate that Navicula sp. has good UV radiation resistance ability by regulating its photosynthetic pigment content, photosynthetic activity, and antioxidant system, making it a potential candidate for the future development of space ecosystems.

UV radiation and temperature increase alter the PSII function and defense mechanisms in a bloom-forming cyanobacterium Microcystis aeruginosa

The aim was to determine the response of a bloom-forming Microcystis aeruginosa to climatic changes. Cultures of M. aeruginosa FACHB 905 were grown at two temperatures (25°C, 30°C) and exposed to high photosynthetically active radiation (PAR: 400-700 nm) alone or combined with UVR (PAR + UVR: 295-700 nm) for specified times. It was found that increased temperature enhanced M. aeruginosa sensitivity to both PAR and PAR + UVR as shown by reduced PSII quantum yields (Fv/Fm) in comparison with that at growth temperature (25°C), the presence of UVR significantly exacerbated the photoinhibition. M. aeruginosa cells grown at high temperature exhibited lower PSII repair rate (Krec) and sustained nonphotochemical quenching (NPQs) induction during the radiation exposure, particularly for PAR + UVR. Although high temperature alone or worked with UVR induced higher SOD and CAT activity and promoted the removal rate of PsbA, it seemed not enough to prevent the damage effect from them showing by the increased value of photoinactivation rate constant (Kpi). In addition, the energetic cost of microcystin synthesis at high temperature probably led to reduced materials and energy available for PsbA turnover, thus may partly account for the lower Krec and the declination of photosynthetic activity in cells following PAR and PAR + UVR exposure. Our findings suggest that increased temperature modulates the sensitivity of M. aeruginosa to UVR by affecting the PSII repair and defense capacity, thus influencing competitiveness and abundance in the future water environment.

Responding to light signals: a comprehensive update on photomorphogenesis in cyanobacteria

Cyanobacteria are ancestors of chloroplast and perform oxygen-evolving photosynthesis similar to higher plants and algae. However, an obligatory requirement of photons for their growth results in the exposure of cyanobacteria to varying light conditions. Therefore, the light environment could act as a signal to drive the developmental processes, in addition to photosynthesis, in cyanobacteria. These Gram-negative prokaryotes exhibit characteristic light-dependent developmental processes that maximize their fitness and resource utilization. The development occurring in response to radiance (photomorphogenesis) involves fine-tuning cellular physiology, morphology and metabolism. The best-studied example of cyanobacterial photomorphogenesis is chromatic acclimation (CA), which allows a selected number of cyanobacteria to tailor their light-harvesting antenna called phycobilisome (PBS). The tailoring of PBS under existing wavelengths and abundance of light gives an advantage to cyanobacteria over another photoautotroph. In this work, we will provide a comprehensive update on light-sensing, molecular signaling and signal cascades found in cyanobacteria. We also include recent developments made in other aspects of CA, such as mechanistic insights into changes in the size and shape of cells, filaments and carboxysomes.

Resilience and Mitigation Strategies of Cyanobacteria under Ultraviolet Radiation Stress

Ultraviolet radiation (UVR) tends to damage key cellular machinery. Cells may adapt by developing several defence mechanisms as a response to such damage; otherwise, their destiny is cell death. Since cyanobacteria are primary biotic components and also important biomass producers, any drastic effects caused by UVR may imbalance the entire ecosystem. Cyanobacteria are exposed to UVR in their natural habitats. This exposure can cause oxidative stress which affects cellular morphology and vital processes such as cell growth and differentiation, pigmentation, photosynthesis, nitrogen metabolism, and enzyme activity, as well as alterations in the native structure of biomolecules such as proteins and DNA. The high resilience and several mitigation strategies adopted by a cyanobacterial community in the face of UV stress are attributed to the activation of several photo/dark repair mechanisms, avoidance, scavenging, screening, antioxidant systems, and the biosynthesis of UV photoprotectants, such as mycosporine-like amino acids (MAAs), scytonemin (Scy), carotenoids, and polyamines. This knowledge can be used to develop new strategies for protecting other organisms from the harmful effects of UVR. The review critically reports the latest updates on various resilience and defence mechanisms employed by cyanobacteria to withstand UV-stressed environments. In addition, recent developments in the field of the molecular biology of UV-absorbing compounds such as mycosporine-like amino acids and scytonemin and the possible role of programmed cell death, signal perception, and transduction under UVR stress are discussed.

Multiple Photolyases Protect the Marine Cyanobacterium Synechococcus from Ultraviolet Radiation

Marine cyanobacteria depend on light for photosynthesis, restricting their growth to the photic zone. The upper part of this layer is exposed to strong UV radiation (UVR), a DNA mutagen that can harm these microorganisms. To thrive in UVR-rich waters, marine cyanobacteria employ photoprotection strategies that are still not well defined. Among these are photolyases, light-activated enzymes that repair DNA dimers generated by UVR. Our analysis of genomes of 81 strains of Synechococcus, Cyanobium, and Prochlorococcus isolated from the world’s oceans shows that they possess up to five genes encoding different members of the photolyase/cryptochrome family, including a photolyase with a novel domain arrangement encoded by either one or two separate genes. We disrupted the putative photolyase-encoding genes in Synechococcus sp. strain RS9916 and discovered that each gene contributes to the overall capacity of this organism to survive UVR. Additionally, each conferred increased survival after UVR exposure when transformed into Escherichia coli lacking its photolyase and SOS response. Our results provide the first evidence that this large set of photolyases endows Synechococcus with UVR resistance that is far superior to that of E. coli, but that, unlike for E. coli, these photolyases provide Synechococcus with the vast majority of its UVR tolerance.

Depletion of m-type thioredoxin impairs photosynthesis, carbon fixation, and oxidative stress in cyanobacteria

Thioredoxins (Trxs) are disulfide oxidoreductases that regulate many biological processes. The m-type thioredoxin (TrxA) is the only Trx present in all oxygenic photosynthetic organisms. Extensive biochemical and proteomic analyses have identified many TrxA target proteins in different photosynthetic organisms. However, the precise function of this essential protein in vivo is still poorly known. In this study, we generated a conditional Synechocystis sp. PCC 6803 mutant strain (STXA2) using an on-off promoter that is able to survive with only 2% of the TrxA level of the wild-type (WT) strain. STXA2 characterization revealed that TrxA depletion results in growth arrest and pronounced impairment of photosynthesis and the Calvin-Benson-Bassham (CBB) cycle. Analysis of the in vivo redox state of the bifunctional enzyme fructose-1,6-bisphosphatase/sedoheptulose-1,7-bisphosphatase showed higher levels of oxidation that affected enzyme activity in STXA2. This result implies that TrxA-mediated redox regulation of the CBB cycle is conserved in both cyanobacteria and chloroplasts, although the targets have different evolutionary origins. The STXA2 strain also accumulated more reactive oxygen species and was more sensitive to oxidative stress than the WT. Analysis of the in vivo redox state of 2-Cys peroxiredoxin revealed full oxidation, corresponding with TrxA depletion. Overall, these results indicate that depletion of TrxA in STXA2 greatly alters the cellular redox state, interfering with essential processes such as photosynthetic machinery operativity, carbon assimilation, and oxidative stress response. The TrxA regulatory role appears to be conserved along the evolution of oxygenic photosynthetic organisms.

