Effect of ultraviolet-B radiation on intact cells of the cyanobacterium Spirulina platensis: characterization of the alterations in the thylakoid membranes

Cell damage repair mechanism in a desert green algae Chlorella sp. against UV-B radiation

The protective ozone layer is continually depleting owing to an increase in the levels of solar UV-B radiation, which has harmful effects on organisms. Algae in desert soil can resist UV-B radiation, but most research on the radiation resistance of desert algae has focused on cyanobacteria. In this study, we found that desert green algae, Chlorella sp., could maintain high photosynthetic activity under UV-B stress. To examine the tolerance mechanism of the desert green algae photosystem, we observed the physiological and transcriptome-level responses of Chlorella sp. to high doses of UV-B radiation. The results showed that the reactive oxygen species (ROS) content first increased and then decreased, while the malondialdehyde (MDA) content revealed no notable lipid peroxidation during the UV-B exposure period. These results suggested that Chlorella sp. may have strong system characteristics for scavenging ROS. The antioxidant enzyme system showed efficient alternate coordination, which exhibited a protective effect against enhanced UV-B radiation. DNA damage and the chlorophyll and soluble protein contents had no significant changes in the early irradiation stage; UV-B radiation did not induce extracellular polysaccharides (EPS) synthesis. Transcriptomic data revealed that a strong photosynthetic system, efficient DNA repair, and changes in the expression of genes encoding ribosomal protein (which aid in protein synthesis and improve resistance) are responsible for the high UV-B tolerance characteristics of Chlorella sp. In contrast, EPS synthesis was not the main pathway for UV-B resistance. Our results revealed the potential cell damage repair mechanisms within Chlorella sp. that were associated with high intensity UV-B stress, thereby providing insights into the underlying regulatory adaptations of desert green algae.

Analysis of morphological change mechanism of linear Arthrospira platensis based on transcriptome results

Arthrospira platensis is a kind of filament cyanobacteria, which is mainly helical with a few linear. The shape of the filaments, such as the length and the pitch, may change with the changes in the environment. Natural Arthrospira platensis FACHB793 is linear, although it has become helical due to a mutation introduced in the process of cultivation. To study the molecular mechanism responsible for the morphological changes of the filaments, two samples were isolated from a natural mutant of Arthrospira platensis FACHB793, which were helical shaped (named A793_H) and linear shaped (named A793_L). Transcriptome sequencing, GO and KEGG enrichment analysis showed that the expression of genes related to or involved in peptidoglycan biosynthesis, beta lactam resistance, photosynthetic antenna protein expression, bacterial secretion, and ABC transporter activity changed between the two samples. The expression of murE and murG in the peptidoglycan biosynthesis pathway and that of oppD in beta lactam resistance were all down-regulated in the helical filaments, which may be related to the longer cell wall and higher peptidoglycan synthesis in linear filaments than helical filaments. In helical filaments, the up-regulation of tatC gene expression in bacterial secretion may be related to the secretion of peptidoglycan degrading enzymes, which may help to change the shape from linear to helical. Moreover, apcA and cpcA in photosynthetic antenna protein expression and nrt and nirA in nitrogen metabolism were all down regulated in the helical filaments, which may be due to the deformed shape of A. platensis FACHB793, resulting in decreased photosynthetic activity in helical filaments. This research provides a foundation for elucidating the possible morphogenetic mechanism of Arthrospira platensis.

Antibiotics promoted the recovery of Microcystis aeruginosa after UV-B radiation at cellular and proteomic levels

