Species Of Photosynthetic Bacteria Research Articles

Carboxysomes: Green Solutions for Sustainable Agriculture and Renewable Energy

Carboxyomes are emerging as potential tools in the sustainable agriculture and biofuel production. They are proteinaceous organelles that are distributed in different species of photosynthetic bacteria and archaea and are significantly involved in carbon fixation. They enable the organisms to thrive in diverse environmental conditions and enhance the efficacy of photosynthesis. They have shown promising potential to address challenging regarding global food security, climate change and renewable energy. These nanocompartments encapsulate Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), an enzyme that catalyzes the first step of the Calvin-Benson-Bassham cycle that is a key pathway in carbon fixation. These specialized structures concentrate CO2 around RuBisCO ad shield it from the competing oxygen, thereby optimizing the efficiency of photosynthetic process and minimize wasteful photorespiration. The photosynthetic organisms having carboxysomes are therefore ideal candidates for sustainable agriculture as they are able to thrive in the harsh conditions such as low CO2 concentrations, water scarcity and high temperatures. They are recently being studied for enhancing the productivity and yield of crops. Efforts are being exerted to introduce carboxysome shell proteins and RuBisCO enzymes into crop genomes. Researchers are aiming to create plants with the ability to efficiently capture and utilize CO2 for photosynthesis. These genetically modified crops can potentially increase crop yields, improve water and nutrient efficiency significantly. Moreover, these carboxysomal transgenes are also able to enhance the resilience of GM plants to different environmental stressors such as drought and heat, and ultimately contribute to global food security and sustainable agriculture. In addition to their applications in agriculture, carboxysomes hold promise for biofuel production as well, as they are capable of carbon fixation. They have enabled researchers to engineer photosynthetic organisms to produce renewable fuels such as ethanol, biodiesel and hydrogen. Furthermore, nanobioreactors that are based on carboxysomes can contribute protectively against climate change as they have the potential to mitigate greenhouse gas emissions by capturing CO2 from industrial emissions and converting it into valuable bioproducts. This approach represents a promising avenue to address global challenges of food insecurity, climate crisis and rapidly declining non-renewable energy sources. If leveraged appropriately, these nanocompartments can significantly contribute in paving a way for more sustainable and resilient future.

Carotenoids are Probably Involved in Singlet Oxygen Generation in the Membranes of Purple Photosynthetic Bacteria under Light Irradiation

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.

Identification and antioxidant activity of carotenoids from superfine powder of Rhodobacter sphaeroides

The interest in carotenoids from the natural-antioxidant point of view has recently risen sharply because of their substantial health benefits. Here, we report the identification and antioxidant activity of carotenoids extracted by superfine grinding from Rhodobacter sphaeroides 3757 which is a species of photosynthetic bacteria. After separation and purification by silica gel column chromatography and reversed-phase high performance liquid chromatography (RP-HPLC), the four major carotenoids from the superfine powder of R. sphaeroide 3757 were identified as bixin, hydroxyspheroidenone, 3,3¢,4¢-tetrahydrospirilloxanthin-20-al, and spheroidenone by reversed phase - high performance liquid chromatography - atmospheric pressure chemical ionization - mass spectrometry (RP-HPLC-APCI-MS). The antioxidant activity of the carotenoids extracted after superfine grinding of dry biomass of R. sphaeroide 3757 was higher than that after ultrasonic treatment. When the ratio solvent-to-solid was 30, 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical-scavenging activity, reducing power, and lipid peroxidation inhibitory activity of the extracts after superfine grinding were 74.0% ± 3.1%, 0.497 ± 0.022, and 77.6% ± 3.2%, respectively. By contrast, the extracts after ultrasonic treatment, these numbers were 61.0% ± 2.5%, 0.328 ± 0.014, and 55.2% ± 2.3%, respectively. These results indicate that carotenoids from R. sphaeroide 3757 show a significant antioxidant activity in vitro in a concentration-dependent manner, and that superfine grinding is the optimal extraction method. We hope to provide scientific guidance for commercial production of natural- antioxidant and functional food from carotenoids of R. sphaeroide.