Enhanced antioxidant capacity and upregulated transporter genes contribute to the UV-B-induced increase in blinin in Conyza blinii

Conyza blinii (C. blinii) is a traditional Chinese medicinal plant mainly grown in Sichuan, China. C. blinii is suitable for studying the mechanism of plant tolerance to UV-B due to its living conditions, characterized by a high altitude and exposure to strong ultraviolet radiation. Our results showed that the growth and photosynthetic activity of C. blinii were improved under a specific intensity of UV-B, rather than being significantly inhibited. Although UV-B increased the content of reactive oxygen species (ROS) in C. blinii, the activities of antioxidative enzymes were elevated, including superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX), which contributed to the elimination of ROS. Additionally, the content of blinin, the characteristic diterpene in C. blinii, was markedly increased by UV-B. Furthermore, RNA sequencing analyses were used to explore the molecular mechanism of UV-B tolerance in C. blinii. According to the results, most of the key enzyme genes in the blinin synthesis pathway were upregulated by UV-B. In addition, 23 upregulated terpene transporter genes were identified, and these genes might participate in blinin transport during the response to UV-B. Taken together, these results implied that enhanced antioxidant capacity and upregulated transporter genes contributed to increased synthesis of blinin in response to UV-B in C. blinii.

Increasing Temperature Counteracts the Negative Effect of UV Radiation on Growth and Photosynthetic Efficiency of Microcystis aeruginosa and Raphidiopsis raciborskii

High temperature can promote cyanobacterial blooms, whereas ultraviolet radiation (UVR) can potentially depress cyanobacterial growth by damaging their photosynthetic apparatus. Although the damaging effect of UVR has been well documented, reports on the interactive effects of UV radiation exposure and warming on cyanobacteria remain scarce. To better understand the combined effects of temperature and UVR on cyanobacteria, two strains of nuisance species, Microcystis aeruginosa (MIRF) and Raphidiopsis raciborskii (formerly Cylindrospermopsis raciborskii, CYRF), were grown at 24°C and 28°C and were daily exposed to UVA + UVB (PAR + UVA+UVB) or only UVA (PAR + UVA) radiation. MIRF and CYRF growth rates were most affected by PAR + UVA+UVB treatment and to a lesser extent by the PAR + UVA treatment. Negative UVR effects on growth, Photosystem II (PSII) efficiency and photosynthesis were pronounced at 24°C when compared to that at 28°C. Our results showed a cumulative negative effect on PSII efficiency in MIRF, but not in CYRF. Hence, although higher temperature ameliorates UVR damage, interspecific differences may lead to deviating impacts on different species, and combined elevated temperature and UVR stress could influence species competition.

Regulation of PSII function in Cyanothece sp. ATCC 51142 during a light-dark cycle

Cyanobacteria, as well as green algae and higher plants, have highly conserved photosynthetic machinery. Cyanothece sp. ATCC 51142 is a unicellular, aerobic, diazotrophic cyanobacterium that fixes N₂ in the dark. In Cyanothece, the psbA gene family is composed of five members, encoding different isoforms of the D1 protein. A new D1 protein has been postulated in the literature, which blocks PSII during the night and allows the fixation of nitrogen. We present data showing changes in PSII function in cells grown in cycles alternating between 12 h of light and dark, respectively, at Cyanothece sp. ATCC 51142. Cyanothece sp. ATCC 51142 uses intrinsic mechanisms to protect its nitrogenase activity in a two-stage process. In Stage I, immediately after the onset of darkness, the cells lose photosynthetic activity in a reversible process, probably by dissociation of water oxidation complex from photosystem II via a mechanism that does not require de novo protein synthesis. In Stage II, a more severe disruption of photosystem II function occurs is in part protein synthesis dependent and it could be a functional signature of the presence of sentinel D1 in a limited number of reaction centers still active or not yet inactivated by the mechanism described in Stage I. This process of inhibition uses light as a triggering signal for both the inhibition of photosynthetic activity and recovery when light returns. The intrinsic mechanism of photosynthetic inactivation during darkness with the interplay of the two mechanisms requires further studies.

Strain specific differences in rates of Photosystem II repair in picocyanobacteria correlate to differences in FtsH protein levels and isoform expression patterns

Picocyanobacteria are the numerically dominant photoautotrophs of the oligotrophic regions of Earth’s oceans. These organisms are characterized by their small size and highly reduced genomes. Strains partition to different light intensity and nutrient level niches, with differing photosynthetic apparatus stoichiometry, light harvesting machinery and susceptibility to photoinactivation. In this study, we grew three strains of picocyanobacteria: the low light, high nutrient strain Prochlorococcus marinus MIT 9313; the high light, low nutrient Prochlorococcus marinus MED 4; and the high light, high nutrient marine Synechococcus strain WH 8102; under low and high growth light levels. We then performed matched photophysiology, protein and transcript analyses. The strains differ significantly in their rates of Photosystem II repair under high light and in their capacity to remove the PsbA protein as the first step in the Photosystem II repair process. Notably, all strains remove the PsbD subunit at the same rate that they remove PsbA. When grown under low light, MIT 9313 loses active Photosystem II quickly when shifted to high light, but has no measurable capacity to remove PsbA. MED 4 and WH 8102 show less rapid loss of Photosystem II and considerable capacity to remove PsbA. MIT 9313 has less of the FtsH protease thought to be responsible for the removal of PsbA in other cyanobacteria. Furthermore, by transcript analysis the predominant FtsH isoform expressed in MIT 9313 is homologous to the FtsH 4 isoform characterized in the model strain Synechocystis PCC 6803, rather than the FtsH 2 and 3 isoforms thought to be responsible for PsbA degradation. MED 4 on the other hand shows high light inducible expression of the isoforms homologous to FtsH 2 and 3, consistent with its faster rate of PsbA removal. MIT 9313 has adapted to its low light environment by diverting resources away from Photosystem II content and repair.

