Biogeochemical processes in the saline meromictic Lake Kaiike, Japan: implications from molecular isotopic evidences of photosynthetic pigments

Stable carbon and nitrogen isotopic compositions were determined for individual photosynthetic pigments isolated and purified from the saline meromictic Lake Kaiike, Japan, to investigate species-independent biogeochemical processes of photoautotrophs in the natural environment. In the anoxic monimolimnion and benthic microbial mats, the carbon isotopic compositions of BChls e and isorenieratene related to brown-coloured strains of green sulfur bacteria are substantially ( approximately 10 per thousand) depleted in (13)C relative to those found in the chemocline. In conjunction with 16S rDNA evidence reported previously, it strongly suggests that Pelodyctyon luteolum inhabited and photosynthesized in the anoxic monimolimnion and benthic microbial mats by using (13)C-depleted regenerated CO(2). By contrast, both Chl a and BChl a in the monimolimnion and microbial mats have similar isotopic compositions as they do in the chemocline, implying that the source organisms live only in the chemocline. In the chemocline, the nitrogen isotopic compositions of BChl e homologues ranges from -7.7 to-6.5 per thousand, whereas that of BChl a is -2.1 per thousand. These isotopic compositions suggest that green sulfur bacteria Chlorobium phaeovibrioides would conduct nitrogen fixation in the chemocline, whereas purple sulfur bacteria Halochromatium sp. and cyanobacteria Synechococcus sp. may assimilate nitrite.     

Chlorobium Tepidum: Insights into the Structure,Physiology, and Metabolism of a Green Sulfur Bacterium Derived from the Complete Genome Sequence

Green sulfur bacteria are obligate, anaerobic photolithoautotrophs that synthesize unique bacteriochlorophylls (BChls) and a unique light-harvesting antenna structure, the chlorosome. One organism, Chlorobium tepidum, has emerged as a model for this group of bacteria primarily due to its relative ease of cultivation and natural transformability. This review focuses on insights into the physiology and biochemistry of the green sulfur bacteria that have been derived from the recently completed analysis of the 2.15-Mb genome of Chl. tepidum. About 40 mutants of Chl. tepidum have been generated within the last 3 years, most of which have been made based on analyses of the genome. This has allowed a nearly complete elucidation of the biosynthetic pathways for the carotenoids and BChls in Chl. tepidum, which include several novel enzymes specific for BChl c biosynthesis. Facilitating these analyses, both BChl c and carotenoid biosynthesis can be completely eliminated in Chl. tepidum. Based particularly on analyses of mutants lacking chlorosome proteins and BChl c, progress has also been made in understanding the structure and biogenesis of chlorosomes. In silico analyses of the presence and absence of genes encoding components involved in electron transfer reactions and carbon assimilation have additionally revealed some of the potential physiological capabilities, limitations, and peculiarities of Chl. tepidum. Surprisingly, some structural components and biosynthetic pathways associated with photosynthesis and energy metabolism in Chl. tepidum are more similar to those in cyanobacteria and plants than to those in other groups of photosynthetic bacteria.     

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.     

Isolation and pigment composition of the antenna system of four species of green sulfur bacteria

New and rapid procedures were developed for the isolation of chlorosomes and FMO-protein from the green sulfur bacteria Prosthecochloris (P.) aestuarii, Chlorobium (Cb.) phaeovibrioides, Cb. tepidum and Cb. vibrioforme. The resulting preparations were free from contaminating pigments and proteins as was shown by absorption spectroscopy, pigment analysis and SDS-PAGE. Two spectrally different types of FMO-protein were found. The first type, present in P. aestuarii and Cb. vibrioforme, has a main absorption band at 6 K at 815 nm, whereas the second type, isolated from Cb. tepidum and Cb. phaeovibrioides, has a strong band at 806 nm. In contrast to what was recently suggested (Tronrud DE and Matthews BW (1993) In: Deisenhofer J and Norris J (eds) The Photosynthetic Reaction Center, Vol 1, pp 13–21. Academic Press, San Diego, CA) the FMO-proteins contained no polar BChl a homologue. The isolated chlorosomes showed a small blue-shift of the QY absorption maximum with respect to intact cells. For the different species, grown under the same light conditions, the homologue composition of BChls c and d was approximately identical whereas for the BChl e in Cb. phaeovibrioides the relative amounts of homologues with larger alkyl substituents at position 8 were considerably larger. Baseplate BChl a was present in all chlorosomes and comprised 1–2% of the chlorosomal BChl. Its QY absorption band was located at about 802 nm and was clearly separated from the major QY absorption band at 6 K. The predominant esterifying alcohol of BChl a in the chlorosomes as well as in the FMO-proteins was phytol, but both antenna complexes also contained small amounts of BChl a esterified with the metabolic intermediates geranylgeraniol, dihydrogeranylgeraniol and tetrahydrogeranylgeraniol, like most purple bacteria. Since the esterifying alcohols of the chlorosomal BChl a and of the main chlorosomal pigments (BChls c, d and e) are different, esterification, and perhaps also the synthesis, of the BChls in the interior of the chlorosome and of the BChls in the baseplate must be spatially and genetically separated processes.     

