Rearrangement of light harvesting bacteriochlorophyll homologues as a response of green sulfur bacteria to low light intensities

The pigment composition of two species of green-colored BChl c-containing green sulfur bacteria (Chlorobium limicola and C. chlorovibrioides) and two species of brown-colored BChl e-containing ones (C. phaeobacteroides and C. phaeovibrioides) incubated at different light intensities have been studied. All species responded to the reduction of light intensity from 50 to 1 Einstein(E) m^–2 s^–1 by an increase in the specific content of light harvesting pigments, bacteriochlorophylls and carotenoids. At critical light intensities (0.5 to 0.1 E m^–2 s^–1) only brown-colored chlorobia were able to grow, though at low specific rates (0.002 days^–1 mg prot^–1). High variations in the relative content of farnesyl-bacteriochlorophyll homologues were found, in particular BChl e ₁ and BChl e ₄, which were tentatively identified as [M, E] and [I, E] BChl_F e, respectively. The former was almost completely lost upon reduction of light intensity from 50 to 0.1 E m^–2 s^–1, whereas the latter increased from 7.2 to 38.4% and from 13.6 to 42.0% in C. phaeobacteroides and C. phaeovibrioides, respectively. This increase in the content of highly alkylated pigment molecules inside the chlorosomes of brown species is interpreted as a physiological mechanism to improve the efficiency of energy transfer towards the reaction center. This study provides some clues for understanding the physiological basis of the adaptation of brown species to extremely low light intensities.Abbreviations BChl bacteriochlorophyll - [M, E] BChl_F e 8-methyl, 12-ethyl BChl e, esterified with farnesol (F). Analogously: I - isobutyl - Pr propyl - Car carotenoids - Chlb chlorobactene - HPLC high performance liquid chromatography - Isr isorenieratene - LHP light harvesting pigments - PDA photodiode array detector - RC reaction center - RCH relative content of homologues     

Identification of the major chlorosomal bacteriochlorophylls of the green sulfur bacteria Chlorobium vibrioforme and Chlorobium phaeovibrioides; their function in lateral energy transfer

The chlorosomal bacteriochlorophyll (BChl) composition of the green sulfur bacteria Chlorobium vibrioforme and Chlorobium phaeovibrioides was investigated by means of normal-phase high-performance liquid chromatography. From both species a number of homologues was isolated, which were identified by absorption and ²⁵²Cf-plasma desorption mass spectroscopy. Besides BChl d, C. vibrioforme contained a significant amount of BChl c, which may provide an explanation for the previous observation of at least two spectrally different pools of BChl in the chlorosomes of green sulfur bacteria (Otte et al. 1991). C. phaeovibrioides contained various homologues of BChl e only. Absorption spectra in acetone of BChl c, d and e, as well as bacteriopheophytin e are presented. No systematic differences were found for the various homologues of each pigment. In addition to farnesol, the mass spectra revealed the presence of various minor esterifying alcohols in both species, including phytol, oleol, cetol and 4-undecyl-2-furanmethanol, as well as an alcohol of low molecular mass, which is tentatively assumed to be decenol.Abbreviations BChl bacteriochlorophyll - BPh bacteriopheophytin (used as a general name for the Mg-free compound, irrespective of the esterifying alcohol) - HPLC high-performance liquid chromatography     

A comparative study of the optical characteristics of intact cells of photosynthetic green sulfur bacteria containing bacteriochlorophyll c,d or e

Energy transfer and pigment arrangement in intact cells of the green sulfur bacteria Prosthecochloris aestuarii, Chlorobium vibrioforme and chlorobium phaeovibrioides, containing bacteriochlorophyll (BChl) c, d or e as main light harvesting pigment, respectively, were studied by means of absorption, fluorescence, circular dichroism and linear dichroism spectroscopy at low temperature. The results indicate a very similar composition of the antenna in the three species and a very similar structure of main light harvesting components, the chlorosome and the membrane-bound BChl a protein. In all three species the Q_y transition dipoles of BChl c, d or e are oriented approximately parallel to the long axis of the chlorosome. Absorption and fluorescence excitation spectra demonstrate the presence of at least two BChl c-e pools in the chlorosomes of all three species, long-wavelength absorbing BChls being closest to the membrane. In C. phaeovibrioides, energy from BChl e is transferred with an efficiency of 25% to the chlorosomal BChl a at 6 K, whereas the efficiency of transfer from BChl e to the BChl a protein is 10%. These numbers are compatible with the hypothesis that the chlorosomal BChl a is an intermediary in the energy transfer from the chlorosome to the membrane.Abbreviations BChl bacteriochlorophyll - Chl chlorophyll - CD circular dichroism - LD linear dichroism     