Dose dependent variance in UV-C radiation induced effects on carbon and nitrogen metabolism in the cyanobacterium Nostoc muscorum Meg1

With the intention of getting an insight into the differential effect of UV-C radiation on the N₂-fixing heterocystous cyanobacterium Nostoc muscorum Meg1, various aspects of carbon and nitrogen metabolism was evaluated in the organism. Exposure to different doses of UV-C (6, 12, 18 and 24 mJ/cm²) showed that among various photo-absorbing pigments, phycobiliproteins were most sensitive. Oxygen evolving complex (OEC) activity measured as net oxygen evolution rate decreased by 63% upon 24 mJ/cm² exposure. Western blot analysis established that D1 protein of PSII was highly sensitive and its levels decreased even at a radiation dose as low as 6 mJ/cm². In contrast, levels of the Calvin cycle enzyme RuBisCO was increased at 6 and 12 mJ/cm² doses but the level decreased drastically (84%) at higher dose (24 mJ/cm²). The nitrogenase enzyme activity decreased at all doses but the ammonia assimilating enzyme glutamine synthetase (GS) activity recorded increase at the lower doses. The reactive oxygen species (ROS) and lipid peroxidation increased upon UV-C exposure. Transmission electron microscopic observation revealed damage to ultrastructure especially the thylakoid membrane organization, aggregation of dissolving phycobilisomes and loss of caboxysomes. Interestingly, sub-lethal radiation (6 and 12 mJ/cm²) dose exposures increased the growth rate in the organism when growth was measured over a period of 11 days after radiation exposure.

Effect of temperature on the accumulation of marine biogenic gels in the surface microlayer near the outlet of nuclear power plants and adjacent areas in the Daya Bay, China

The surface microlayer (SML) in marine systems is often characterized by an enrichment of biogenic, gel-like particles, such as the polysaccharide-containing transparent exopolymer particles (TEP) and the protein-containing Coomassie stainable particles (CSP). This study investigated the distribution of TEP and CSP, in the SML and underlying water, as well as their bio-physical controlling factors in Daya Bay, an area impacted by warm discharge from two Nuclear power plants (Npp’s) and aquaculture during a research cruise in July 2014. The SML had higher proportions of cyanobacteria and of pico-size Chl a contrast to the underlayer water, particularly at the nearest outlet station characterized by higher temperature. Diatoms, dinoflagellates and chlorophyll a were depleted in the SML. Both CSP and TEP abundance and total area were enriched in the SML relative to the underlying water, with enrichment factors (EFs) of 1.5–3.4 for CSP numbers and 1.32–3.2 for TEP numbers. Although TEP and CSP showed highest concentration in the region where high productivity and high nutrient concertation were observed, EFs of gels and of dissolved organic carbon (DOC) and dissolved acidic polysaccharide (> 1 kDa), exhibited higher values near the outlet of the Npp’s than in the adjacent waters. The positive relation between EF’s of gels and temperature and the enrichment of cyanobacteria in the SML may be indicative of future conditions in a warmer ocean, suggesting potential effects on adjusting phytoplankton community, biogenic element cycling and air-sea exchange processes.

Algal light sensing and photoacclimation in aquatic environments

Anoxygenic photosynthetic prokaryotes arose in ancient oceans ~3.5 billion years ago. The evolution of oxygenic photosynthesis by cyanobacteria followed soon after, enabling eukaryogenesis and the evolution of complex life. The Archaeplastida lineage dates back ~1.5 billion years to the domestication of a cyanobacterium. Eukaryotic algae have subsequently radiated throughout oceanic/freshwater/terrestrial environments, adopting distinctive morphological and developmental strategies for adaptation to diverse light environments. Descendants of the ancestral photosynthetic alga remain challenged by a typical diurnally fluctuating light supply ranging from ~0 to ~2000 μE m^-2  s^-1 . Such extreme changes in light intensity and variations in light quality have driven the evolution of novel photoreceptors, light-harvesting complexes and photoprotective mechanisms in photosynthetic eukaryotes. This minireview focuses on algal light sensors, highlighting the unexpected roles for linear tetrapyrroles (bilins) in the maintenance of functional chloroplasts in chlorophytes, sister species to streptophyte algae and land plants.

The Different Resistance of Two Astragalus Plants to UV-B Stress is Tightly Associated with the Organ-specific Isoflavone Metabolism

In this work, the changes in isoflavone levels and the expression of genes involved in their biosynthesis were studied in two Astragalus by UPLC-MS and real-time PCR after 10 days of UV-B treatment (λ_max  = 313 nm, 804 J m^-2 ). Isoflavones were significantly induced by UV-B irradiation. The influence might be activated by the regulation of these target genes. Our results indicate that (1) the resistance of Astragalus membranaceus might not be as good as Astragalus mongholicus in the enhanced UV-B radiation environment; (2) the enhanced accumulation of calycosin and calycosin-7-glucoside with UV-B treatment in roots of A. mongholicus might be derived from formononetin which is synthesized in the leaves; (3) the glycosylation process could be stimulated and activated by the enhanced UV-B radiation in both A. mongholicus and A. membranaceus. In other words, glycosylation of isoflavones might play a crucial role for two Astragalus plants in response to UV-B stress. Overall, this study offered a feasible elicitation strategy to understand the accumulation pattern of isoflavone in A. mongholicus and A. membranaceus, and also provided a reference for the changes in isoflavone levels of Astragalus in UV-B enhanced environment in the future.

Crystal structure and redox properties of a novel cyanobacterial heme protein with a His/Cys heme axial ligation and a Per-Arnt-Sim (PAS)-like domain

Photosystem II catalyzes light-induced water oxidation leading to the generation of dioxygen indispensable for sustaining aerobic life on Earth. The Photosystem II reaction center is composed of D1 and D2 proteins encoded by psbA and psbD genes, respectively. In cyanobacteria, different psbA genes are present in the genome. The thermophilic cyanobacterium Thermosynechococcus elongatus contains three psbA genes: psbA1, psbA2, and psbA3, and a new c-type heme protein, Tll0287, was found to be expressed in a strain expressing the psbA2 gene only, but the structure and function of Tll0287 are unknown. Here we solved the crystal structure of Tll0287 at a 2.0 Å resolution. The overall structure of Tll0287 was found to be similar to some kinases and sensor proteins with a Per-Arnt-Sim-like domain rather than to other c-type cytochromes. The fifth and sixth axial ligands for the heme were Cys and His, instead of the His/Met or His/His ligand pairs observed for most of the c-type hemes. The redox potential, E_½, of Tll0287 was -255 ± 20 mV versus normal hydrogen electrode at pH values above 7.5. Below this pH value, the E_½ increased by ≈57 mV/pH unit at 15 °C, suggesting the involvement of a protonatable group with a pK_red = 7.2 ± 0.3. Possible functions of Tll0287 as a redox sensor under microaerobic conditions or a cytochrome subunit of an H₂S-oxidizing system are discussed in view of the environmental conditions in which psbA2 is expressed, as well as phylogenetic analysis, structural, and sequence homologies.