Identification of a gene essential for the first committed step in the biosynthesis of bacteriochlorophyll c

Bacteriochlorophylls (BChls) c, d, and e are the major chlorophylls in chlorosomes, which are the largest and one of the most efficient antennae produced by chlorophototrophic organisms. In the biosynthesis of these three BChls, a C-13(2)-methylcarboxyl group found in all other chlorophylls (Chls) must be removed. This reaction is postulated to be the first committed step in the synthesis of these BChls. Analyses of gene neighborhoods of (B)Chl biosynthesis genes and distribution patterns in organisms producing chlorosomes helped to identify a gene (bciC) that appeared to be a good candidate to produce the enzyme involved in this biochemical reaction. To confirm that this was the case, a deletion mutant of an open reading frame orthologous to bciC, CT1077, was constructed in Chlorobaculum tepidum, a genetically tractible green sulfur bacterium. The CT1077 deletion mutant was unable to synthesize BChl c but still synthesized BChl a and Chl a. The deletion mutant accumulated large amounts of various (bacterio)pheophorbides, all of which still had C-13(2)-methylcarboxyl groups. A C. tepidum strain in which CT1077 was replaced by an orthologous gene, Cabther_B0081 [corrected] from "Candidatus Chloracidobacterium thermophilum" was constructed. Although the product of Cabther_B0081 [corrected] was only 28% identical to the product of CT1077, this strain synthesized BChl c, BChl a, and Chl a in amounts similar to wild-type C. tepidum cells. To indicate their roles in the first committed step of BChl c, d, and e biosynthesis, open reading frames CT1077 and Cabther_B0081 [corrected] have been redesignated bciC. The potential mechanism by which BciC removes the C-13(2)-methylcarboxyl moiety of chlorophyllide a is discussed.     

Fluorescence detected magnetic resonance of monomers and aggregates of bacteriochlorophylls of green sulfur bacteria Chlorobium sp.

Fluorescence detected magnetic resonance (FDMR) was used to study the lowest triplet state of bacteriochlorophylls (BChls) c and d in Chlorobium (Chl.) tepidum and Chl. vibrioforme, respectively. These pigments were studied both in the oligomeric form (in whole cells) and in the monomeric form (after conversion using a 1% 1-hexanol treatment). Fluorescence spectra show the presence of lower-state aggregates, apart from monomers, in samples treated with 1-hexanol. Values of the zero field splitting (ZFS) parameter D, obtained from FDMR spectra, were found to decrease with an increasing aggregate size. The observed ZFS trends are explained by a delocalization of the triplet spins, including a charge resonance (CR) contribution, over the aggregate. A simple model is presented relating the changes of D and E as a result of monomer aggregation to the aggregate geometry. Application of this model to BChls c and d indicates approximately diagonal stacking of the monomers in the dimer. Results for oligomeric BChl c and d were compared with those previously obtained for oligomeric BChl e. FDMR transitions of BChls c, d and e differ both in frequencies and in signs. The D and E values of Car's and BChl a (in whole cells) agree well with those reported for Chl. phaeobacteroides and Chl. limicola.     

Living off the Sun: chlorophylls,bacteriochlorophylls and rhodopsins

Pigments absorbing 350–1,050 nm radiation have had an important role on the Earth for at least 3.5 billion years. The ion pumping rhodopsins absorb blue and green photons using retinal and pump ions across cell membranes. Bacteriochlorophylls (BChl), absorbing in the violet/blue and near infra red (NIR), power anoxygenic photosynthesis, with one photoreaction centre; and chlorophylls (Chl), absorbing in the violet/blue and red (occasionally NIR) power oxygenic photosynthesis, with two photoreaction centres. The accessory (bacterio)chlorophylls add to the spectral range (bandwidth) of photon absorption, e.g., in algae living at depth in clear oceanic water and in algae and photosynthetic (PS) bacteria in microbial mats. Organism size, via the package effect, determines the photon absorption benefit of the costs of synthesis of the pigment–protein complexes. There are unresolved issues as to the evolution of Chls vs. BChls and the role of violet/blue and NIR radiation in PS bacteria.     