Observation of hybrid artificial photosynthetic membranes using peripheral and core antennae from two different species of photosynthetic bacteria by AFM and fluorescence micro-spectroscopy

The primary process of photosynthesis involves the absorption of sun-light by antenna complexes and subsequent transfer of excitation energy toward the reaction center where a charge separation takes place. This process is usually performed in photosynthetic membranes. The photosynthetic apparatus from purple photosynthetic bacteria contains the reaction center (RC) and two types of antenna complexes, the core antenna light-harvesting 1 (LH1) complex and the peripheral light-harvesting 2 (LH2) complex, and is embedded in these membranes. The arrangement of these light-harvesting pigment–protein complexes in the photosynthetic membranes does have an influence on the light-harvesting performance in the primary process of photosynthesis. In this study, we have constructed hybrid artificial photosynthetic membranes by introducing both LH2 complexes (from Rhodopseudomonas acidophila) and LH1-RC core complexes (from Blastochloris viridis) to a lipid bilayer system composed of Egg PC. The architecture of the artificial photosynthetic membranes can be varied by controlling the ratio of added LH2 to LH1-RC core complexes (LH1-RC/LH2 = 1/1, 1/2, 1/5, 1/12). Previous studies using transmission electron microscopy (TEM) showed that the packing of LH1-RC in a trigonal lattice was changed to orthorhombic by increasing the ratio of LH2 to LH1-RC. We further investigated the local arrangement of the pigment–protein complexes using high-resolution atomic force microscopy and postulate some types of arrangement depending on the condition of the sample preparation. LH2 to LH1 energy transfer was studied using confocal laser fluorescence microscopy with 400 nm spatial resolutions. We found a difference in the ratio of the fluorescence intensity emitted from LH2 and LH1 depending on the local arrangement of these antenna complexes in the hybrid artificial photosynthetic membranes. The effect of the arrangement of the pigment–protein complexes on the light harvesting efficiency is discussed.

Interdependence of carotenoid biosynthesis and assembly of pigment-protein complexes in Ectothiorhodospira haloalkaliphila cells

In the majority of species of photosynthetic bacteria, the lightharvesting antenna consists of two typesof pigment–protein complexes—peripheral LH2 andnearcentral LH1—which are located in the innercytoplasmic membrane [1]. The LH1 complex surrounds the reaction center (RC), forming a “core”complex with it (or the LH1–RC complex). Bothlightharvesting complexes were obtained in the pureform and are well studied [2–6]. Depending on thespecies of bacteria, the LH1 complex has the majorabsorption maximum at ~865–890 nm in the nearinfrared region of the spectrum, whereas the LH2 complex has two maxima (at ~800 and ~850 nm) [2, 3]. Inthe purple bacteria, these complexes are constructedaccording to a similar modular principle and consist oftwo types of polypeptides (α and β), bacteriochlorophyll (BChl), and carotenoids [4–6]. Carotenoids areadditional pigments. They perform several importantfunctions in vivo (lightharvesting, protecting, andstructural functions) [7, 8]. It was stated that the LH2complexes are not assembled in the cells of carotenoidless mutants of purple nonsulfur bacteria [9].There is another approach to obtaining carotenoidless LH2 complexes—the use of an inhibitor ofcarotenogenesis during cell culturing. Using theinhibitor diphenylamine (DPA), we obtained carotenoidless cells (DPA cells) of the purple sulfur bacterium

Static and Dynamic Protein Impact on Electronic Properties of Light-Harvesting Complex LH2

A comparative analysis of the temperature dependence of the absorption spectra of the LH2 complexes from different species of photosynthetic bacteria, i.e., Rhodobacter sphaeroides, Rhodoblastus acidophilus, and Phaeospirillum molischianum, was performed in the temperature range from 4 to 300 K. Qualitatively, the temperature dependence is similar for all of the species studied. The spectral bandwidths of both B800 and B850 bands increases with temperature while the band positions shift in opposite directions: the B800 band shifts slightly to the red while the B850 band to the blue. These results were analyzed using the modified Redfield theory based on the exciton model. The main conclusion drawn from the analysis was that the spectral density function (SDF) is the main factor underlying the strength of the temperature dependence of the bandwidths for the B800 and B850 electronic transitions, while the bandwidths themselves are defined by the corresponding inhomogeneous distribution function (IDF). Slight variation of the slope of the temperature dependence of the bandwidths between species can be attributed to the changes of the values of the reorganization energies and characteristic frequencies determining the SDF. To explain the shift of the B850 band position with temperature, which is unusual for the conventional exciton model, a temperature dependence of the IDF must be postulated. This dependence can be achieved within the framework of the modified (dichotomous) exciton model. The slope of the temperature dependence of the B850 bandwidth is then defined by the value of the reorganization energy and by the difference between the transition energies of the dichotomous states of the pigment molecules. The equilibration factor between these dichotomous states mainly determines the temperature dependence of the peak shift.