Circular dichroism of green bacterial chlorosomes

Positive and negative bands in previously measured circular dichroism (CD) spectra of Chlorobium limicola chlorosomes appeared to be sign-reversed relative to those of Chloroflexus aurantiacus chlorosomes in the 740–750 nm spectral region where bacteriochlorophyll (BChl) c absorbs maximally. It was not clear, however, whether this difference was intrinsic to the chlorosomes or was due to differences in the procedures used to prepare them. We therefore repeated the CD measurements using chlorosomes isolated from both Cb. limicola f. thiosulfatophilum and Cf. aurantiacus using the method of Gerola and Olson (1986, Biochim. Biophys. Acta 848: 69–76). Contrary to the earlier results, both types of chlorosomes had very similar CD spectra, suggesting that both have similar arrangements of BChl c molecules. The previously reported difference between the CD spectra of Chlorobium and Chloroflexus chlorosomes is due to the instability of Chlorobium chlorosomes, which can undergo a hypsochromic shift in their near infrared absorption maximum accompanied by an apparent inversion in their near infrared CD spectrum during isolation. Treating isolated chlorosomes with the strong ionic detergent sodium dodecylsulfate, which removes BChl a, does not alter the arrangement of BChl c molecules in either Chloroflexus or Chlorobium chlorosomes, as indicated by the lack of an effect on their CD spectra.Abbreviations BChl bacteriochlorophyll - Cb. Chlorobium - CD circular dichroism - Cf. Chloroflexus - NIR near infrared     

Excitation Energy Transfer Dynamics and Excited-State Structure in Chlorosomes of Chlorobium phaeobacteroides

The excited-state relaxation within bacteriochlorophyll (BChl) e and a in chlorosomes of Chlorobium phaeobacteroides has been studied by femtosecond transient absorption spectroscopy at room temperature. Singlet-singlet annihilation was observed to strongly influence both the isotropic and anisotropic decays. Pump intensities in the order of 10¹¹ photons × pulse⁻¹ × cm⁻² were required to obtain annihilation-free conditions. The most important consequence of applied very low excitation doses is an observation of a subpicosecond process within the BChl e manifold (~200–500 fs), manifesting itself as a rise in the red part of the Q_y absorption band of the BChl e aggregates. The subsequent decay of the kinetics measured in the BChl e region and the corresponding rise in the baseplate BChl a is not single-exponential, and at least two components are necessary to fit the data, corresponding to several BChl e→BChl a transfer steps. Under annihilation-free conditions, the anisotropic kinetics show a generally slow decay within the BChl e band (10–20 ps) whereas it decays more rapidly in the BChl a region (~1 ps). Analysis of the experimental data gives a detailed picture of the overall time evolution of the energy relaxation and energy transfer processes within the chlorosome. The results are interpreted within an exciton model based on the proposed structure.     

Excitation energy transfer in chlorosomes of Chlorobium phaeobacteroides strain CL1401: the role of carotenoids

The role of carotenoids in chlorosomes of the green sulfur bacterium Chlorobium phaeobacteroides, containing bacteriochlorophyll (BChl) e and the carotenoid (Car) isorenieratene as main pigments, was studied by steady-state fluorescence excitation, picosecond single-photon timing and femtosecond transient absorption (TA) spectroscopy. In order to obtain information about energy transfer from Cars in this photosynthetic light-harvesting antenna with high spectral overlap between Cars and BChls, Car-depleted chlorosomes, obtained by inhibition of Car biosynthesis by 2-hydroxybiphenyl, were employed in a comparative study with control chlorosomes. Excitation spectra measured at room temperature give an efficiency of 60–70% for the excitation energy transfer from Cars to BChls in control chlorosomes. Femtosecond TA measurements enabled an identification of the excited state absorption band of Cars and the lifetime of their S₁ state was determined to be 10 ps. Based on this lifetime, we concluded that the involvement of this state in energy transfer is unlikely. Furthermore, evidence was obtained for the presence of an ultrafast (>100 fs) energy transfer process from the S₂ state of Cars to BChls in control chlorosomes. Using two time-resolved techniques, we further found that the absence of Cars leads to overall slower decay kinetics probed within the Q_y band of BChl e aggregates, and that two time constants are generally required to describe energy transfer from aggregated BChl e to baseplate BChl a.This revised version was published online in October 2005 with corrections to the Cover Date.     