Dynamics of energy conversion in reaction center core complexes of the green sulfur bacterium Prosthecochloris aestuarii at low temperature.

Excited-state and electron-transfer dynamics at cryogenic temperature in reaction center core (RCC) complexes of the photosynthetic green sulfur bacterium Prosthecochloris aestuarii were studied by means of time-resolved absorption spectroscopy, using selective excitaton of bacteriochlorophyll (BChl) a and of chlorophyll (Chl) a 670. The results indicate that the BChls a of the RCC complex form an excitonically coupled system. Relaxation of the excitation energy within the ensemble of BChl a molecules occurred within 2 ps. A time constant of about 25 ps was ascribed to charge separation. Absorption changes in the 670 nm region, where Chl a 670 absorbs, were fairly complicated. They showed various time constants and were dependent on the wavelength of excitation and they did not lead to a simple picture of the electron acceptor reaction. Energy transfer from Chl a 670 to BChl a occurred with a time constant of 1.5 ps. However, upon excitation of Chl a 670 the amount of oxidized primary electron donor, P840(+), formed relative to that of excited BChl a was considerably larger than upon direct excitation of BChl a. This indicates the existence of an alternative pathway for charge separation which does not involve excited BChl a.     

The bchU gene of Chlorobium tepidum encodes the c-20 methyltransferase in bacteriochlorophyll c biosynthesis

Bacteriochlorophylls (BChls) c and d, two of the major light-harvesting pigments in photosynthetic green sulfur bacteria, differ only by the presence of a methyl group at the C-20 methine bridge position in BChl c. A gene potentially encoding the C-20 methyltransferase, bchU, was identified by comparative analysis of the Chlorobium tepidum and Chloroflexus aurantiacus genome sequences. Homologs of this gene were amplified and sequenced from Chlorobium phaeobacteroides strain 1549, Chlorobium vibrioforme strain 8327d, and C. vibrioforme strain 8327c, which produce BChls e, d, and c, respectively. A single nucleotide insertion in the bchU gene of C. vibrioforme strain 8327d was found to cause a premature, in-frame stop codon and thus the formation of a truncated, nonfunctional gene product. The spontaneous mutant of this strain that produces BChl c (strain 8327c) has a second frameshift mutation that restores the correct reading frame in bchU. The bchU gene was inactivated in C. tepidum, a BChl c-producing species, and the resulting mutant produced only BChl d. Growth rate measurements showed that BChl c- and d-producing strains of the same organism (C. tepidum or C. vibrioforme) have similar growth rates at high and intermediate light intensities but that strains producing BChl c grow faster than those with BChl d at low light intensities. Thus, the bchU gene encodes the C-20 methyltransferase for BChl c biosynthesis in Chlorobium species, and methylation at the C-20 position to produce BChl c rather than BChl d confers a significant competitive advantage to green sulfur bacteria living at limiting red and near-infrared light intensities.     

Impact of esterified bacteriochlorophylls on the biogenesis of chlorosomes in Chloroflexus aurantiacus

A chlorosome is an antenna complex located on the cytoplasmic side of the inner membrane in green photosynthetic bacteria that contains tens of thousands of self-assembled bacteriochlorophylls (BChls). Green bacteria are known to incorporate various esterifying alcohols at the C-17 propionate position of BChls in the chlorosome. The effect of these functional substitutions on the biogenesis of the chlorosome has not yet been fully explored. In this report, we address this question by investigating various esterified bacteriochlorophyll c (BChl c) homologs in the thermophilic green non-sulfur bacterium Chloroflexus aurantiacus. Cultures were supplemented with exogenous long-chain alcohols at 52 °C (an optimal growth temperature) and 44 °C (a suboptimal growth temperature), and the morphology, optical properties and exciton transfer characteristics of chlorosomes were investigated. Our studies indicate that at 44 °C Cfl. aurantiacus synthesizes more carotenoids, incorporates more BChl c homologs with unsaturated and rigid polyisoprenoid esterifying alcohols and produces more heterogeneous BChl c homologs in chlorosomes. Substitution of phytol for stearyl alcohol of BChl c maintains similar morphology of the intact chlorosome and enhances energy transfer from the chlorosome to the membrane-bound photosynthetic apparatus. Different morphologies of the intact chlorosome versus in vitro BChl aggregates are suggested by small-angle neutron scattering. Additionally, phytol cultures and 44 °C cultures exhibit slow assembly of the chlorosome. These results suggest that the esterifying alcohol of BChl c contributes to long-range organization of BChls, and that interactions between BChls with other components are important to the assembly of the chlorosome. Possible mechanisms for how esterifying alcohols affect the biogenesis of the chlorosome are discussed.     