Construction of hybrid photosynthetic units using peripheral and core antennae from two different species of photosynthetic bacteria: detection of the energy transfer from bacteriochlorophyll a in LH2 to bacteriochlorophyll b in LH1

Typical purple bacterial photosynthetic units consist of supra-molecular arrays of peripheral (LH2) and core (LH1-RC) antenna complexes. Recent atomic force microscopy pictures of photosynthetic units in intact membranes have revealed that the architecture of these units is variable (Scheuring et al. (2005) Biochim Bhiophys Acta 1712:109-127). In this study, we describe methods for the construction of heterologous photosynthetic units in lipid-bilayers from mixtures of purified LH2 (from Rhodopseudomonas acidophila) and LH1-RC (from Rhodopseudomonas viridis) core complexes. The architecture of these reconstituted photosynthetic units can be varied by controlling ratio of added LH2 to core complexes. The arrangement of the complexes was visualized by electron-microscopy in combination with Fourier analysis. The regular trigonal array of the core complexes seen in the native photosynthetic membrane could be regenerated in the reconstituted membranes by temperature cycling. In the presence of added LH2 complexes, this trigonal symmetry was replaced with orthorhombic symmetry. The small lattice lengths for the latter suggest that the constituent unit of the orthorhombic lattice is the LH2. Fluorescence and fluorescence-excitation spectroscopy was applied to the set of the reconstituted membranes prepared with various proportions of LH2 to core complexes. Remarkably, even though the LH2 complexes contain bacteriochlorophyll a, and the core complexes contain bacteriochlorophyll b, it was possible to demonstrate energy transfer from LH2 to the core complexes. These experiments provide a first step along the path toward investigating how changing the architecture of purple bacterial photosynthetic units affects the overall efficiency of light-harvesting.

Composition and localization of bacteriochlorophyll a intermediates in the purple photosynthetic bacterium Rhodopseudomonas sp. Rits

Rhodopseudomonas sp. Rits is a recently isolated new species of photosynthetic bacteria and found to accumulate a significantly high amount of bacteriochlorophyll (BChl) a intermediates possessing non-, di- and tetra-hydrogenated geranylgeranyl groups at the 17-propionate as well as normal phytylated BChl a (Mizoguchi T et al. (2006) FEBS Lett 580:137-143). A phylogenetic analysis showed that this bacterium was closely related to Rhodopseudomonas palustris. The strain Rits synthesizes light-harvesting complexes 2 and 4 (LH2/4), as peripheral antennas, as well as the reaction center and light-harvesting 1 core complex (RC-LH1 core). The amounts of these complexes were dependent upon the incident light intensities, which was also a typical behavior of Rhodopseudomonas palustris. HPLC analyses of extracted pigments indicated that all four BChls a were associated with the purified photosynthetic pigment-protein, as complexes described above. The results suggested that this bacterium could use these pigments as functional molecules within the LH2/4 and RC-LH1 core. Pigment compositional analyses in several purple photosynthetic bacteria showed that such BChl a intermediates were always detected and were more widely distributed than expected. Long chains in the propionate moiety of BChl a would be one of the important factors for assembly of LH systems in purple photosynthetic bacteria.

Is There a Conserved Interaction between Cardiolipin and the Type II Bacterial Reaction Center?

In a recent publication, the structural details of an interaction between the Rhodobacter sphaeroides reaction center and the anionic phospholipid diphosphatidyl glycerol (cardiolipin) were described (K. E. McAuley, P. K. Fyfe, J. P. Ridge, N. W. Isaacs, R. J. Cogdell, and M. R. Jones, 1999, Proc. Natl. Acad. Sci. U.S.A. 96:14706–14711). This was the first crystallographic description of an interaction between this biologically important lipid and an integral membrane protein and was also the first piece of evidence that the reaction center has a specific interaction with cardiolipin. We have examined the extent to which the residues that interact with the cardiolipin are conserved in other species of photosynthetic bacteria with this type of reaction center and discuss the possibility that this cardiolipin binding site is a conserved feature of these reaction centers. We look at how sequence variations that would affect the shape of the cardiolipin binding site might affect the protein-cardiolipin interaction, by modeling the binding of cardiolipin to the reaction center from Rhodopseudomonas viridis.