Elevated UV-B radiation due to ozone layer depletion may prevent the growth of bloom-forming cyanobacteria in aquatic environments, while antibiotic contaminants may cause effects opposite to that of UV-B due to hormesis. This study investigated the influence of a quaternary antibiotic mixture on Microcystis aeruginosa after UV-B radiation through a 15-day exposure test. UV-B radiation extended the lag phase of M. aeruginosa at doses of 600 and 900 mJ/cm², and significantly (p < 0.05) reduced the growth rate and the F_v/F_m value at doses of 300-900 mJ/cm². Although UV-B radiation significantly (p < 0.05) stimulated the microcystin production ability in each cyanobacterial cell, the total microcystin concentration still significantly (p < 0.05) decreased due to the reduction of cell density. Mixed antibiotics and UV-B regulated the proteomic expression profile of M. aeruginosa in different manners. UV-B radiation upregulated 19 proteins and downregulated 49 proteins in M. aeruginosa, while mixed antibiotics upregulated 45 proteins and downregulated 25 proteins in UV-B treated cells. Mixed antibiotics significantly (p < 0.05) stimulated growth and photosynthesis, increased cell density and microcystin concentration, and reduced oxidative stress in UV-B treated cells through the upregulation of proteins involved in photosynthesis, biosynthesis, cell division, oxidation-reduction, gene expression and microcystin synthesis. This study verified the hypothesis that antibiotics accelerated the recovery of M. aeruginosa from UV-B induced damage. A safe threshold of 20 ng/L was suggested for mixed antibiotics (5 ng/L for each antibiotic), in order to eliminate the stimulatory effects of antibiotics on bloom-forming cyanobacteria.

Coordinated downregulation of the photosynthetic apparatus as a protective mechanism against UV exposure in the diatom Corethron hystrix

The effect of ultraviolet radiation (UVR) on photosynthetic efficiency and the resulting mechanisms against UV exposure employed by phytoplankton are not completely understood. To address this knowledge gap, we developed a novel close-coupled, wavelength-configurable platform designed to produce precise and repeatable in vitro irradiation of Corethron hystrix, a member of a genera found abundantly in the Southern Ocean where UV exposure is high. We aimed to determine its metabolic, protective, and repair mechanisms as a function of varying levels of specific electromagnetic energy. Our results show that the physiological responses to each energy level of UV have a negative linear decrease in the photosynthetic efficiency of photosystem II proportional to UV intensity, corresponding to a large increase in the turnover time of quinone reoxidation. Gene expression changes of photosystem II-related reaction center proteins D1, CP43, and CP47 showed coordinated downregulation whereas the central metabolic pathway demonstrated mixed expression of up and downregulated transcripts after UVR exposure. These results suggest that while UVR may damage photosynthetic machinery, oxidative damage may limit production of new photosynthetic and electron transport complexes as a result of UVR exposure.

Prasanna K. Mohanty (1934-2013): a great photosynthetiker and a wonderful human being who touched the hearts of many

Prasanna K. Mohanty, a great scientist, a great teacher and above all a great human being, left us more than a year ago (on March 9, 2013). He was a pioneer in the field of photosynthesis research; his contributions are many and wide-ranging. In the words of Jack Myers, he would be a "photosynthetiker" par excellence. He remained deeply engaged with research almost to the end of his life; we believe that generations of researchers still to come will benefit from his thorough and enormous work. We present here his life and some of his contributions to the field of Photosynthesis Research. The response to this tribute was overwhelming and we have included most of the tributes, which we received from all over the world. Prasanna Mohanty was a pioneer in the field of "Light Regulation of Photosynthesis", a loving and dedicated teacher-unpretentious, idealistic, and an honest human being.

Effects of ultraviolet radiation (UVA+UVB) on young gametophytes of Gelidium floridanum: growth rate, photosynthetic pigments, carotenoids, photosynthetic performance, and ultrastructure

This study investigated the effects of radiation (PAR+UVA+UVB) on the development and growth rates (GRs) of young gametophytes of Gelidium floridanum. In addition, photosynthetic pigments were quantified, carotenoids identified, and photosynthetic performance assessed. Over a period of 3 days, young gametophytes were cultivated under laboratory conditions and exposed to photosynthetically active radiation (PAR) at 80 μmol photons m(-2) s(-1) and PAR+UVA (0.70 W m(-2))+UVB (0.35 W m(-2)) for 3 h per day. The samples were processed for light and electron microscopy to analyze the ultrastructure features, as well as carry out metabolic studies of GRs, quantify the content of photosynthetic pigments, identify carotenoids and assess photosynthetic performance. PAR+UVA+UVB promoted increase in cell wall thickness, accumulation of floridean starch grains in the cytoplasm and disruption of chloroplast internal organization. Algae exposed to PAR+UVA+UVB also showed a reduction in GR of 97%. Photosynthetic pigments, in particular, phycoerythrin and allophycocyanin contents, decreased significantly from UV radiation exposure. This result agrees with the decrease in photosynthetic performance observed after exposure to ultraviolet radiation, as measured by a decrease in the electron transport rate (ETR), where values of ETRmax declined approximately 44.71%. It can be concluded that radiation is a factor that affects the young gametophytes of G. floridanum at this stage of development.