Different physiological responses of cyanobacteria to ultraviolet-B radiation under iron-replete and iron-deficient conditions: Implications for underestimating the negative effects of UV-B radiation

Iron deficiency has been considered one of the main limiting factors of phytoplankton productivity in some aquatic systems including oceans and lakes. Concomitantly, solar ultraviolet-B radiation has been shown to have both deleterious and positive impacts on phytoplankton productivity. However, how iron-deficient cyanobacteria respond to UV-B radiation has been largely overlooked in aquatic systems. In this study, physiological responses of four cyanobacterial strains (Microcystis and Synechococcus), which are widely distributed in freshwater or marine systems, were investigated under different UV-B irradiances and iron conditions. The growth, photosynthetic pigment composition, photosynthetic activity, and nonphotochemical quenching of the different cyanobacterial strains were drastically altered by enhanced UV-B radiation under iron-deficient conditions, but were less affected under iron-replete conditions. Intracellular reactive oxygen species (ROS) and iron content increased and decreased, respectively, with increased UV-B radiation under iron-deficient conditions for both Microcystis aeruginosa FACHB 912 and Synechococcus sp. WH8102. On the contrary, intracellular ROS and iron content of these two strains remained constant and increased, respectively, with increased UV-B radiation under iron-replete conditions. These results indicate that iron-deficient cyanobacteria are more susceptible to enhanced UV-B radiation. Therefore, UV-B radiation probably plays an important role in influencing primary productivity in iron-deficient aquatic systems, suggesting that its effects on the phytoplankton productivity may be underestimated in iron-deficient regions around the world.

Photoinactivation of Photosystem II in Prochlorococcus and Synechococcus

The marine picocyanobacteria Synechococcus and Prochlorococcus numerically dominate open ocean phytoplankton. Although evolutionarily related they are ecologically distinct, with different strategies to harvest, manage and exploit light. We grew representative strains of Synechococcus and Prochlorococcus and tracked their susceptibility to photoinactivation of Photosystem II under a range of light levels. As expected blue light provoked more rapid photoinactivation than did an equivalent level of red light. The previous growth light level altered the susceptibility of Synechococcus, but not Prochlorococcus, to this photoinactivation. We resolved a simple linear pattern when we expressed the yield of photoinactivation on the basis of photons delivered to Photosystem II photochemistry, plotted versus excitation pressure upon Photosystem II, the balance between excitation and downstream metabolism. A high excitation pressure increases the generation of reactive oxygen species, and thus increases the yield of photoinactivation of Photosystem II. Blue photons, however, retained a higher baseline photoinactivation across a wide range of excitation pressures. Our experiments thus uncovered the relative influences of the direct photoinactivation of Photosystem II by blue photons which dominates under low to moderate blue light, and photoinactivation as a side effect of reactive oxygen species which dominates under higher excitation pressure. Synechococcus enjoyed a positive metabolic return upon the repair or the synthesis of a Photosystem II, across the range of light levels we tested. In contrast Prochlorococcus only enjoyed a positive return upon synthesis of a Photosystem II up to 400 μmol photons m-2 s-1. These differential cost-benefits probably underlie the distinct photoacclimation strategies of the species.

UV-B-Heavy Metal Interaction: Comparative Evaluation on Photosynthetic Properties of Three Cyanobacteria

Gradual increase in UV-B component reaching earth surface together with heavy metal contamination appears to be a serious environmental concern. Differential sensitivity in photosynthetic characters of Anabaena doliolum, Microcystis sp., and Nostoc muscorum were observed following exposure to UV-B radiation and heavy metals (Cd and Zn) which displayed reduced photosynthesis with maximum inhibition in Anabaena. PSII was the most sensitive component of the electron transport chain depicting 84, 80, and 70 % inhibition in A. doliolum, Microcystis sp., and N. muscorum, respectively. Cadmium and UV-B-induced inhibition of DCPIP photoreduction could not reversed by artificial electron donors in all the strains. However, they substantially reversed the inhibition caused by Zn as well as Zn + UV-B interactively in N. muscorum, not observed in other two strains. Absorption spectra of all the strains showed differential decrement in chl a peak in treated than the control cells (P < 0.25, r = -0.942). Significantly decreased peaks (P < 0.05) by different states of stresses to all the three cyanobacteria were observed in emission spectra. Excitation spectra of the test strains suggest disorganization or delinking of phycobilisomes from the PSII reaction center, also attested by strong negative correlation between the treatment of stress and phycocyanin (P < 0.025, r = -0.971). The present study qualified N. muscorum as most tolerant followed by Microcystis sp., whereas A. doliolum emerged as the most sensitive one and also endorses high toxicity of Cd as compared to Zn.

Comparison of D1´- and D1-containing PS II reaction centre complexes under different environmental conditions in Synechocystis sp. PCC 6803

In oxygenic photosynthesis, the D1 protein of Photosystem II is the primary target of photodamage and environmental stress can accelerate this process. The cyanobacterial response to stress includes transcriptional regulation of genes encoding D1, including low-oxygen-induction of psbA1 encoding the D1´ protein in Synechocystis sp. PCC 6803. The psbA1 gene is also transiently up-regulated in high light, and its deletion has been reported to increase ammonium-induced photoinhibition. Therefore we investigated the role of D1´-containing PS II centres under different environmental conditions. A strain containing only D1´-PS II centres under aerobic conditions exhibited increased sensitivity to ammonium chloride and high light compared to a D1-containing strain. Additionally a D1´-PS II strain was outperformed by a D1-PS II strain under normal conditions; however, a strain containing low-oxygen-induced D1´-PS II centres was more resilient under high light than an equivalent D1 strain. These D1´-containing centres had chlorophyll a fluorescence characteristics indicative of altered forward electron transport and back charge recombination with the donor side of PS II. Our results indicate D1´-PS II centres are important in the reconfiguration of thylakoid electron transport in response to high light and low oxygen.

Natural isoforms of the Photosystem II D1 subunit differ in photoassembly efficiency of the water-oxidizing complex

Oxygenic photosynthesis efficiency at increasing solar flux is limited by light-induced damage (photoinhibition) of Photosystem II (PSII), primarily targeting the D1 reaction center subunit. Some cyanobacteria contain two natural isoforms of D1 that function better under low light (D1:1) or high light (D1:2). Herein, rates and yields of photoassembly of the Mn4CaO5 water-oxidizing complex (WOC) from the free inorganic cofactors (Mn(2+), Ca(2+), water, electron acceptor) and apo-WOC-PSII are shown to differ significantly: D1:1 apo-WOC-PSII exhibits a 2.3-fold faster rate-limiting step of photoassembly and up to seven-fold faster rate to the first light-stable Mn(3+) intermediate, IM1*, but with a much higher rate of photoinhibition than D1:2. Conversely, D1:2 apo-WOC-PSII assembles slower but has up to seven-fold higher yield, achieved by a higher quantum yield of charge separation and slower photoinhibition rate. These results confirm and extend previous observations of the two holoenzymes: D1:2-PSII has a greater quantum yield of primary charge separation, faster [P680 (+) Q A (-) ] charge recombination and less photoinhibition that results in a slower rate and higher yield of photoassembly of its apo-WOC-PSII complex. In contrast, D1:1-PSII has a lower quantum yield of primary charge separation, a slower [P680 (+) Q A (-) ] charge recombination rate, and faster photoinhibition that together result in higher rate but lower yield of photoassembly at higher light intensities. Cyanobacterial PSII reaction centers that contain the high- and low-light D1 isoforms can tailor performance to optimize photosynthesis at varying light conditions, with similar consequences on their photoassembly kinetics and yield. These different efficiencies of photoassembly versus photoinhibition impose differential costs for biosynthesis as a function of light intensity.