Cross-species conservation of lipid binding sites on the intramembrane surface of the bacterial reaction centre

X-ray crystallography has shown that the Rhodobacter sphaeroides reaction centre engages in a specific interaction with a molecule of the anionic phospholipid (diphosphatidyl glycerol). We have used extensive sequence alignments to determine the degree to which the residues that interact with the are conserved in other species of photosynthetic bacteria with this type of reaction centre. We discuss the possibility that this cardiolipin binding site is a conserved feature of these reaction centres. We also discuss the cross-species conservation of residues involved in lipid binding in a recently-reported X-ray crystal structure for the reaction centre from Thermochromatium tepidum, and compare the structure of this reaction centre with the complex from Rhodobacter sphaeroides.

Cloning, sequencing and analysis of the pucC genes from Rubrivivax gelatinosus strain 151 and Rhodopseudomonas acidophila strain 10050.

The pucC genes of Rubrivivax gelatinosus strain 151 and Rhodopseudomonas acidophila strain 10050 have been identified, cloned and sequenced. In Rubrivivax gelatinosus the arrangement of the pucC gene with regard to the pucBA genes was shown to differ from that found in other species of photosynthetic bacteria. The Rhodopseudomonas acidophila pucC was found downstream of four new pucBA gene pairs, bringing the sequenced pucBA pairs to a total of eight in this strain. The predicted PucC protein sequences were compared to those of PucC from other species and showed high similarity. Similarity was also seen to more distantly related proteins LhaA and orf428 of Rhodobacter capsulatus, orf G115 of Rhodospirillum rubrum and 'orf428' from Synechocystis sp. PCC6803. An analysis of the predicted secondary structure of these proteins is given, and their structural similarity to proteins in the Major Facilitator Superfamily is discussed with regard to their possible function.

Temperature dependence of the light-induced infrared difference spectra of chromatophores and reaction centers from photosynthetic bacteria

Temperature dependences of the light-induced infrared difference spectra have been observed between 300 and 80 K for chromatophores from several species of photosynthetic bacteria ( Rhodobacter sphaeroides, Rhodobacter capsulatus, Chromatium vinosum, Rhodopseudomonas palustris and Rhodopseudomonas viridis) and for isolated reaction centers from Rhodobacter sphaeroides as well. Particular attention is paid to the light-induced (positive) bands in the C 9 keto carbonyl stretching region, which are due to the radical cation of the bacteriochlorophyll special pair (P +). The observed bands are decomposed into two to four Gaussian curves, depending on the bacterial species. The positions of the resultant component bands are correlated with the degree of localization/delocalization of an unpaired electron over the two bacteriochlorophylls in the special pair on the time scale of infrared absorption (∼ 10 −13 s). The intensities of most of the component bands show temperature dependences, which confirm the existence of multiple structures for the moiety involving P + and its environment. The water contents in some samples affect the intensities of the light-induced bands. This effect depends on bacterial species as well as on the state of the sample (whether the sample is isolated RCs or chromatophores).

The sulfolipid sulfoquinovosyldiacylglycerol is not required for photosynthetic electron transport in Rhodobacter sphaeroides but enhances growth under phosphate limitation.

All photosynthetic organisms, with the exception of several species of photosynthetic bacteria, are thought to contain the sulfolipid 6-sulfo-alpha-D-quinovosyldiacylglycerol. The association of this lipid with photosynthetic membranes has led to the assumption that it plays some role in photosynthesis. Stable null mutants of the photosynthetic bacterium Rhodobacter sphaeroides completely lacking sulfolipid were obtained by disruption of the sqdB gene. The ratios of the various components of the photosynthetic electron transport chain, as well as the electron transfer rates during cyclic electron transport, were not altered in the mutants, when grown under optimal conditions. Growth rates of wild type and mutants were identical under a variety of growth conditions, with the exception of phosphate limitation, which resulted in reduced growth of the mutants. Phosphate limitation of the wild type caused a significant reduction in the amount of all phospholipids and an increased amount of sulfolipid. By contrast, the sulfolipid-deficient mutant had reduced levels of phosphatidylcholine and phosphatidylethanolamine but maintained a normal level of phosphatidylglycerol. In addition, two unidentified lipids lacking phosphorus accumulated in the membranes of both wild-type and mutant strains under phosphate limitation. We conclude that sulfolipid plays no significant unique role in photoheterotrophic growth or photosynthetic electron transport in R. sphaeroides but may function as a surrogate for phospholipids, particularly phosphatidylglycerol, under phosphate-limiting conditions.