Effect of cerium on photosynthetic pigments and photochemical reaction activity in soybean seedling under ultraviolet-B radiation stress

Effects of cerium (Ce) on photosynthetic pigments and photochemical reaction activity in soybean (Glycine max L.) under ultraviolet-B (UV-B) radiation stress were studied under laboratory conditions. UV-B radiation caused the decrease in chlorophyll content, net photosynthetic rate, Hill reaction activity, photophosphorylation rate and Mg(2+)-ATPase activity. Ce (III) (20 mg L(-1)) could alleviate UV-B-induced inhibition to these photosynthetic parameters because values of these photosynthetic parameters in Ce (III) + UV-B treatment were obviously higher than those with UV-B treatment alone. Dynamic changes of the above photosynthetic parameters show that Ce (III) could slow down the decrease rate of these photosynthetic parameters during a 5-day UV-B radiation and quicken the restoration during recovery period. The final restoration degree of five parameters mentioned above in leaves exposed to low level of UV-B radiation (0.15 W m(2)) was higher than that exposed to high level (0.45 W m(2)). Correlating net photosynthetic rate with other four parameters, we found that the regulating mechanisms Ce (ΠΙ) on photosynthesis under various level of UV-B radiation were not the same. The protective effects of Ce (III) on photosynthesis in plants were influenced by the intensity of UV-B radiation.

Photoregulation of morphological structure and its physiological relevance in the cyanobacterium Arthrospira (Spirulina) platensis

The spiral structure of the cyanobacterium Arthrospira (Spirulina) platensis (Nordst.) Gomont was previously found to be altered by solar ultraviolet radiation (UVR, 280-400 nm). However, how photosynthetic active radiation (PAR, 400-700 nm) and UVR interact in regulating this morphological change remains unknown. Here, we show that the spiral structure of A. platensis (D-0083) was compressed under PAR alone at 30 degrees C, but that at 20 degrees C, the spirals compressed only when exposed to PAR with added UVR, and that UVR alone (the PAR was filtered out) did not tighten the spiral structure, although its presence accelerated morphological regulation by PAR. Their helix pitch decreased linearly as the cells received increased PAR doses, and was reversible when they were transferred back to low PAR levels. SDS-PAGE analysis showed that a 52.0 kDa periplasmic protein was more abundant in tighter filaments, which may have been responsible for the spiral compression. This spiral change together with the increased abundance of the protein made the cells more resistant to high PAR as well as UVR, resulting in a higher photochemical yield.

Effect of moderate UV-B irradiation on Synechocystis PCC 6803 biliproteins

In the present study, we investigated the mechanism of UV-B radiation induced damage to the light harvesting apparatus of the cyanobacterium Synechocystis 6803. Liquid chromatography analysis and spectroscopy investigations performed on phycobilisomes or isolated biliproteins irradiated with moderate UV-B intensity (1.3 W/m(2)) revealed rapid destruction of beta-phycocyanin and a slower damage of the other biliproteins, alpha-phycocyanin and both alpha and beta-allophycocyanin. EPR spin trapping measurements revealed that carbon centered adducts of the spin trap DMPO were formed. This evidence indicates that free radicals produced from bilins probably attack the polypeptide chain of protein inducing its degradation. Our results show that the bilin chromophore is the main target of UV-B irradiation, causing structural changes, which in turn induce reaction of the chromophore with atmospheric oxygen and lead to production of reactive radicals. Our results also demonstrate that beta-phycocyanin is the most affected biliprotein, probably due to the presence of two bilins as chromophore.

