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DPA membranes from Allochromatium vinosum cells, in which carotenoid biosynthesis was inhibited using diphenylamine (DPA) were obtained, into which rhodopin was incorporated. The LH2 complex with rhodopin content of 85% was isolated. Using a test for the thermal stability of LH2 complexes (DPA and with incorporated rhodopin), it was established that carotenoids of the early stages of biosynthesis (≤1 molecules per complex) did not interfere with rhodopin incorporation. It was found that when the LH2 complex with incorporated rhodopin was irradiated with light at the wavelength of 502 nm, BChl850 was photobleached at a rate close to that in the control LH2 complex. This indicates that rhodopin, after being incorporated into the DPA LH2 complex, is capable of generating singlet oxygen under illumination. Previously obtained data on heterogeneity of the carotenoid composition in DPA LH2 complexes (variation in the number of individual carotenoids molecules per complex in the general population) and our earlier suggestion about the structural role of carotenoids, namely, their ability to stabilize the LH2 complexes, were confirmed. Based on analysis of our results, as well as of the literature data, the interaction of singlet oxygen and carotenoids is discussed. [ABSTRACT FROM AUTHOR]

The interaction of singlet oxygen with bacteriochlorophyll (BChl) in the membranes, LH2 light-harvesting complexes and pigment extracts from Allochromatium (Alc.) vinosum strain MSU (previously Alc. minutissimum) was studied. The ability of carotenoids illuminated with light within their absorption region to generate singlet oxygen, which oxidizes BChl, was confirmed; BChl itself was not active in this process and acted a target for singlet oxygen. A sharp decrease in the amount of oxidized BChl in the LH2 complex under illumination in the carotenoid region was noted only at the carotenoid/complex ratio of about 0.1, i.e., when there is less than 1 carotenoid molecule per 10 LH2 complexes. It is assumed that carotenoids from the early stages of biosynthesis (neurosporene, ζ-carotene) are more active in the generation of singlet oxygen under illumination than more mature carotenoids. BChl photooxidation in LH2 complexes occurred only in the presence of oxygen. A decrease in oxygen concentration by 7 or more times resulted in a slowdown of the process or its complete stop. In model systems (pigment extract of the LH2 complex retaining the BChl/carotenoids ratio), BChl under illumination becomes both the main generator of singlet oxygen and a target for the action of the latter. Under these conditions, carotenoids do not protect BChl from oxidation. The ability of BChl850 in the LH2 complex to be oxidized by singlet oxygen suggests that it can be used as a natural sensor for this agent. [ABSTRACT FROM AUTHOR]

The possibility of singlet oxygen generation in five species of photosynthetic bacteria under irradiation with light was studied using the Singlet Oxygen Sensor Green (SOSG) dye that, when singlet oxygen is bound, is transformed into a fluorescent endoperoxide form. Irradiation was carried out using light of different spectral composition: red (absorption of bacteriochlorophyll, BChl) and white (absorption of BChl and carotenoids). No singlet oxygen generation was detected under red light irradiation, which indicated that BChl was not involved in this process. Under white light, as the irradiation time increased, an increase in SOSG fluorescence was recorded in the membranes of four bacterial species: Allochromatium vinosum MSU, Rhodobacter sphaeroides G1C, Rba. blasticus K-1 and Rhodopseudomonas faecalis. In Rba. sphaeroides, no increase in SOSG fluorescence was observed. It is assumed that in the white light this process occurs due to the pigments that absorb in the blue-green region of the spectrum, i.e. carotenoids with 8‒11 conjugated double bonds (neurosporene, spheroidene, lycopene, and rhodopin). Similar to Trolox, SOSG may slow down the bleaching process of BChl850 in the membranes of Alc. vinosum strain MSU and may therefore effectively perform the role of a singlet oxygen trap. [ABSTRACT FROM AUTHOR]

The direct action of singlet oxygen on the bacteriochlorophyll (BChl) of light-harvesting complexes in the membranes of four species of purple non-sulfur and sulfur photosynthesizing bacteria with and without carotenoids was studied. It was found that BChl in carotenoidless samples is generally more resistant to the action of singlet oxygen compared to the control. It is assumed that carotenoids are not required to protect BChl of bacterial light-harvesting complexes from singlet oxygen, and in the classic work by Griffith et al. [1] the apoptosis process in carotenoidless mutant cells, which involves the destruction of complexes, the appearance of monomeric BChl, and the generation of singlet oxygen caused by BChl, followed by BChl oxidation, was mistakenly attributed to the protective function of carotenoids. [ABSTRACT FROM AUTHOR]