Biogeochemistry of microbial mats under Precambrian environmental conditions: a modelling study

Microbial mats have arguably been the most important ecosystem on Earth over its 3.5 Gyr inhabitation. Mats have persisted as consortia for billions of years and occupy some of Earth's most hostile environments. With rare exceptions (e.g. microbial mats developed on geothermal springs at Yellowstone National Park, USA), today's mats do not exist under conditions analogous to Precambrian habitats with substantially lower oxygen and sulphate concentrations. This study uses a numerical model of a microbial mat to investigate how mat composition in the past might have differed from modern mats. We present a numerical model of mat biogeochemistry that simulates the growth of cyanobacteria (CYA), colourless sulphur bacteria (CSB), and purple sulphur bacteria (PSB), with sulphate‐reducing bacteria (SRB) and heterotrophic bacteria represented by parameterized sulphate reduction rates and heterotrophic consumption rates, respectively. Variations in the availability of light, oxygen, sulphide, and sulphate at the upper boundary of the mat are the driving forces in the model. Mats with remarkably similar biomass and chemical profiles develop in models under oxygen boundary conditions ranging from 2.5 × 10^?13 to 0.25 mm and sulphate boundary concentrations ranging from 0.29 to 29 mm , designed to simulate various environments from Archean to modern. The modelled mats show little sensitivity to oxygen boundary conditions because, independent of the overlying oxygen concentrations, cyanobacterial photosynthesis creates similar O₂ concentrations of 0.45–0.65 mm in the upper reaches of the mat during the photoperiod. Varying sulphate boundary conditions have more effect on the biological composition of the mat. Sulphide generated from sulphate reduction controls the magnitude and distribution of the PSB population, and plays a part in the distribution of CSB. CSB are the most sensitive species to environmental change, varying with oxygen and sulphide.     

Archaean metabolic evolution of microbial mats

Microbial mats of coexisting bacteria and archaea date back to the early Archaean: many of the major steps in early evolution probably took place within them. The earliest mats may have formed as biofilms of cooperative chemolithotrophs in hyperthermophile settings, with microbial exploitation of diversifying niches. Anoxygenic photosynthesis using bacteriochlorophyll could have allowed mats, including green gliding bacteria, to colonize anaerobic shallow-water mesothermophile habitats. Exploitation of the Calvin–Benson cycle by purple bacteria allowed diversification of microbial mats, with some organisms in more aerobic habitats, while green sulphur bacteria specialized in anaerobic niches. Cyanobacterial evolution led to more complex mats and plankton, allowing widespread colonization of the globe and the creation of further aerobic habitat. Microbial mat structure may reflect this evolutionary development in broad terms, with anaerobic lower levels occupied by archaeal and bacterial respirers, fermenters and green bacteria, while the higher levels contain aerobic purple bacteria and are dominated by cyanobacteria. A possible origin of eukaryotes is from a fusion of symbiotic partners living across a redox boundary in a mat. The geological record of Archaean mats may be present as isotopic fingerprints: with the presence of cyanobacteria, mats may have had a nearly modern structure as early as 3.5 Ga ago (1 Ga = 10⁹ years).     

Internal structure of chlorosomes from brown-colored chlorobium species and the role of carotenoids in their assembly

Chlorosomes are the main light harvesting complexes of green photosynthetic bacteria. Recently, a lamellar model was proposed for the arrangement of pigment aggregates in Chlorobium tepidum chlorosomes, which contain bacteriochlorophyll (BChl) c as the main pigment. Here we demonstrate that the lamellar organization is also found in chlorosomes from two brown-colored species (Chl. phaeovibrioides and Chl. phaeobacteroides) containing BChl e as the main pigment. This suggests that the lamellar model is universal among green sulfur bacteria. In contrast to green-colored Chl. tepidum, chlorosomes from the brown-colored species often contain domains of lamellar aggregates that may help them to survive in extremely low light conditions. We suggest that carotenoids are localized between the lamellar planes and drive lamellar assembly by augmenting hydrophobic interactions. A model for chlorosome assembly, which accounts for the role of carotenoids and secondary BChl homologs, is presented.     