Electrochromic spectral band shift of carotenoids in the photosynthetic membranes of Rhodospirillum molischianum and Rhodospirillum photometricum

Light-induced and diffusion-potential-induced absorption changes of carotenoids were studied in two species of photosynthetic bacteria in the genus Rhodospirillum. In Rs. photometricum, illumination or inside-negative diffusion potentials elicited a red shift of the carotenoid spectrum. However, in Rs. molischianum the spectral shift was in the opposite direction: i.e., a blue shift. The major carotenoids in both species were lycopene and rhodopin. The opposite shift direction can be explained by an opposite polarity of the permanent fields around the carotenoid molecules, which probably originates from the surrounding polypeptides.

A novel directly coupled gradostat

The original bidirectional compound chemostat (gradostat) described by Lovitt and Wimpenny has been simplified by making a more compact apparatus in which chemical gradients are established by diffusion between adjacent culture chambers. The experimental model (diffusion coupled (DC) gradostat) consisted of five chambers whose contents could be agitated by turbines rotating in the horizontal plane on a common shaft. Two biological experiments were designed to reveal the value of the DC gradostat. A methylotroph ( Methylophilus methylotrophus) grown in a methanol gradient showed expected changes in cell viability as a function of position in the five vessel array. Cells of two species of photosynthetic bacteria ( Rhodobacter capsulata and Rhodopseudomonas marina/agilis) with different salt sensitivities could be mixed and subsequently separated by the DC gradostat operating with a NaCl gradient of 0–3% w/v.

Second-site mutation at M43 (Asn?Asp) compensates for the loss of two acidic residues in the QB site of the reaction center

Two acidic residues, L212Glu and L213Asp, in the QB binding sites of the photosynthetic reaction centers of Rhodobacter capsulatus and Rhodobacter sphaeroides are thought to play central roles in the transfer of protons to the quinone anion(s) generated by photoinduced electron transfer. We constructed the site-specific double mutant L212Ala-L213Ala in R. capsulatus, that is incapable of growth under photosynthetic conditions. A photocompetent derivative of that strain has been isolated that carries the original L212Ala-L213Ala double mutation and a second-site suppressor mutation at residue M43 (Asn→Asp), outside of the QB binding site, that is solely responsible for restoring the photosynthetic phenotype. The Asp,Asn combination of residues at the L213 and M43 positions is conserved in the five species of photosynthetic bacteria whose reaction center sequences are known. In R. capsulatus and R. sphaeroides, the pair is L213Asp-M43Asn. But, the reaction centers of Rhodopseudomonas viridis, Rhodospirillum rubrum and Chloroflexus aurantiacus reverse the combination to L213Asn-M43Asp. In this respect, the QB site of the suppressor strain resembles that of the latter three species in that it couples an uncharged residue at L213 with an acidic residue at M43. These reaction centers, in which L213 is an amide, must employ an alternative proton transfer pathway. The observation that the M43Asn→Asp mutation in R. capsulatus compensates for the loss of both acidic residues at L212 and L213 suggests that M43Asp is involved in a new proton transfer route in this species that resembles the one normally used in reaction centers of Rps. virddis, Rsp. rubrum and C. aurantiacus.

I. Langmuir-blodgett monolayer films of bacterial photosynthetic membranes and isolated reaction centers: preparation, spectrophotometric and electrochemical characterization

The Langmuir-Blodgett (LB) film technique has been successfully applied to the construction of stable and photo-active films of chromatophore membranes and isolated reaction centers from two species of photosynthetic bacteria, Rhodobacter sphaeroides and Rhodopseudomonas viridis. LB films of these preparations were characterized at the air/water interface through compression isotherms and film stabilities. Films deposited on glass slides were analyzed by spectrophotometric and redox potentiometric techniques. The results obtained indicate that the in vivo properties of the photosynthetic apparatus in the deposited films are essentially unchanged. Furthermore, the pigments and redox cofactors in the films are highly oriented and offer a unique opportunity for structural and functional studies of the kind described in the accompanying paper (Biochim. Biophys. Acta 1057 (1991) 258-272).