The effect of singlet oxygen was studied in the system of Allochromatium (Alc.) vinosum MSU membranes, rose bengal, and light (547−600nm) with and without a quencher. In the system without a quencher, bleaching of the BChl850 band of the LH2 light-harvesting complex was observed and an absorption peak emerged at 698 nm, which belonged to the oxidized product, 3-acetyl-chlorophyll. The efficiency of five quenchers of singlet oxygen for neutralization of the effect of singlet oxygen on bacteriochlorophyll in the light-harvesting complexes of the membranes Alc. vinosum MSU was studied: sodium ascorbate, histidine, imidazole, trolox, and sodium azide. All of them, with the exception of sodium azide, neutralized successively the effect of singlet oxygen. [ABSTRACT FROM AUTHOR]

Seven different carotenoids with the number of conjugated double bonds (N) from 5 to 11 were incorporated in vitro into carotenoidless complexes LH2 of the sulfur bacterium Allochromatium vinosum strain MSU. The efficiency of their incorporation varied from 4 to 99%. The influence of N in the carotenoid molecules on the energy transfer efficiency from these pigments to bacteriochlorophyll (BChl) in the modified LH2 complexes was studied for the first time. The highest level of energy transfer was recorded for rhodopin (N = 11) and neurosporene (N = 7) (37 and 51%, respectively). In the other carotenoids, this parameter ranged from 11 to 33%. In the LH2 complexes studied, we found no direct correlation between the decrease in N in carotenoids and increase in the energy transfer efficiency from these pigments to BChl. [ABSTRACT FROM AUTHOR]

We investigated the formation of the B800-850 complex in cells of the bacterium Rhodopseudomonas palustris AB illuminated by red and blue light under anaerobic growth conditions. Under red illumination, the B800-850 complex was assembled with a reduced absorption band at 850 nm. The results of re-electrophoresis of the B800-850 complex and oxidation in the presence of potassium iridate suggest its heterogeneity. It may be a mixture of two complexes (B800 and B800-850). The B800-850 complex lacks the capacity for conformational transitions if assembled under blue illumination. Accordingly, the light-harvesting complex assembled in the blue light contains polypeptides that are not synthesized under normal conditions or at increased or decreased light intensities. The mechanism of regulation of the synthesis of the polypeptides of light-harvesting the B800-850 complex and its dependence on the spectral composition of the light is discussed. [ABSTRACT FROM AUTHOR]

The article presents research on the carotenoid-photosensitized oxidation of bacteriochlorophyll (BChl) dimers in complexes B800-850 that are light-harvesting in "Allochromatium minutissimum" cells. The study examined the photooxidation of bacteriochlorophyll dimers that are sensitized by light absorbed by carotenoids in Alc. minutissimum cells. Results of the study include the high resistance of BChl to white light in pigment-protein complexes in which carotenoid synthesis is inhibited.

Treatment of some sulfur bacteria ( Allochromatium minutissimum, Thiorhodospira sibirica, and Ectothiorhodospira halovacuolata WN22) with dioxane results in formation of the bacteriochlorophyll form B820 in the light harvesting complex LH2. This form characterized by absorption maximum at 820 nm has the same absorption spectrum as B820 subcomplex from LH1 complex. Appearance of the B820 form was accompanied by a sharp decrease in absorption in the carotenoid region. This phenomenon observed in all LH2 complexes investigated may be attributed to formation of colorless carotenoid aggregates. This is very similar to the previously reported dissociation of the LH1 complex with carotenoids into B820 subcomplexes. Although the B820 form corresponded the bacteriochlorophyll dimer, its circular dichroism spectrum showed that pigment molecules in this dimer exhibit different interaction than those in the B820 subcomplex. The dioxane treatment of LH2 complexes isolated from Rhodopseudomonas palustris bacteria grown under normal or low intensity illumination did not result in formation of such dimers. It is suggested that bacteriochlorophyll B820 formation is related to unique structure of LH2 complexes from the sulfur bacteria. [ABSTRACT FROM AUTHOR]

Chromatophores and peripheral light-harvesting complexes B800–850 with a trace of carotenoids were isolated from Chromatium minutissimum cells in which carotenoid biosynthesis was inhibited by diphenylamine. Three methods previously used for the reconstitution of carotenoids into either the light-harvesting (LH1) type complexes or reaction centers (RC) of carotenoidless mutants were examined for the possibility of carotenoid reconstitution into the carotenoid depleted chromatophores. All these methods were found to be unsuitable because carotenoid depleted complex B800–850 from Chr. minutissimum is characterized by high lability. We have developed a novel method maintaining the native structure of the complexes and allowing reconstitution of up to 80% of the carotenoids as compared to the control. The reconstituted complex has a similar CD spectrum in the carotenoid region as the control, and its structure restores its stability. These data give direct proof for the structural role of carotenoids in bacterial photosynthesis. [ABSTRACT FROM AUTHOR]

Presents a study which discussed the importance of carotenoids in the protection of bacteriochlorophyll clusters against photooxidation in light-harvesting complexes of photosynthetic bacteria. Role of carotenoids in photosynthesis; Analysis of the protective function of carotenoids; Mechanism of the protective effect of carotenoids against photooxidation.