Identification of the bacteriochlorophylls, carotenoids, quinones, lipids, and hopanoids of "Candidatus Chloracidobacterium thermophilum"

"Candidatus Chloracidobacterium thermophilum" is a recently discovered chlorophototroph from the bacterial phylum Acidobacteria, which synthesizes bacteriochlorophyll (BChl) c and chlorosomes like members of the green sulfur bacteria (GSB) and the green filamentous anoxygenic phototrophs (FAPs). The pigments (BChl c homologs and carotenoids), quinones, lipids, and hopanoids of cells and chlorosomes of this new chlorophototroph were characterized in this study. "Ca. Chloracidobacterium thermophilum" methylates its antenna BChls at the C-8(2) and C-12(1) positions like GSB, but these BChls were esterified with a variety of isoprenoid and straight-chain alkyl alcohols as in FAPs. Unlike the chlorosomes of other green bacteria, "Ca. Chloracidobacterium thermophilum" chlorosomes contained two major xanthophyll carotenoids, echinenone and canthaxanthin. These carotenoids may confer enhanced protection against reactive oxygen species and could represent a specific adaptation to the highly oxic natural environment in which "Ca. Chloracidobacterium thermophilum" occurs. Dihydrogenated menaquinone-8 [menaquinone-8(H(2))], which probably acts as a quencher of energy transfer under oxic conditions, was an abundant component of both cells and chlorosomes of "Ca. Chloracidobacterium thermophilum." The betaine lipid diacylglycerylhydroxymethyl-N,N,N-trimethyl-β-alanine, esterified with 13-methyl-tetradecanoic (isopentadecanoic) acid, was a prominent polar lipid in the membranes of both "Ca. Chloracidobacterium thermophilum" cells and chlorosomes. This lipid may represent a specific adaptive response to chronic phosphorus limitation in the mats. Finally, three hopanoids, diploptene, bacteriohopanetetrol, and bacteriohopanetetrol cyclitol ether, which may help to stabilize membranes during diel shifts in pH and other physicochemical conditions in the mats, were detected in the membranes of "Ca. Chloracidobacterium thermophilum."     

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     

Self-assembly of natural light-harvesting bacteriochlorophylls of green sulfur photosynthetic bacteria in silicate capsules as stable models of chlorosomes

Naturally occurring bacteriochlorophyll(BChl)s-c, -d, and -e from green sulfur photosynthetic bacteria were self-assembled in an aqueous solution in the presence of octadecyltriethoxysilane and tetraethoxysilane, followed by polycondensation of the alkoxysilanes by incubation for 50 h at 25 degrees C. The resulting BChl self-assemblies in silicate capsules exhibited visible absorption and circular dichroism spectra similar to the corresponding natural light-harvesting systems (chlorosomes) of green sulfur bacteria. Dynamic light scattering measurements indicated that the silicate capsules had an average hydrodynamic diameter of several hundred nanometers. BChl self-aggregates in silicate capsules were significantly stable to a nonionic surfactant Triton X-100, which was apt to decompose the BChl aggregates to their monomeric form, compared with conventional micelle systems. BChls in silicate capsules were more tolerant to demetalation of the central magnesium under acidic conditions than the natural systems.     

Two-dimensional mapping of photopigment distribution and activity of Chloroflexus-like bacteria in a hypersaline microbial mat

Pigment analysis in an intact hypersaline microbial mat by hyperspectral imaging revealed very patchy and spatially uncorrelated distributions of photopigments Chl a and BChl a/c, which are characteristic photopigments for oxygenic (diatoms and cyanobacteria) and anoxygenic phototrophs (Chloroflexaceae). This finding is in contrast to the expectation that these biomarker pigments should be spatially correlated, as oxygenic phototrophs are thought to supply the Chloroflexaceae members with organic substrates for growth. We suggest that the heterogeneous occurrence is possibly due to sulfide, whose production by sulfate-reducing bacteria may be spatially heterogeneous in the partially oxic photic zone of the mat. We furthermore mapped the near-infra-red-light controlled respiration of Chloroflexaceae under light and dark conditions and found that Chloroflexaceae are responsible for a major part of oxygen consumption at the lower part of the oxic zone in the mat. The presence of Chloroflexaceae was further confirmed by FISH probe and 16S rRNA gene clone library analysis. We assume that species related to the genera Oscillochloris and 'Candidatus Chlorothrix', in contrast to those related to Chloroflexus and Roseiflexus, depend less on excreted photosynthates but more on the presence of free sulfide, which may explain their presence in deeper parts of the mat.     