Suitability of different photosynthetic organisms for an extraterrestrial biological life support system

In the present era of intensive space and planetary research, efficient life support systems (LSSs) are needed to maintain suitable living conditions when humans move into space, i.e. away from the Earth's atmosphere. Thus far, such suitable conditions on various space flights and on the space stations (Mir and the International Space Station) have been maintained solely via physical and chemical means (transport of O2, H2O and food from the Earth, cleaning and recycling of air and water). However, for long-duration missions to distant destinations, such as exploratory missions to Mars, biological life support systems (BLSSs) may be needed to convert local CO2 and H2O to O2, and to food. As on earth, this conversion process would need to be based on photosynthesis. Use of higher plants and microalgae as BLSS organisms has been intensively studied. Here we review the growth requirements of these two types of photosynthetic organisms, with particular attention to their suitability for use in harsh Martian conditions, i.e. low temperatures, low atmospheric pressure, high CO2 concentration, high UV radiation and dryness.

Effects of solar UV radiation on morphology and photosynthesis of filamentous cyanobacterium Arthrospira platensis

To study the impact of solar UV radiation (UVR) (280 to 400 nm) on the filamentous cyanobacterium Arthrospira (Spirulina) platensis, we examined the morphological changes and photosynthetic performance using an indoor-grown strain (which had not been exposed to sunlight for decades) and an outdoor-grown strain (which had been grown under sunlight for decades) while they were cultured with three solar radiation treatments: PAB (photosynthetically active radiation [PAR] plus UVR; 280 to 700 nm), PA (PAR plus UV-A; 320 to 700 nm), and P (PAR only; 400 to 700 nm). Solar UVR broke the spiral filaments of A. platensis exposed to full solar radiation in short-term low-cell-density cultures. This breakage was observed after 2 h for the indoor strain but after 4 to 6 h for the outdoor strain. Filament breakage also occurred in the cultures exposed to PAR alone; however, the extent of breakage was less than that observed for filaments exposed to full solar radiation. The spiral filaments broke and compressed when high-cell-density cultures were exposed to full solar radiation during long-term experiments. When UV-B was screened off, the filaments initially broke, but they elongated and became loosely arranged later (i.e., there were fewer spirals per unit of filament length). When UVR was filtered out, the spiral structure hardly broke or became looser. Photosynthetic O(2) evolution in the presence of UVR was significantly suppressed in the indoor strain compared to the outdoor strain. UVR-induced inhibition increased with exposure time, and it was significantly lower in the outdoor strain. The concentration of UV-absorbing compounds was low in both strains, and there was no significant change in the amount regardless of the radiation treatment, suggesting that these compounds were not effectively used as protection against solar UVR. Self-shading, on the other hand, produced by compression of the spirals over adaptive time scales, seems to play an important role in protecting this species against deleterious UVR. Our findings suggest that the increase in UV-B irradiance due to ozone depletion not only might affect photosynthesis but also might alter the morphological development of filamentous cyanobacteria during acclimation or over adaptive time scales.

Protective effect of supplemental low intensity white light on ultraviolet-B exposure-induced impairment in cyanobacterium Spirulina platensis: formation of air vacuoles as a possible protective measure

Intact trichomes of Spirulina platensis were exposed to 1-5 h of low (0.2 mW cm(-2)) or high (0.6 mW cm(-2)) intensity UV-B (280-320 nm) radiation, alone or with photosynthetically active radiation (PAR) of supplemental 50 muE m(-2) s(-1) white light (WL). The mitigating effect of supplemental WL on UV-B induced alterations in Spirulina were investigated by monitoring time-dependent change in photosystem (PS) II mediated O(2) evolution, absorption, circular dichroism (CD) spectra, and ultrastructure. At low intensity, UV-B induced loss in PS II-catalyzed O(2) evolution, but caused no change in the absorption spectrum. At high intensity, UV-B caused a decrease in absorption by phycobilisomes (PBsomes), which was only partly prevented by the presence of low-intensity supplemental WL. The CD spectral analysis revealed that UV-B exposure caused time-dependent enhancement of the negative psi-type bands at 452 and 689 nm, reflecting alterations in the macroaggregation of chlorophyll-protein complexes. This enhancement of negative PS II-type bands was substantially arrested by the presence of supplemental WL exposure, even when UV-B exposure was continued for 5 h. These changes in UV-B-induced CD spectrum suggest alterations in the antenna structure of Spirulina involving both PBsomes and Chlorophyll a. Thus, supplemental low intensity WL arrests, to large extent, the macroaggregation of pigment-protein complexes. Furthermore, the electron micrographs of Spirulina revealed that UV-B exposure caused disorganization of the cellular ultrastructure, while the inclusion of supplemental WL enhanced the formation of air vacuoles in Spirulina. We suggest that the formation of vacuoles by supplemental WL is a protective feature against UV-B.