The effect of the qualitative and quantitative content of carotenoids on the assembly of light-harvesting complexes isolated from the heat-resistant purple sulfur bacterium Thermochromatium (T.) tepidum was studied. To decrease the content of carotenoids in the photosynthetic membranes and complexes, an inhibitor of carotenonegesis, diphenylamine (DPA), was used. The minimum carotenoid content of 40% was observed in T. tepidum cells grown in the presence of 120 μM DPA. At higher DPA concentrations, the growth of the culture stopped. Photosynthetic membranes and LH2 and LH1-RC complexes were isolated from DPA-treated cells; carotenoid content of these structures was 45 and 55%, respectively. Their spectral characteristics in the near IR region did not depend on the amount of carotenoids in the complexes and were similar to those of the native wild-type complexes. Simultaneously with a general decrease in the content of carotenoids, carotenoids from earlier stages of biosynthesis, such as neurosporene, ζ-carotene, and their hydroxy derivatives, appeared in the DPA-treated complexes. Emergence in DPA-treated complexes of carotenoids with a shorter chain of conjugated double bonds than in the control resulted in an increase in the efficiency of energy transfer from carotenoids to bacteriochlorophyll. [ABSTRACT FROM AUTHOR]

Regardless of cultivation conditions, two types of LH2 complexes are always assembled in photosynthetic membranes of Thiorhodospira (T.) sibirica: short-wave B800−830 and long-wave B800−850. The present work studied the effect of qualitative and quantitative content of carotenoids on assembling of these complexes. Diphenylamine (DPA), an inhibitor of carotenoid biosynthesis, was used to decrease the content of carotenoids in photosynthetic membranes and the complexes. In the cells of T. sibirica strain A12T, inhibition of carotenoid biosynthesis ranged from 40 to 50%. Complete suppression of carotenoid biosynthesis was achieved only once, which resulted in absorption spectrum changes of the DPA-LH2 complexes and lower stability of their structure; they were often destroyed during the isolation. The use of DPA with the cells of T. sibirica strain Kir-3 was more efficient. From inhibited cells of this strain, B800−850 and B800−830 complexes were isolated, in which the carotenoid content decreased to 80, 50, and 10%. Their spectral characteristics in the near infrared region were independent of the carotenoid quantity in the complexes and were similar to those of native complexes of the wild-type. At higher DPA concentration in the cultivation medium, accompanied by a decrease of the overall carotenoid content, such carotenoids of the earlier stages of biosynthesis as neurosporene, ζ-carotene, and their hydroxy derivatives, were found in the complexes. The efficiency of energy transfer from carotenoids to bacteriochlorophyll in all LH2 complexes was not shown to depend on the composition and quantity of carotenoids in the samples and was ~40%. Since the near-infrared spectrum of the LH2-complex of the B800−850 type was similar to that of the B800−850 complexes from Rhodospirillum molishianum, with 8 pairs of α/β-heterodimers forming the basis of their structure, it is possible to assume that the structure of the studied B800−850 complex from T. sibirica also consists of 8 pairs of α/β-heterodimers. A single carotenoid molecule is known to occur per the α/β-heterodimer; therefore, on average less than one carotenoid molecule (8 × 0.1 = 0.8) occurs per an LH2 complex from T.sibirica with 10% carotenoid content. This implies that the population of LH2 complexes contains both carotenoidless LH2 complexes and the complexes containing one or more carotenoid molecules. Obviously, LH2 complexes of both types are assembled regardless of the quantity of carotenoids per a complex, and carotenoids are not the required component for their correct assembly in vivo. [ABSTRACT FROM AUTHOR]