Regulatory properties of the ADPglucose pyrophosphorylase from Rhodopseudomonas sphaeroides and from Rhodopseudomonas gelatinosa

Highly purified fractions of chlorosomes and cytoplasmic membranes were isolated from Chloroflexus aurantiacus Ok-70-fl and Chlorobium limicola 6230. These fractions were comparatively analyzed for their pigmentation, phospholipid, glycolipid, and cytochrome c content as well as for their specific activities of succinate dehydrogenase and NADH-oxidase. The data showed that there are some differences in pigmentation and phospholipid content between the isolated fractions of Chloroflexus and Chlorobium. Chlorosomes of Chloroflexus contained a specific BChl a-complex with a characteristic absorption maximum at about 790 nm. This BChl a-complex could not be detected in spectra of chlorosomes from Chlorobium. The near infrared region of the spectra of the isolated cytoplasmic membranes of both organisms revealed considerable differences: The BChl a-complexes of Chloroflexus membranes exhibited peaks at 806 and 868 nm whereas the membranes of Chlorobium had a single BChl a-peak at 710 nm. In contrast to the findings with Chlorobium the chlorosomes of Chloroflexus contained at least twice as much phospholipids as did the cytoplasmic membranes. In Chlorobium the phospholipid content of cytoplasmic membranes is three times that of their chlorosomes. The distribution of all other components (carotenoid composition, enzyme activities, cytochrome c content, and glycolipids) was about the same in both strains. From the data it was concluded that differences in the organization of the photosynthetic apparatus are mainly based on differences of the organization of the photosynthetic units in the cytoplasmic membrane and probably the kind of linkage of the light harvesting system in the chlorosomes with the reaction center in the cytoplasmic membranes.Abbreviations BChl c bacteriochlorophyll c - BChl a bacteriochlorophyll a - DSM Deutsche Sammlung von Mikrorganismen     

Structural investigation of oxidized chlorosomes from green bacteria using multifrequency electron paramagnetic resonance up to 330 GHz

Chemical oxidation of the chlorosomes from Chloroflexus aurantiacus and Chlorobium tepidum green bacteria produces bacteriochlorophyll radicals, which are characterized by an anomalously narrow EPR signal compared to in vitro monomeric BChl c ^.+ [Van Noort PI, Zhu Y, LoBrutto R and Blankenship RE (1997) Biophys J 72: 316–325]. We have performed oxidant concentration and temperature-dependent X-band EPR measurements in order to elucidate the line narrowing mechanism. The linewidth decreases as the oxidant concentration is increased only for Chloroflexus indicating that for this system Heisenberg spin exchange is at least partially responsible for the EPR spectra narrowing. For both species the linewidth is decreasing on increasing the temperature. This indicates that temperature-activated electron transfer is the main narrowing mechanism for BChl radicals in chlorosomes. The extent of the electron transfer process among different BChl molecules has been evaluated and a comparison between the two species representative of the two green bacteria families has been made. In parallel, high frequency EPR experiments have been performed on the oxidized chlorosomes of Chloroflexus and Chlorobium at 110 and 330 GHz in the full temperature range investigated at X-band. The g-tensor components obtained from the simulation of the 330 GHz EPR spectrum from Chlorobium show the same anisotropy as those of monomeric Chl a ^.+ [Bratt PJ, Poluektov OG, Thurnauer MC, Krzystek J, Brunel LC, Schrier J, Hsiao YW, Zerner M and Angerhofer A (2000) J Phys Chem B 104: 6973–6977]. The spectrum of Chloroflexus has a nearly axial g-tensor with reduced anisotropy compared to Chlorobium and monomeric Chl a in vitro. g-tensor values and temperature dependence of the linewidth have been discussed in terms of the differences in the local structure of the chlorosomes of the two families.This revised version was published online in October 2005 with corrections to the Cover Date.     