The Effects of Salt Stress on Photosynthetic Electron Transport and Thylakoid Membrane Proteins in the Cyanobacterium Spirulina platensis

The response of Spirulina (Arthrospira) platensis to high salt stress was investigated by incubating the cells in light of moderate intensity in the presence of 0.8 M NaCl. NaCl caused a decrease in photosystem II (PSII) mediated oxygen evolution activity and increase in photosystem I (PSI) activity and the amount of P700. Similarly maximal efficiency of PSII (Fv/Fm) and variable fluorescence (Fv/Fo) were also declined in salt-stressed cells. Western blot analysis reveal that the inhibition in PSII activity is due to a 40 % loss of a thylakoid membrane protein, known as D1, which is located in PSII reaction center. NaCl treatment of cells also resulted in the alterations of other thylakoid membrane proteins: most prominently, a dramatic diminishment of the 47-kDa chlorophyll protein (CP) and 94-kDa protein, and accumulation of a 17-kDa protein band were observed in SDS-PAGE. The changes in 47-kDa and 94-kDa proteins lead to the decreased energy transfer from light harvesting antenna to PSII, which was accompanied by alterations in the chlorophyll fluorescence emission spectra of whole cells and isolated thylakoids. Therefore we conclude that salt stress has various effects on photosynthetic electron transport activities due to the marked alterations in the composition of thylakoid membrane proteins.

UV-B effects on Antarctic Chlorella sp cells

Growth of Antarctic Chlorella sp cells was measured in cultures irradiated with 30 kJ m(-2) UV-B (280-320 nm). The specific growth rate immediately after the lag phase was 0.36+/-0.06 and 0.26+/-0.03 day(-1) for unirradiated cultures and cultures irradiated with UV-B, respectively, UV-B irradiation significantly decreased ascorbate content by 54.5%, and increased the ascorbyl radical content/ascorbate content ratio by 2.25-fold in algae cultures in log phase. UV-B exposure significantly decreased by 95, 62 and 71% the content of alpha-tocopherol, beta-carotene and total thiols, respectively, in cells in log phase of development. The cellular content of alpha-tocopherol, beta-carotene and total thiols was reduced by 78, 43 and 44%, respectively in stationary phase, as compared to the antioxidant content in the cells during log phase of development. UV-B exposure reduced the content of alpha-tocopherol and total thiols in stationary phase of development by 64 and 91%, respectively, as compared to unirradiated cells. The content of beta-carotene in stationary phase was not affected by UV-B exposure. The results presented here suggest that increased UV-B radiation was responsible for the development of oxidative stress conditions, assessed as the ascorbyl radical content/ascorbate content ratio, in Antarctic Chlorella sp cells. Moreover, a significant decrease in the content of both lipid and water soluble antioxidants might contribute to establish oxidative stress in the cells.

Determination of photosystem II subunits by matrix-assisted laser desorption/ionization mass spectrometry

Photosystem II of higher plants and cyanobacteria is composed of more than 20 polypeptide subunits. The pronounced hydrophobicity of these proteins hinders their purification and subsequent analysis by mass spectrometry. This paper reports the results obtained by application of matrix-assisted laser desorption/ionization mass spectrometry directly to isolated complexes and thylakoid membranes prepared from cyanobacteria and spinach. Changes in protein contents following physiopathological stimuli are also described. Good correlations between expected and measured molecular masses allowed the identification of the main, as well as most of the minor, low molecular weight components of photosystem II. These results open up new perspectives for clarifying the functional role of the various polypeptide components of photosystems and other supramolecular integral membrane complexes.