The results of assembling the light-harvesting complexes in the cells of the purple sulfur bacterium Thiorhodospira (T.) sibirica strain Kir-3 while suppressing the biosynthesis of carotenoids with diphenylamine (DPA) were studied. LH2 complexes (B800-850 and B800-830) with different carotenoid composition were isolated from the cells obtained. Maximum inhibition of carotenoid biosynthesis (~90% of the control) was reached at an inhibitor concentration of 53.25 μM (9 mg/L). It was established that changes in the qualitative and quantitative composition of carotenoids do not affect the assembly of B800-830 and B800-850 complexes. It is assumed that, in the population of DPA-LH2 complexes from T. sibirica strain Kir-3, both the carotenoidless complexes and the complexes containing one or two carotenoid molecules can be assembled. These results support the hypothesis that carotenoids are not required for assembling B800-850 and B800-830 complexes. [ABSTRACT FROM AUTHOR]

The effect of light with different spectral composition: white, red and blue-green (the first one is absorbed by all the pigments of the cell, and the second and the third ones are absorbed by bacteriochlorophyll and carotenoids, respectively) on culture growth, carotenoid synthesis, and assembly of the light-harvesting complexes was studied for the purple sulfur bacteria Allochromatium (Alc.) minutissimum MSU and Alc. vinosum ATCC 17899. The working hypothesis on the growth of bacteria under blue-green illumination (absorbed by carotenoids) resulting in the inhibition of cell growth was tested. When equalizing the light by luxes, the intensity of illumination for each luminous flux was 1800 lx (white and red light, 4 W/m2; bluegreen light, 0.4 W/m2). The growth of the cells was recorded in white and red light, while in blue-green light an insignificant increase was observed only for Alc. vinosum at the end of the experiment (7-9 days). Regardless of the spectral composition of the light the B800-850 type LH2 complex was always assembled in Alc. minutissimum membranes, and two short-wave LH2 complexes of В800-820 and В800-840 type were assembled in the membranes of Alc. vinosum. Upon smoothing and increasing the luminous flux up to 6 W/m2 for every illumination mode, both cultures grew with approximately equal rates in blue-green light. In the membranes of Alc. minutissimum and Alc. vinosum the same types of LH2 complexes were assembled as in the case of 1800 lx illumination. It was found that blue-green light did not inhibit cell growth. At illumination of the cells collected at the end of the experiment with blue-green light for 6 h, no photooxidation of BChl850 was registered. However, in the membranes from the cells oxygen-saturated at isolation, ~50% of BChl850 was oxidized after 30 minutes of illumination. In the course of cell growth, oxygen is probably completely consumed and anaerobic conditions develop inside the cell. Under these conditions, formation of reactive oxygen species, BChl photooxidation and inhibition of the cell growth become impossible. [ABSTRACT FROM AUTHOR]

Singlet fission of carotenoid excitation is studied in purple phototrophic bacterium Thermochromatium tepidum. Using time-resolved EPR and magnetic field-induced modulation of fluorescence yield it is shown that the concept of intramolecular excitation fission developed in a number of publications is not supported by the experimental results. The obtained data favor intermolecular fission mechanism involving two carotenoid molecules. [ABSTRACT FROM AUTHOR]

An alkaliphilic nonsulfur purple bacterial (NPB) strain “Green” was isolated from sediments of the littoral zone of the soda lake (mineralization 22 g/l, pH 9.5) in the Barguzin River valley (Eastern Siberia). The cells of the new isolate are ovoid or polymorphic at latter stages. The photosynthetic membrane structures are of vesicular type. Bacteriochlorophyll a and carotenoids of both spheroidene and spirilloxanthin type are the photosynthetic pigments. Two light-harvesting systems (LH1 and LH2) are present. The new isolate is a photoheterotroph and a facultative aerobe. It grows well in the dark on organic substrates; anaerobic phototrophic growth is poor. The isolate is alkaliphilic with pH optimum of 8.5–9.5. The most abundant cell growth occurred at 5–40 g/l NaCl (optimum at 10 g/l) and 30 °C. The DNA G+C base content was 69.9 mol %. Analysis of 16S rRNA gene sequences revealed a 10% difference with the most closely related NPB ( Rhodobacter species). Rubrimonas cliftoensis, a bacteriochlorophyll a-containing bacterium, is the closest relative (93.3% similarity). It is proposed that strain “Green” should be placed in the new genus and new species Rubribacterium polymorphum gen. nov., sp. nov. GenBank accession number: 16S rRNA-EU857676. [ABSTRACT FROM AUTHOR]