The size of the photosynthetic unit in purple bacteria

Pigment analysis was performed by means of normal phase HPLC on a number of bacteriochlorophyll a and b containing species of purple bacteria that contain a core antenna only. At least 99% of the bacteriochlorophyll in Rhodobacter sphaeroides R26, Rhodopseudomonas viridis and Thiocapsa pfennigii was esterified with phytol (BChl a _p and BChl b _p, respectively). Rhodospirillum rubrum contained only BChl a esterified with geranyl-geraniol (BChl a _GG). Rhodospirillum sodomense and Rhodopseudomonas marina contained, in addition to BChl a _p, small amounts of BChl a _GG, and presumably also of BChl a esterified with dihydro and tetrahydro geranyl-geraniol (2,10,14-phytatrienol and probably 2,14-phytadienol). In all species bacteriopheophytin (BPhe) esterified with phytol was present. The BChl/BPhe ratio indicated that in these species a constant number of 25 ± 3 antenna BChls is present per reaction centre. This number supports a model in which the core antenna consists of 12 - heterodimers surrounding the reaction centre. Determination of the in vivo extinction coefficient of BChl in the core-reaction centre complex yielded a value of ca. 140 mM^–1 cm^–1 for BChl a containing species and of 130 mM^–1 cm^–1 for Rhodopseudomonas viridis.Abbreviations BChl bacteriochlorophyll - BPhe bacteriopheophytin - GG geranyl-geraniol - LHI and LHII core and peripheral antenna complexes - P phytol - RC reaction centre Dedicated to the memory of Professor D.I. Arnon.     

The molar extinction coefficient of bacteriochlorophyll e and the pigment stoichiometry in Chlorobium phaeobacteroides

We have determined the molar extinction coefficient of bacteriochlorophyll (BChl) e, the main light-harvesting pigment from brown-coloured photosynthetic sulfur bacteria. The extinction coefficient was determined using pure [Pr,E]BChl e_F isolated by reversed-phase HPLC from crude pigment extracts of Chlorobium (Chl.) phaeobacteroides strain CL1401. The extinction coefficients at the Soret and Q_y bands were determined in four organic solvents. The extinction coefficient of BChl e differs from those of other related Chlorobium chlorophylls (BChl c and BChl d) but is similar to that of chlorophyll b. The determined extinction coefficient was used to calculate the stoichiometric BChl e to BChl a and BChl e to carotenoids ratios in whole cells and isolated chlorosomes from Chl. phaeobacteroides strain CL1401 using the spectrum-reconstruction method (SRCM) described by Naqvi et al. (1997) (Spectrochim Acta A Mol Biomol Spectrosc 53: 2229–2234) . In isolated chlorosomes, BChl a content was ca. 1% of the total BChl content and the stoichiometric ratio of BChl e to carotenoids was 6. In whole cells, however, BChl a content was 3–4%, owing to the presence of BChl a-containing elements, i.e. FMO protein and reaction centre. An average of 5 BChl e molecules per carotenoid was determined in whole cells.     

Hole burning study of excited state structure and energy transfer dynamics of bacteriochlorophyll c in chlorosomes of green sulphur photosynthetic bacteria

Results of low temperature fluorescence and spectral hole burning experiments with whole cells and isolated chlorosomes of the green sulfur bacterium Chlorobium limicola containing BChl c are reported. At least two spectral forms of BChl c (short-wavelength and long-wavelength absorbing BChl c) were identified in the second derivative fluorescence spectra. The widths of persistent holes burned in the fluorescence spectrum of BChl c are determined by excited state lifetimes due to fast energy transfer. Different excited state lifetimes for both BChl c forms were observed. A site distribution function of the lowest excited state of chlorosomal BChl c was revealed. The excited state lifetimes are strongly influenced by redox conditions of the solution. At anaerobic conditions the lifetime of 5.3 ps corresponds to the rate of energy transfer between BChl c clusters. This time shortens to 2.6 ps at aerobic conditions. The shortening may be caused by introducing a quencher. Spectral bands observed in the fluorescence of isolated chlorosomes were attributed to monomeric and lower state aggregates of BChl c. These forms are not functionally connected with the chlorosome.Abbreviations BChl bacteriochlorophyll - EET electronic energy transfer - FWHM full width at half maximum - SDF site distribution function - RC reaction centre     