Chlorobium tepidum mutant lacking bacteriochlorophyll c made by inactivation of the bchK gene,encoding bacteriochlorophyll c synthase

The gene encoding bacteriochlorophyll (BChl) c synthase was identified by insertional inactivation in the photosynthetic green sulfur bacterium Chlorobium tepidum and was named bchK. The bchK mutant of C. tepidum was rusty-orange in color and completely lacked BChl c. Because of the absence of the BChl c antenna, the mutant grew about seven times slower than the wild type at light intensities that were limiting to the wild type (< 90 micromol m(-2) s(-1)). Various pheophorbides, which probably represent precursors of BChl c which had lost magnesium, accumulated in the mutant cells. A small fraction of these pheophorbides were apparently esterified by the remaining chlorophyll (Chl) a and BChl a synthases in cells. The amounts of BChl a, Chl a, isoprenoid quinones, carotenoids, Fenna-Matthews-Olson protein, and chlorosome envelope protein CsmA were not significantly altered on a cellular basis in the mutant compared to in the wild type. This suggests that the BChl a antennae, photosynthetic reaction centers, and remaining chlorosome components were essentially unaffected in the mutant. Electron microscopy of thin sections revealed that the mutant lacked normal chlorosomes. However, a fraction containing vestigial chlorosomes, denoted "carotenosomes," was partly purified by density centrifugation; these structures contained carotenoids, isoprenoid quinones, and a 798-nm-absorbing BChl a species that is probably protein associated. Because of the absence of the strong BChl c absorption found in the wild type, the bchK mutant should prove valuable for future analyses of the photosynthetic reaction center and of the roles of BChl a in photosynthesis in green bacteria. An evolutionary implication of our findings is that the photosynthetic ancestor of green sulfur bacteria could have evolved without chlorosomes and BChl c and instead used only BChl a-containing proteins as the major light-harvesting antennae.     

Protein structure,electron transfer and evolution of prokaryotic photosynthetic reaction centers

Photosynthetic reaction centers from a variety of organisms have been isolated and characterized. The groups of prokaryotic photosynthetic organisms include the purple bacteria, the filamentous green bacteria, the green sulfur bacteria and the heliobacteria as anoxygenic representatives as well as the cyanobacteria and prochlorophytes as oxygenic representatives. This review focuses on structural and functional comparisons of the various groups of photosynthetic reaction centers and considers possible evolutionary scenarios to explain the diversity of existing photosynthetic organisms.Abbreviations BChl bacteriochlorophyll - Chl chlorophyll - Rb Rhodobacter - Rp Rhodopseudomonas     

Antenna organization in green photosynthetic bacteria. 1. Oligomeric bacteriochlorophyll c as a model for the 740 nm absorbing bacteriochlorophyll c in Chloroflexus aurantiacus chlorosomes

Bacteriochlorophyll (BChl) c was extracted from Chloroflexus aurantiacus and purified by reverse-phase high-pressure liquid chromatography. This pigment consists of a complex mixture of homologues, the major component of which is 4-ethyl-5-methylbacteriochlorophyll c stearyl ester. Unlike previously characterized BChls c, the pigment from C. aurantiacus is a racemic mixture of diastereoisomers with different configurations at the 2a chiral center. Diluting a concentrated methylene chloride solution of BChl c with hexane produces an oligomer with absorption maxima at 740-742 and at 460-462 nm. Both the absorption spectrum and the fluorescence emission spectrum (maximum at 750 nm) of this oligomer closely match those of BChl c in chlorosomes. Further support for this model comes from the ability of alcohols, which disrupt BChl c oligomers by ligating the central Mg atom, to convert BChl c in chlorosomes to a monomeric form when added in low concentrations. The lifetime of fluorescence from the 740 nm absorbing BChl c oligomer is about 80 ps. Although exciton quenching might be unusually fast in the in vitro BChl c oligomer because of its large size and/or the presence of minor impurities, this result suggests that energy transfer from the BChl c antenna in chlorosomes must be very fast if it is to be efficient.