A novel strain, SHET, of aerobic bacteriochlorophyll a-containing bacteria was isolated from the surface layer of bottom sediments from the soda lake Shuluutai-Ekhe-Torom (Chita oblast, Eastern Siberia, Russia). The lake water has a total mineralization of 30 g/l and a pH of 9.2. The cells of strain SHET are cocci or short rods, which reproduce by uniform division. The cells are motile by means of flagella. The cell wall structure is of the gram-negative type. Sparse intracytoplasmic membrane vesicles are located close to the cell wall. The new isolate is an obligate aerobe and facultative alkaliphile which grows in a pH range of 7.5–9.5 (with an optimum at pH 8.5–9.0). The best growth of strain SHET occurred at 2.0 g/l NaCl and 23–28°C. Photosynthetic pigments are represented by bacteriochlorophyll a, with the maximum absorption at 865 nm in the in vivo spectrum, and carotenoids (spirilloxanthin derivatives). Analysis of the 16S rRNA gene sequences demonstrated that strain SHET is closely related to Roseococcus thiosulfatophilus of the α-1 subclass of Proteobacteria (98.6 % similarity). The DNA G+C base content is 69.1 mol %. Unlike Rsc. thiosulfatophilus, strain SHET grows well on sugars and glycerol and is not capable of utilizing thiosulfate as an energy source. The new isolate is a facultative alkaliphile and reduces nitrates to nitrites. On the basis of its phenotypic and genetic characteristics, strain SHET was described as a new species of the genus Roseococcus, Rsc. suduntuyensis sp. nov. [ABSTRACT FROM AUTHOR]

A novel strain, alga-05, of alkaliphilic purple nonsulfur bacteria was isolated from sediments of a small saline (60 g/l) soda lake near Lake Algin (Barguzin Valley, Buryat Republic, Russia). These bacteria contain bacteriochlorophyll a and carotenoids of the alternative spirilloxanthin group with predominating demethylspheroidenone. They are facultative anaerobes; their photosynthetic structures are of the vesicular type and arranged along the cell periphery. Growth of this strain is possible in a salinity range of 5–80 g/l NaCl, with an optimum at 20 g/l NaCl. Best growth occurred at 20–35°C. Analysis of the 16S rRNA gene sequences demonstrated that the studied isolate is closely related to the alkaliphilic purple nonsulfur bacterium Rhodobaca bogoriensis (99% similarity) isolated from soda lakes of the African Rift Zone. According to the results of DNA-DNA hybridization, strain alga-05 has a 52% similarity with the type species of the genus Rhodobaca. On the basis of the obtained genotypic data and some phenotypic properties (dwelling in a hypersaline soda lake of Siberia, moderate halophily, ability to grow at relatively low temperatures, etc.), the isolated strain of purple bacteria was described as a new species of the genus Rhodobaca, Rca. barguzinensis sp. nov. [ABSTRACT FROM AUTHOR]

A new nonmotile purple sulfur bacterium (strain M9) was isolated from the steppe soda lake Lake Dzun Uldziin Nur (pH 9.4; mineralization, 3.3%) situated in southeastern Mongolia. Individual cells appear as vibrios 0.3–0.5 × 0.7–1 μm in size. The dividing cells often do not separate from each other, forming an almost closed ring. The internal photosynthetic membranes are represented by concentric lamellae lining the cell wall. Photosynthetic pigments are bacteriochlorophyll a and carotenoids of the spirilloxanthin series. The main carotenoid (>96%) is spirilloxanthin. Two typical light-harvesting complexes (LH1 and LH2) are present in the membranes in a 1 : 1 ratio. The bacterium is an anaerobe and facultative photoorganoheterotroph. Photolithoautotrophic growth on sulfide is scarce. Thiosulfate is utilized as an electron donor only in the presence of organic matter. Globules of elemental sulfur are formed as an intermediary product of sulfide and thiosulfate oxidation and are deposited outside the cells. The end product of oxidation is sulfate. In the presence of sulfide and carbonates, acetate, lactate, malate, pyruvate, propionate, succinate, and fumarate are used as additional sources of carbon in anoxygenic photosynthesis. Vitamins are not required. The bacterium is an alkaliphile, the pH optimum is at 8.3–9.1, the pH range is 7.6–10.1. The optimum NaCl concentration in the medium is 1 to 7%; the range is 0.5 to 0.9%. The optimum carbonate content in the medium is 2%; the range is 1 to 10%. The best growth occurs at 30–35°C. The DNA G+C content is 57.5 mol %. According to the results of analysis of the 16S rRNA gene sequences, the new isolate M9 belongs to the phylogenetic cluster containing representatives of the family Ectothiorhodospiraceae within the class “Gammaproteobacteria.” In this class, the new isolate forms a new branch, which occupies an intermediate position between the representatives of the genera Ectothiorhodospira and Thiorhodospira. Based on the phenotypic and genetic characteristics, the new purple sulfur bacterium was assigned to a new species of a new genus of the family Ectothiorhodospiraceae, Ectothiorhodosinus mongolicum gen. nov., sp. nov. [ABSTRACT FROM AUTHOR]