Atomic structure of the bacteriochlorophyll c assembly in intact chlorosomes from Chlorobium limicola determined by solid-state NMR

Green sulfur photosynthetic bacteria optimize their antennas, chlorosomes, especially for harvesting weak light by organizing bacteriochlorophyll (BChl) assembly without any support of proteins. As it is difficult to crystallize the organelles, a high-resolution structure of the light-harvesting devices in the chlorosomes has not been clarified. We have determined the structure of BChl c assembly in the intact chlorosomes from Chlorobium limicola on the basis of ¹³C dipolar spin-diffusion solid-state NMR analysis of uniformly ¹³C-labeled chlorosomes. About 90 intermolecular C–C distances were obtained by the simultaneous assignment of distance correlations and the structure optimization preceded by the polarization-transfer matrix analysis. An atomic structure was obtained, using these distance constraints. The determined structure of the chlorosomal BChl c assembly is built with the parallel layers of piggyback-dimers. This supramolecular structure would provide insights into the mechanism of weak-light capturing.     

Strongly exciton-coupled BChle chromophore system in the chlorosomal antenna of intact cells of the green bacteriumChlorobium phaeovibrioides: A spectral hole burning study

Spectral hole burning studies of intact cells of the green bacteriumChlorobium phaeovibrioides have proven that the Q_y-absorption system of antenna bacteriochlorophylle (BChle) should be interpreted in terms of the delocalized exciton level structure of an aggregate. For the first time the 0-0 band of the lowest exciton state of BChle aggregates has been directly detected as the lowest energy inhomogeneously broadened band (FWHM 100 cm^–1; position of maximum, at 739 nm) of the near-infrared BChle band in the 1.8 K excitation spectrum (FWHM=750 cm^–1; position of maximum, at 715 nm). The comparative analysis of the hole spectra, measured for the three species of BChlc- ande-containing green bacteria, has shown that the 0-0 transition bands of the lowest exciton state of BChlc ande aggregates display fundamentally similar spectral features: (1) the magnitude of inhomogeneous broadening of these bands is about 100 cm^–1; (2) at the wavelength of the maximum of each band, the amplitude of the preburnt excitation spectrum makes up 20% of the maximum amplitude of the spectrum; (3) the spectral position of each band coincides with the spectral position of the longest wavelength band of the circular dichroism spectrum; (4) the width of these bands is 2.3-times less than that of monomeric BChl in vitro.     

Specific Gene bciD for C7-Methyl Oxidation in Bacteriochlorophyll e Biosynthesis of Brown-Colored Green Sulfur Bacteria

The gene named bciD, which encodes the enzyme involved in C7-formylation in bacteriochlorophyll e biosynthesis, was found and investigated by insertional inactivation in the brown-colored green sulfur bacterium Chlorobaculum limnaeum (previously called Chlorobium phaeobacteroides). The bciD mutant cells were green in color, and accumulated bacteriochlorophyll c homologs bearing the 7-methyl group, compared to C7-formylated BChl e homologs in the wild type. BChl-c homolog compositions in the mutant were further different from those in Chlorobaculum tepidum which originally produced BChl c: (3¹ S)-8-isobutyl-12-ethyl-BChl c was unusually predominant.     