Temperature and light interactively modulate gene expression in Saccharina latissima (Phaeophyceae)

Macroalgae of the order Laminariales (kelp) are important components of cold-temperate coastal ecosystems. Major factors influencing their distribution are light (including UV radiation) and temperature. Therefore, future global environmental changes potentially will impact their zonation, distribution patterns, and primary productivity. Many physiological studies were performed on UV radiation and temperature stress in kelp but combinatory effects have not been analyzed and so far no study is available on the molecular processes involved in acclimation to these stresses. Therefore, sporophytes of Saccharina latissima were exposed for 2 weeks to 12 combinations of photosynthetically active radiation (PAR), UV radiation and temperature. Subsequently, microarray hybridizations were performed to determine changes in gene expression patterns. Several effects on the transcriptome were observed after exposure experiments. The strongest effect of temperature on gene expression was observed at 2°C. Furthermore, UV radiation had stronger effects on gene expression than high PAR, and caused stronger induction genes correlated with categories such as photosynthetic components and vitamin B6 biosynthesis. Higher temperatures ameliorated the negative effects of UV radiation in S. latissima. Regulation of reactive oxygen species (ROS) scavenging seems to work in a compartment specific way. Gene expression profiles of ROS scavengers indicated a high amount of oxidative stress in response to the 2°C condition as well as to excessive light at 12°C. Interestingly, stress levels that did not lead to physiological alterations already caused by a transcriptomic response.

Taxonomic and functional diversity provides insight into microbial pathways and stress responses in the saline Qinghai Lake, China

Microbe-mediated biogeochemical cycles contribute to the global climate system and have sensitive responses and feedbacks to environmental stress caused by climate change. Yet, little is known about the effects of microbial biodiversity (i.e., taxonmic and functional diversity) on biogeochemical cycles in ecosytems that are highly sensitive to climate change. One such sensitive ecosystem is Qinghai Lake, a high-elevation (3196 m) saline (1.4%) lake located on the Tibetan Plateau, China. This study provides baseline information on the microbial taxonomic and functional diversity as well as the associated stress response genes. Illumina metagenomic and metatranscriptomic datasets were generated from lake water samples collected at two sites (B and E). Autotrophic Cyanobacteria dominated the DNA samples, while heterotrophic Proteobacteria dominated the RNA samples at both sites. Photoheterotrophic Loktanella was also present at both sites. Photosystem II was the most active pathway at site B; while, oxidative phosphorylation was most active at site E. Organisms that expressed photosystem II or oxidative phosphorylation also expressed genes involved in photoprotection and oxidative stress, respectively. Assimilatory pathways associated with the nitrogen cycle were dominant at both sites. Results also indicate a positive relationship between functional diversity and the number of stress response genes. This study provides insight into the stress resilience of microbial metabolic pathways supported by greater taxonomic diversity, which may affect the microbial community response to climate change.

Responses of a rice-field cyanobacterium Anabaena siamensis TISTR-8012 upon exposure to PAR and UV radiation

The effects of PAR and UV radiation and subsequent responses of certain antioxidant enzymatic and non-enzymatic defense systems were studied in a rice field cyanobacterium Anabaena siamensis TISTR 8012. UV radiation resulted in a decline in growth accompanied by a decrease in chlorophyll a and photosynthetic efficiency. Exposure of cells to UV radiation significantly affected the differentiation of vegetative cells into heterocysts or akinetes. UV-B radiation caused the fragmentation of the cyanobacterial filaments conceivably due to the observed oxidative stress. A significant increase of reactive oxygen species in vivo and DNA strand breaks were observed in UV-B exposed cells followed by those under UV-A and PAR radiation, respectively. The UV-induced oxidative damage was alleviated due to an induction of antioxidant enzymatic/non-enzymatic defense systems. In response to UV irradiation, the studied cyanobacterium exhibited a significant increase in antioxidative enzyme activities of superoxide dismutase, catalase and peroxidase. Moreover, the cyanobacterium also synthesized some UV-absorbing/screening substances. HPLC coupled with a PDA detector revealed the presence of three compounds with UV-absorption maxima at 326, 331 and 345 nm. The induction of the biosynthesis of these UV-absorbing compounds was found under both PAR and UV radiation, thus suggesting their possible function as an active photoprotectant.

Engineered Photosystem II reaction centers optimize photochemistry versus photoprotection at different solar intensities

The D1 protein of Photosystem II (PSII) provides most of the ligating amino acid residues for the Mn4CaO5 water-oxidizing complex (WOC) and half of the reaction center cofactors, and it is present as two isoforms in the cyanobacterium Synechococcus elongatus PCC 7942. These isoforms, D1:1 and D1:2, confer functional advantages for photosynthetic growth at low and high light intensities, respectively. D1:1, D1:2, and seven point mutations in the D1:2 background that are native to D1:1 were expressed in the green alga Chlamydomonas reinhardtii. We used these nine strains to show that those strains that confer a higher yield of PSII charge separation under light-limiting conditions (where charge recombination is significant) have less efficient photochemical turnover, measured in terms of both a lower WOC turnover probability and a longer WOC cycle period. Conversely, these same strains under light saturation (where charge recombination does not compete) confer a correspondingly faster O2 evolution rate and greater protection against photoinhibition. Taken together, the data clearly establish that PSII primary charge separation is a trade-off between photochemical productivity (water oxidation and plastoquinone reduction) and charge recombination (photoprotection). These trade-offs add up to a significant growth advantage for the two natural isoforms. These insights provide fundamental design principles for engineering of PSII reaction centers with optimal photochemical efficiencies for growth at low versus high light intensities.

Flavodiiron protein Flv2/Flv4-related photoprotective mechanism dissipates excitation pressure of PSII in cooperation with phycobilisomes in Cyanobacteria

Oxygenic photosynthesis evolved with cyanobacteria, the ancestors of plant chloroplasts. The highly oxidizing chemistry of water splitting required concomitant evolution of efficient photoprotection mechanisms to safeguard the photosynthetic machinery. The role of flavodiiron proteins (FDPs), originally called A-type flavoproteins or Flvs, in this context has only recently been appreciated. Cyanobacterial FDPs constitute a specific protein group that evolved to protect oxygenic photosynthesis. There are four FDPs in Synechocystis sp. PCC 6803 (Flv1 to Flv4). Two of them, Flv2 and Flv4, are encoded by an operon together with a Sll0218 protein. Their expression, tightly regulated by CO2 levels, is also influenced by changes in light intensity. Here we describe the overexpression of the flv4-2 operon in Synechocystis sp. PCC 6803 and demonstrate that it results in improved photochemistry of PSII. The flv4-2/OE mutant is more resistant to photoinhibition of PSII and exhibits a more oxidized state of the plastoquinone pool and reduced production of singlet oxygen compared with control strains. Results of biophysical measurements indicate that the flv4-2 operon functions in an alternative electron transfer pathway from PSII, and thus alleviates PSII excitation pressure by channeling up to 30% of PSII-originated electrons. Furthermore, intact phycobilisomes are required for stable expression of the flv4-2 operon genes and for the Flv2/Flv4 heterodimer-mediated electron transfer mechanism. The latter operates in photoprotection in a complementary way with the orange carotenoid protein-related nonphotochemical quenching. Expression of the flv4-2 operon and exchange of the D1 forms in PSII centers upon light stress, on the contrary, are mutually exclusive photoprotection strategies among cyanobacteria.