Photoinhibition by flash and continuous light of oxygen uptake by intact photosynthetic bacteria

The inhibition of respiration by continuous or flashing light has been studied in intact cells of different species of photosynthetic bacteria. For Rhodopseudomonas palustris, Rhodopseudomonas sphaeroides and Rhodopseudomonas capsulata, the inhibition by short actinic flashes shows a remarkable periodicity of two: each flash induces an inhibition of respiration, but a stimulation is observed after an even number of flashes. On the other hand, no oscillation is observed for Rhodospirillum rubrum and Rhodopseudomonas viridis cells. These different behaviours are explained by a difference in the redox state of the secondary electron acceptor as shown by the effect of ortho-phenanthroline on the amperometric signal. Addition of uncouplers (carbonyl cyanide m-chlorophenylhydrazone) or of an ATPase inhibitor (tri- N-butyl tin), has little effect on the oscillatory pattern induced by flash excitation. However, inhibition of respiration by continuous light is suppressed in the presence of carbonyl cyanide m-chlorophenylhydrazone. In the presence of tri- N-butyl tin the steady-state level is reached more rapidly than in the control experiment for a given light intensity. These results are interpreted as evidence of two modes of light inhibition of respiration in photosynthetic bacteria. A first type of inhibition, clearly shown under flash excitation, is due to interaction between respiratory and photosynthetic chains at the level of electron carriers. After each flash, an electron is diverted from the respiratory chain to the photooxidized reaction center. Because of the gating mechanism at the level of the secondary acceptor, the respiration is stimulated after an even number of flashes. The second mode of inhibition prevails under continuous illumination. Under these conditions, the rate of respiration is controlled essentially by the photoinduced proton electrochemical gradient.

Effect of oxygen on acetylene reduction by photosynthetic bacteria

The effect of dissolved oxygen concentration on nitrogenase activity was studied in three species of photosynthetic bacteria. The O2 concentration in the cell suspension was measured with an O2 electrode inserted into the reaction vessel. Acetylene reduction by whole cells of Rhodopseudomonas capsulata, Rhodospirillum rubrum, and Chromatium vinosum strain D was inhibited 50% by 0.73, 0.32, and 0.26 microM O2, respectively. The inhibition of the activity by O2 in R. capsulata usually was reversed completely by reestablishing anaerobic conditions. In R. rubrum and C. vinosum the inhibition was only partially reversible. The respiration rate of R. capsulata was the highest of the three, that of R. rubrum was intermediate, and that of C. vinosum was lowest. R. capsulata and R. rubrum cells were broken after their acetylene reduction activity in vivo had been completely inhibited by O2, and nitrogenase was found to be active in vitro. A concentration of cyanide that did not affect acetylene reduction activity, but which inhibited 75 to 90% of the O2 uptake by whole cells of R. capsulata, shifted the O2 concentration causing 50% inhibition of nitrogenase activity from 0.73 microM to 2.03 microM. These results are in accordance with the assumption that within a limited range of O2 concentrations, the respiratory activity of the cells is enough to scavenge the O2 and to keep the interior of the cells essentially anaerobic. It is suggested that O2 inhibits nitrogenase activity by competing for a limited supply of electrons. When cyanide is present, respiration is slower but is adequate to keep the nitrogenase environment in the cell anaerobic. The lower respiration rate may allow a greater proportion of the electrons to be used for acetylene reduction.

Flash-induced changes in the in vivo bacteriochlorophyll fluorescence yield at low temperatures and low redox potentials in carotenoid-containing strains of photosynthetic bacteria

The changes in the in vivo bacteriochlorophyll fluorescence induced by a Xenon flash at low temperatures (77–200 K) with the “primary” acceptor X chemically prereduced have been examined in whole cells of several species of photosynthetic bacteria which contain carotenoids absorbing in the visible part of the absorption spectrum. Two groups of species with different behaviour could be distinguished. In both cases a flash-induced rise of the fluorescence yield was observed with X prereduced at 77 K; as the temperature was increased the ratio of the maximum fluorescence ( F M) and the basal fluorescence ( F o) decreased and the kinetics of the decay of the high fluorescent state, as observed during the tail of the flash, apparently accelerated. Of the species examined the flash-induced changes in fluorescence-yield kinetics appeared to occur at higher temperatures in the members of one group ( Chromatium vinosum, Rhodopseudomonas gelatinosa and Rhodopseudomonas palustris) than in the members of the other ( Rhodopseudomonas sphaeroides and Rhodospirillum rubrum). These effects are interpreted in terms of the light-induced generation of triplet states within the reaction centre. It is suggested that the species-dependent differences may reflect differences in the molecular organisation of the reaction centre. It was found that in all species the reaction centre carotenoid triplet does not act as a fluorescence quencher under these conditions.