Bacteriochlorophyll organization and energy transfer kinetics in chlorosomes from Chloroflexus aurantiacus depend on the light regime during growth

We have used measurements of fluorescence and circular dichroism (CD) to compare chlorosome-membrane preparations derived from the green filamentous bacterium Chloroflexus aurantiacus grown in continuous culture at two different light-intensities. The cells grown under low light (6 mol m^–2 s^–1) had a higher ratio of bacteriochlorophyll (BChl) c to BChl a than cells grown at a tenfold higher light intensity; the high-light-grown cells had much more carotenoid per bacteriochlorophyll.The anisotropy of the Q_Y band of BChl c was calculated from steady-state fluorescence excitation and emission spectra with polarized light. The results showed that the BChl c in the chlorosomes derived from cells grown under high light has a higher structural order than BChl c in chlorosomes from low-light-grown cells. In the central part of the BChl c fluorescence emission band, the average angles between the transition dipole moments for BChl c molecules and the symmetry axis of the chlorosome rod element were estimated as 25° and 17° in chlorosomes obtained from the low- and high-light-grown cells, respectively.This difference in BChl organization was confirmed by the decay associated spectra of the two samples obtained using picosecond single-photon-counting experiments and global analysis of the fluorescence decays. The shortest decay component obtained, which probably represents energy-transfer from the chlorosome bacteriochlorophylls to the BChl a in the baseplate, was 15 ps in the chlorosomes from high-light-grown cell but only 7 ps in the preparation from low-light grown cells. The CD spectra of the two preparations were very different: chlorosomes from low-light-grown cells had a type II spectrum, while those from high-light-grown cells was of type I (Griebenow et al. (1991) Biochim Biophys Acta 1058: 194–202). The different shapes of the CD spectra confirm the existence of a qualitatively different organization of the BChl c in the two types of chlorosome.Abbreviations BChl bacteriochlorophyll - CD circular dichroism - DAS decay associated spectrum - PMSF phenylmethylsulfonyl fluoride     

The Model of Pigment Aggregation in the Chlorosomal Antenna of the Green Bacterium Chloroflexus aurantiacus

Independent experimental and theoretical evaluation was performed for the adequacy of our previously proposed general molecular model of the structural organization of light-harvesting pigments in chlorosomal bacteriochlorophyll (BChl) /d/e-containing superantennae of different green bacteria. Measurement of the temperature dependence of steady-state fluorescence spectra of BChl c was accomplished in intact cells of a photosynthetic green bacterium Chloroflexus aurantiacus; this allows in vivodetermination of the structure of exciton levels of BChl c oligomers in this natural antenna. Experimental data confirm our model of organization of oligomeric pigments in chlorosomal BChl c antenna of green bacterium Chloroflexus aurantiacus. This model implies that the unit building block of the antenna is a cylindrical assembly containing six excitonically coupled linear pigment chains, whose exciton structure with intense upper levels provides for the optimal spectral properties of the light-harvesting antenna.     

Studies of the location and function of isoprenoid quinones in chlorosomes from green sulfur bacteria

The chlorosome antenna of the green sulfur bacterium Chlorobium tepidum essentially consists of aggregated bacteriochlorophyll (BChl) c enveloped in a glycolipid monolayer. Small amounts of protein and the isoprenoid quinones chlorobiumquinone (CK) and menaquinone-7 (MK-7) are also present. Treatment of isolated chlorosomes from Cb. tepidum with sodium dodecyl sulfate (SDS) did not affect the quinones, demonstrating that these are located in a site which is inaccessible to SDS, probably in the interior of the chlorosomes. About half of the quinones were removed by Triton X-100. The non-ionic character of Triton probably allowed it to extract components from within the chlorosomes. MK-10 in chlorosomes from the green filamentous bacterium Chloroflexus aurantiacus was likewise found to be located in the chlorosome interior. The excitation transfer in isolated chlorosomes from Cb. tepidum is redox-regulated. We found a ratio of BChl c fluorescenceintensity under reducing conditions (Fred) to that under oxidizing conditions (Fox) of approximately 40. The chlorosomal BChl a fluorescence was also redox-regulated. When the chlorosomal BChl c–BChl c interactions were disrupted by 1-hexanol, the BChl c Fred/Fox ratiodecreased to approximately 3. When CK and MK-7 were extracted from isolated chlorosomes with hexane, the BChl c Fred/Fox ratio also decreased to approximately 3. A BChl c Fred/Fox ratio of 3–5 was furthermore observed in aggregates of pure BChl c and in chlorosomes from Cfx. aurantiacus which do not contain CK. We therefore suggest that BChl c aggregates inherently exhibit a small redox-dependent fluorescence (Fred/Fox 3) and that the large redox-dependent fluorescence observed in chlorobial chlorosomes (Fred/Fox 40) is CK-dependent.     