Green algae in alpine biological soil crust communities: acclimation strategies against ultraviolet radiation and dehydration

Green algae are major components of biological soil crusts in alpine habitats. Together with cyanobacteria, fungi and lichens, green algae form a pioneer community important for the organisms that will succeed them. In their high altitudinal habitat these algae are exposed to harsh and strongly fluctuating environmental conditions, mainly intense irradiation, including ultraviolet radiation, and lack of water leading to desiccation. Therefore, green algae surviving in these environments must have evolved with either avoidance or protective strategies, as well as repair mechanisms for damage. In this review we have highlighted these mechanisms, which include photoprotection, photochemical quenching, and high osmotic values to avoid water loss, and in some groups flexibility of secondary cell walls to maintain turgor pressure even in water-limited situations. These highly specialized green algae will serve as good model organisms to study desiccation tolerance or photoprotective mechanisms, due to their natural capacity to withstand unfavorable conditions. We point out the urgent need for modern phylogenetic approaches in characterizing these organisms, and molecular methods for analyzing the metabolic changes involved in their adaptive strategies.

Kinetics of photosynthetic response to ultraviolet and photosynthetically active radiation in Synechococcus WH8102 (cyanobacteria)

The picoplanktonic cyanobacteria, Synechococcus spp., (Nägeli) are important contributors to global ocean primary production that can be stressed by solar radiation, both in the photosynthetically active (PAR) and ultraviolet (UV) range. We studied the responses of PSII quantum yield (active fluorescence), carbon fixation ((14)C assimilation) and oxygen evolution (membrane inlet mass spectrometry) in Synechococcus WH8102 under moderate UV and PAR. PSII quantum yield decreased during exposure to moderate UV and UV+PAR, with response to the latter being faster (6.4 versus 2.8 min, respectively). Repair processes were also faster when UV+PAR exposure was followed by moderate PAR (1.68 min response time) than when UV was followed by very low PAR (10.5 min response time). For the UV+PAR treatment, the initial decrease in quantum yield was followed by a 50% increase ("rebound") after 7 min exposure, showing an apparent photoprotection induction. While oxygen uptake increased with PAR, it did not change under UV, suggesting that this oxygen-dependent mechanism of photoprotection, which may be acting as an electron sink, is not an important strategy against UV. We used propyl gallate, an antioxidant, to test for plastid terminal oxidase (ptox) or ptox-like enzymes activity, but it caused nonspecific and toxic effects on Synechococcus WH8102.

Mechanisms of photosynthetic inactivation on growth suppression of Microcystis aeruginosa under UV-C stress

This study aims to investigate the effects of UV-C irradiation on photosynthetic processes of Microcystis aeruginosa to unravel the mechanism(s) involved in how and in what ways UV-C mediates growth suppression and cellular recovery. Changes in the concentration of photosynthetic pigments, photochemical efficiency, PS II core protein (D1) content, and the coding genes expressions were measured. The results indicate that UV-C doses at 20-200 mJ cm(-2) lead to rapid reduction in gene expression of both psbA (for D1) and cpc (for phycocyanin), but the suppression was short term and recoverable within 3 d of post-UV incubation. Conversely, UV-C doses at ≥50 mJ cm(-2) could induce marked decline in photochemical efficiency (represented by the optimal PS II quantum yield, FV/FM, and the effective PS II quantum yield, Y) as well as decreases in D1 content and water soluble pigments (phycoerythrins, phycocyanins, allophycocyanins) in M. aeruginosa during the post UV-C incubation period. The results suggest that interruption of both the light energy harvesting apparatus (especially the water soluble pigments) and the photochemical process mainly accounted for the growth suppression effect in UV-C irradiated M. aeruginosa.

A unique regulation of the expression of the psbA, psbD, and psbE genes, encoding the 01, 02 and cytochrome b559 subunits of the Photosystem II complex in the chlorophyll d containing cyanobacterium Acaryochloris marina

Photosynthetic electron transport, chromatic photoacclirnation and expression of the genes encoding the 01, 02, and cytochrome b559 subunits of the Photosystem II complex were studied in the chlorophyll d containing cyanobacterium Acaryochloris marina MBIC11017 under various environmental conditions. During oxygen deprivation and inhibition of photosynthetic electron transport by dibromothymoquinone the psbA1 gene encoding a 01' isoform was induced. All of the three psbA and one of the three psbD (psbD2) genes, encoding two different isoforms of the 01 and the abundant isoform of the 02 proteins, respectively were induced under exposure to UV-B radiation and high intensity visible light. Under far red light the amount of Photosystem II complexes increased, and expression of the psbE2 gene encoding the alpha-subunit of cytochrome b559 was enhanced. However, the psbF and psbE1 genes encoding the beta- and another isoform of alpha-cytochrome b559, respectively remained lowly expressed under all conditions. Far red light also induced the psbD3 gene encoding a 02' isoform whose primary structure is different from the abundant 02 isoform. psbD3 was also induced under low intensity visible light, when chromatic photoacclimation was indicated by a red-shifted absorption of chlorophyll d. Our results show that differential expression of multigene families encoding different isoforms of 01 and 02 plays an important role in the acclimation of A. marina to contrasting environmental conditions. Moreover, the disproportionate quantity of transcripts of the alpha and beta subunits of cytochrome b559 implies the existence of an alpha-alpha homodimer organization of cytochrome b559 in Photosystem II complexes.