Effect of quinones on formation and properties of bacteriochlorophyll c aggregates

Chlorosomes of green photosynthetic bacterium Chlorobium tepidum contain aggregates of bacteriochlorophyll c (BChl c) with carotenoids and isoprenoid quinones. BChl aggregates with very similar optical properties can be prepared also in vitro either in non-polar solvents or in aqueous buffers with addition of lipids and/or carotenoids. In this work, we show that the aggregation of BChl c in aqueous buffer can be induced also by quinones (vitamin K₁ and K₂), provided they are non-polar due to a hydrophobic side-chain. Polar vitamin K_3, which possess the same functional group as K₁ and K₂, does not induce the aggregation. The results confirm a principal role of the hydrophobic interactions as a driving force for the aggregation of chlorosomal BChls. The chlorosomal quinones play an important role in a redox-dependent excitation quenching, which may protect the cells against damage under oxygenic conditions. We found that aggregates of BChl c with vitamin K₁ and K₂ exhibit an excitation quenching as well. The amplitude of the quenching depends on quinone concentration, as determined from fluorescence measurements. No lipid is necessary to induce the quenching, which therefore originates mainly from interactions of BChl c with quinones incorporated in the aggregate structure. In contrast, only a weak quenching was observed for dimers of BChl c in buffer (either with or without vitamin K₃) and also for BChl c aggregates prepared with a lipid (lecithin). Thus, the weak quenching seems to be a property of BChl c itself.     

Photosynthetic bacteria in meromictic lakes and stratified fjords of the Vestfold Hills,Antarctica

Thirteen meromictic lakes and two permanently stratified fjords in the Vestfold Hills, Antarctica, were surveyed in 1983 for photosynthetic bacteria. Burton Lake and Ellis Fjord were sampled throughout the year to determine seasonal variations. Physical and chemical parameters were recorded and related to the species present. The dominant species in waters with salinities of 100.7 g kg^–1 were Chlorobium vibrioforme and Chlorobium limicola with populations at the O₂–H₂S interface in the range 0.3 to 6.7 × 10⁶ ml^–1. Neither of these species was found at higher salinities. Thiocapsa roseopersicina and a Chromatium sp. were found in low numbers (< 10⁵ ml^–1) in most of the same waters as the Chlorobium spp. These bacterial phototrophs developed in a narrow band below the O₂–H₂S interface where both light and H₂S were available. Very low numbers (< 10² ml^–1) of Rhodopseudomonas palustris were found in both oxic and anoxic waters having salinity 148 g kg^–1. The dominance of the Chlorobium spp. is ascribed to their more efficient maintenance metabolism during the darkness, their faster growth at low light intensities (< 1 µE m^–2 s^–1) and the lack of selective filtering of incident light. The Chlorobium spp. grew well at –2 °C, but not –5°C in hypersaline waters. The concentration of H₂S had no apparent effect on the development of the bacterial flora. Viable cells were found to depths of 100 m in Ellis Fjord indicating that viability in total darkness could have been maintained for periods of the order of 1700 days.     

Different Sensitivities to Oxygen Between Two Strains of the Photosynthetic Green Sulfur Bacterium Chlorobium vibrioforme NCIB 8327 with Bacteriochlorophyll c and d

Two sub-strains of the anoxygenic photosynthetic green sulfur bacterium Chlorobium vibrioforme NCIB 8327 were derived from the same clone and could be discriminated only by their possession of either bacteriochlorophyll (BChl) c or d as the major pigment in the peripheral light-harvesting antenna system, chlorosome (Saga Y et al. (2003) Anal Sci 19: 1575–1579). In the presence of a proper amount of oxygen in the initial culture medium, the BChl d strain showed longer retardation on its growth initiation than the BChl c strain, indicating that the latter was advantageous for survival under aerobic light conditions which produced reactive oxygen species in vivo. The result would be ascribable to the difference of the midpoint potentials between two kinds of chlorosomes formed by self-aggregates of BChl c and d as measured by their fluorescence quenching.