Effects of UVB Radiation on competition between the bloom-forming cyanobacterium Microcystis aeruginosa and the Chlorophyceae Chlamydomonas microsphaera(1)

The growth, photosynthetic characteristics, and competitive ability of three algal strains were investigated under different doses of ultraviolet-B (UVB) radiation (0, 0.285, and 0.372 W · m(-2) ). The organisms were the toxic bloom-forming cyanobacterium Microcystis aeruginosa FACHB 912, nontoxic M. aeruginosa FACHB 469, and the green microalga Chlamydomonas microsphaera FACHB 52. In monocultures, the growth of all three strains was inhibited by UVB. In mixed cultures, enhanced UVB radiation resulted in decreased percentages of the two M. aeruginosa strains (19%-22% decrease on d 12 of the competition experiment). UVB radiation resulted in increased contents of chlorophyll a, b, and carotenoids (CAR) in C. microsphaera, and decreased contents of allophycocyanin (APC) or phycocyanin in the two Microcystis strains. All three strains showed increased levels of UVabsorbing compounds and intracellular reactive oxygen species under 0.372 W · m(-2) UVB radiation, and decreased light compensation points, dark respiratory rates, and maximal quantum efficiency of PSII. After a 20 h recovery, the photosynthetic oxygen evolution of C. microsphaera was restored to its maximum value, but that of Microcystis strains continued to decrease. Nonphotochemical quenching was increased by UVB radiation in C. microsphaera, but was unaffected in the two M. aeruginosa strains. Our results indicated that C. microsphaera has a competitive advantage relative to Microcystis during exposure to UVB irradiation.

Natural variants of photosystem II subunit D1 tune photochemical fitness to solar intensity

Photosystem II (PSII) is composed of six core polypeptides that make up the minimal unit capable of performing the primary photochemistry of light-driven charge separation and water oxidation in all oxygenic phototrophs. The D1 subunit of this complex contains most of the ligating amino acid residues for the Mn(4)CaO(5) core of the water-oxidizing complex (WOC). Most cyanobacteria have 3-5 copies of the psbA gene coding for at least two isoforms of D1, whereas algae and plants have only one isoform. Synechococcus elongatus PCC 7942 contains two D1 isoforms; D1:1 is expressed under low light conditions, and D1:2 is up-regulated in high light or stress conditions. Using a heterologous psbA expression system in the green alga Chlamydomonas reinhardtii, we have measured growth rate, WOC cycle efficiency, and O(2) yield as a function of D1:1, D1:2, or the native algal D1 isoform. D1:1-PSII cells outcompete D1:2-PSII cells and accumulate more biomass in light-limiting conditions. However, D1:2-PSII cells easily outcompete D1:1-PSII cells at high light intensities. The native C. reinhardtii-PSII WOC cycles less efficiently at all light intensities and produces less O(2) than either cyanobacterial D1 isoform. D1:2-PSII makes more O(2) per saturating flash than D1:1-PSII, but it exhibits lower WOC cycling efficiency at low light intensities due to a 40% faster charge recombination rate in the S(3) state. These functional advantages of D1:1-PSII and D1:2-PSII at low and high light regimes, respectively, can be explained by differences in predicted redox potentials of PSII electron acceptors that control kinetic performance.

Prochlorococcus and Synechococcus have Evolved Different Adaptive Mechanisms to Cope with Light and UV Stress

Prochlorococcus and Synechococcus, which numerically dominate vast oceanic areas, are the two most abundant oxygenic phototrophs on Earth. Although they require solar energy for photosynthesis, excess light and associated high UV radiations can induce high levels of oxidative stress that may have deleterious effects on their growth and productivity. Here, we compared the photophysiologies of the model strains Prochlorococcus marinus PCC 9511 and Synechococcus sp. WH7803 grown under a bell-shaped light/dark cycle of high visible light supplemented or not with UV. Prochlorococcus exhibited a higher sensitivity to photoinactivation than Synechococcus under both conditions, as shown by a larger drop of photosystem II (PSII) quantum yield at noon and different diel patterns of the D1 protein pool. In the presence of UV, the PSII repair rate was significantly depressed at noon in Prochlorococcus compared to Synechococcus. Additionally, Prochlorococcus was more sensitive than Synechococcus to oxidative stress, as shown by the different degrees of PSII photoinactivation after addition of hydrogen peroxide. A transcriptional analysis also revealed dramatic discrepancies between the two organisms in the diel expression patterns of several genes involved notably in the biosynthesis and/or repair of photosystems, light-harvesting complexes, CO(2) fixation as well as protection mechanisms against light, UV, and oxidative stress, which likely translate profound differences in their light-controlled regulation. Altogether our results suggest that while Synechococcus has developed efficient ways to cope with light and UV stress, Prochlorococcus cells seemingly survive stressful hours of the day by launching a minimal set of protection mechanisms and by temporarily bringing down several key metabolic processes. This study provides unprecedented insights into understanding the distinct depth distributions and dynamics of these two picocyanobacteria in the field.

Light history influences the response of the marine cyanobacterium Synechococcus sp. WH7803 to oxidative stress

Marine Synechococcus undergo a wide range of environmental stressors, especially high and variable irradiance, which may induce oxidative stress through the generation of reactive oxygen species (ROS). While light and ROS could act synergistically on the impairment of photosynthesis, inducing photodamage and inhibiting photosystem II repair, acclimation to high irradiance is also thought to confer resistance to other stressors. To identify the respective roles of light and ROS in the photoinhibition process and detect a possible light-driven tolerance to oxidative stress, we compared the photophysiological and transcriptomic responses of Synechococcus sp. WH7803 acclimated to low light (LL) or high light (HL) to oxidative stress, induced by hydrogen peroxide (H₂O₂) or methylviologen. While photosynthetic activity was much more affected in HL than in LL cells, only HL cells were able to recover growth and photosynthesis after the addition of 25 μM H₂O₂. Depending upon light conditions and H₂O₂ concentration, the latter oxidizing agent induced photosystem II inactivation through both direct damage to the reaction centers and inhibition of its repair cycle. Although the global transcriptome response appeared similar in LL and HL cells, some processes were specifically induced in HL cells that seemingly helped them withstand oxidative stress, including enhancement of photoprotection and ROS detoxification, repair of ROS-driven damage, and regulation of redox state. Detection of putative LexA binding sites allowed the identification of the putative LexA regulon, which was down-regulated in HL compared with LL cells but up-regulated by oxidative stress under both growth irradiances.

Strategies for psbA gene expression in cyanobacteria, green algae and higher plants: from transcription to PSII repair

The Photosystem (PS) II of cyanobacteria, green algae and higher plants is prone to light-induced inactivation, the D1 protein being the primary target of such damage. As a consequence, the D1 protein, encoded by the psbA gene, is degraded and re-synthesized in a multistep process called PSII repair cycle. In cyanobacteria, a small gene family codes for the various, functionally distinct D1 isoforms. In these organisms, the regulation of the psbA gene expression occurs mainly at the level of transcription, but the expression is fine-tuned by regulation of translation elongation. In plants and green algae, the D1 protein is encoded by a single psbA gene located in the chloroplast genome. In chloroplasts of Chlamydomonas reinhardtii the psbA gene expression is strongly regulated by mRNA processing, and particularly at the level of translation initiation. In chloroplasts of higher plants, translation elongation is the prevalent mechanism for regulation of the psbA gene expression. The pre-existing pool of psbA transcripts forms translation initiation complexes in plant chloroplasts even in darkness, while the D1 synthesis can be completed only in the light. Replacement of damaged D1 protein requires also the assistance by a number of auxiliary proteins, which are encoded by the nuclear genome in green algae and higher plants. Nevertheless, many of these chaperones are conserved between prokaryotes and eukaryotes. Here, we describe the specific features and fundamental differences of the psbA gene expression and the regeneration of the PSII reaction center protein D1 in cyanobacteria, green algae and higher plants. This article is part of a Special Issue entitled Photosystem II.

Functional characterization and quantification of the alternative PsbA copies in Thermosynechococcus elongatus and their role in photoprotection

The D1 protein (PsbA) of photosystem II (PSII) from Thermosynechococcus elongatus is encoded by a psbA gene family that is typical of cyanobacteria. Although the transcription of these three genes has been studied previously (Kós, P. B., Deák, Z., Cheregi, O., and Vass, I. (2008) Biochim. Biophys. Acta 1777, 74-83), the protein quantification had not been possible due to the high sequence identity between the three PsbA copies. The successful establishment of a method to quantify the PsbA proteins on the basis of reverse phase-LC-electrospray mass ionization-MS/MS (RP-LC-ESI-MS/MS) enables an accurate comparison of transcript and protein level for the first time ever. Upon high light incubation, about 70% PsbA3 could be detected, which closely corresponds to the transcript level. It was impossible to detect any PsbA2 under all tested conditions. The construction of knock-out mutants enabled for the first time a detailed characterization of both whole cells and also isolated PSII complexes. PSII complexes of the ΔpsbA1/psbA2 mutant contained only copy PsbA3, whereas only PsbA1 could be detected in PSII complexes from the ΔpsbA3 mutant. In whole cells as well as in isolated complexes, a shift of the free energy between the redox pairs in the PsbA3 complexes in comparison with PsbA1 could be detected by thermoluminescence and delayed fluorescence measurements. This change is assigned to a shift of the redox potential of pheophytin toward more positive values. Coincidentally, no differences in the Q(A)-Q(B) electron transfer could be observed in flash-induced fluorescence decay or prompt fluorescence measurements. In conclusion, PsbA3 complexes yield a better protection against photoinhibition due to a higher probability of the harmless dissipation of excess energy.

Cyanobacteria and ultraviolet radiation (UVR) stress: mitigation strategies

Cyanobacteria are primitive photosynthetic oxygen-evolving prokaryotes that appeared on the Earth when there was no ozone layer to protect them from damaging ultraviolet radiation (UVR). UVR has both direct and indirect effects on the cyanobacteria due to absorption by biomolecules and UVR-induced oxidative stress, respectively. However, these organisms have developed several lines of mitigation strategies/defense mechanisms such as avoidance, scavenging, screening, repair and programmed cell death to counteract the damaging effects of UVR. This review presents an update on the effects of UVR on cyanobacteria and the defense mechanisms employed by these prokaryotes to withstand UVR stress. In addition, recent developments in the field of molecular biology of UV-absorbing compounds such as mycosporine-like amino acids and scytonemin, are also added and the possible role of programmed cell death, signal perception as well their transduction under UVR stress is being discussed.

The long-term effects of UV exclusion on the microbial composition and photosynthetic competence of bacteria in hot-spring microbial mats

The primary objective of this study was to determine whether the long-term exclusion of ultraviolet (UV) radiation (UVR) from hot-spring microbial mats resulted in an alteration of microbial composition, such as a shift to more UV-sensitive species. Over a 1-3-month period, microbial mats in two alkaline geothermal streams in Yellowstone National Park were covered with filters that excluded or transmitted UVR. Over some, 25% transmission neutral density screens were also used. In the 40-47 degrees C range, there were no apparent changes in community composition during the summer with or without high or low UVR, as assessed by denaturing gradient gel electrophoresis (DGGE) profiles after polymerase chain reaction amplification of 16S-rRNA genes with general Bacteria and Cyanobacteria primers. Major bands were purified from the DGGE gels and sequenced. Only one of the cyanobacterial sequences matched known strains in the database; the others appear to be unique. Although the bacterial composition of these communities was apparently stable, surface layers of cyanobacteria protected from UVR were not as competent photosynthetically as those that had been maintained under UVR. This decrease in competence was expressed as a loss of the ability to perform at a maximum rate under full UVR plus visible irradiance. However, even +UV-maintained cyanobacteria performed better when UVR was excluded during the photosynthesis tests. It is probable that the large differences in photosynthetic competence observed reflect changes at the level of gene expression in the dominant species rather than changes in species composition.

Cyanobacterial psbA gene family: optimization of oxygenic photosynthesis

The D1 protein of Photosystem II (PSII), encoded by the psbA genes, is an indispensable component of oxygenic photosynthesis. Due to strongly oxidative chemistry of PSII water splitting, the D1 protein is prone to constant photodamage requiring its replacement, whereas most of the other PSII subunits remain ordinarily undamaged. In cyanobacteria, the D1 protein is encoded by a psbA gene family, whose members are differentially expressed according to environmental cues. Here, the regulation of the psbA gene expression is first discussed with emphasis on the model organisms Synechococcus sp. and Synechocystis sp. Then, a general classification of cyanobacterial D1 isoforms in various cyanobacterial species into D1m, D1:1, D1:2, and D1' forms depending on their expression pattern under acclimated growth conditions and upon stress is discussed, taking into consideration the phototolerance of different D1 forms and the expression conditions of respective members of the psbA gene family.

Function and evolution of the psbA gene family in marine Synechococcus: Synechococcus sp. WH7803 as a case study

In cyanobacteria, the D1 protein of photosystem II (PSII) is encoded by the psbA multigene family. In most freshwater strains, a D1:1 isoform of this protein is exchanged for a D1:2 isoform in response to various stresses, thereby altering PSII photochemistry. To investigate PSII responses to stress in marine Synechococcus, we acclimated cultures of the WH7803 strain to different growth irradiances and then exposed them to high light (HL) or ultraviolet (UV) radiation. Measurement of PSII quantum yield and quantitation of the D1 protein pool showed that HL-acclimated cells were more resistant to UV light than were low light- (LL) or medium light- (ML) acclimated cells. Both UV and HL induced the expression of psbA genes encoding D1:2 and the repression of the psbA gene encoding D1:1. Although three psbA genes encode identical D1:2 isoforms in Synechococcus sp. WH7803, only one was strongly stress responsive in our treatment conditions. Examination of 11 marine Synechococcus genomic sequences identified up to six psbA copies per genome, with always a single gene encoding D1:1. In phylogenetic analyses, marine Synechococcus genes encoding D1:1 clustered together, while the genes encoding D1:2 grouped by genome into subclusters. Moreover, examination of the genomic environment of psbA genes suggests that the D1:2 genes are hotspots for DNA recombination. Collectively, our observations suggest that while all psbA genes follow a concerted evolution within each genome, D1:2 coding genes are subject to intragenome homogenization most probably mediated by gene conversion.