Species Of Green Sulfur Bacteria Research Articles

Phylogeny and taxonomy of Chlorobiaceae

Based on phylogenetic relationships found according to gene sequences of the 16S rRNA and the FMO (Fenna-Matthews-Olson protein) genes, and supported by the G + C content of the DNA and sequence signatures, the strains and species of green sulfur bacteria have been grouped into a phylogenetic system. Since properties used previously for classification such as cell morphology, photosynthetic pigments and substrate utilization do not conform with their phylogeny, a reassignment of strains to species, and a rearrangement among the species were necessary. The comparison of the traditional classification system of these bacteria with their phylogenetic relationship yielded a confusing picture. As a consequence of this rearrangement, species of the green sulfur bacteria were classified into the genera Chlorobium, Chlorobaculum, Prosthecochloris, and Chloroherpeton. Strains were assigned to the species according to their phylogenetic similarity and a number of new combinations, and new species were defined. New isolates and also environmental gene sequences fit very well into the established groups or may form new species, some of which have been described and others are awaiting their description. New strains and available gene sequences are included into the phylogenetic system, and a taxonomic classification on the species level is proposed.

Chirality-Based Signatures of Local Protein Environments in Two-Dimensional Optical Spectroscopy of Two Species Photosynthetic Complexes of Green Sulfur Bacteria: Simulation Study

Two-dimensional electronic chirality-induced signals of excitons in the photosynthetic Fenna-Matthews-Olson complex from two species of green sulfur bacteria ( Chlorobium tepidum and Prosthecochloris aestuarii) are compared. The spectra are predicted to provide sensitive probes of local protein environment of the constituent bacteriochlorophyll a chromophores and reflect electronic structure variations (site energies and couplings) of the two complexes. Pulse polarization configurations are designed that can separate the coherent and incoherent exciton dynamics contributions to the two-dimensional spectra.

Variable fluorescence in green sulfur bacteria

Green sulfur bacteria possess a complex photosynthetic machinery. The dominant light harvesting systems are chlorosomes, which consist of bacteriochlorophyll c, d or e oligomers with small amounts of protein. The chlorosomes are energetically coupled to the membrane-embedded iron sulfur-type reaction center via a bacteriochlorophyll a-containing baseplate protein and the Fenna–Matthews–Olson (FMO) antenna protein. The fluorescence yield and spectral properties of these photosynthetic complexes were investigated in intact cells of several species of green sulfur bacteria under physiological, anaerobic conditions. Surprisingly, green sulfur bacteria show a complex modulation of fluorescence yield upon illumination that is very similar to that observed in oxygenic phototrophs. Within a few seconds of illumination, the fluorescence reaches a maximum, which decreases within a minute of illumination to a lower steady state. Fluorescence spectroscopy reveals that the fluorescence yield during both processes is primarily modulated on the FMO-protein level, while the emission from chlorosomes remains mostly unchanged. The two most likely candidates that modulate bacteriochlorophyll fluorescence are (1) direct excitation quenching at the FMO-protein level and (2) indirect modulation of FMO-protein fluorescence by the reduction state of electron carriers that are part of the reaction center.

Phototrophic consortia: model systems for symbiotic interrelations between prokaryotes.

Most symbiotic prokaryotes known to date have been found in association with eukaryotes, whereas only few (3.5%) bacteria or archaea are known that have established interactions with other prokaryotes. As revealed by direct microscopic investigations, however, multiple morphotypes of highly structured associations of different prokaryotes exist in nature. These so-called consortia appear to represent the most developed type of bacterial interaction. Phototrophic consortia are associations of green sulfur bacteria that surround a central chemotrophic bacterium. In some natural environments, almost all cells of green sulfur bacteria occur in the associated state, i.e. as epibionts of phototrophic consortia. In contrast to earlier speculations, the central bacterium belongs to the beta-Proteobacteria. Within the consortia, the green sulfur bacterial epibionts grow photolithoautotrophically, utilizing exogenous sulfide as photosynthetic electron donor. The entire consortium does not appear to be independent of organic carbon compounds, since it exhibits chemotaxis towards 2-oxoglutarate and, as demonstrated by microautoradiography, can also incorporate this compound. Intact consortia exhibit a scotophobic response in which the bacteriochlorophylls of the epibionts function as light sensors, whereas the chemotrophic central bacterium confers motility upon the association. Hence, a signal exchange must occur between the different bacteria. Based on their 16S rRNA gene sequences, the epibionts represent distinct phylotypes that are often only distantly related to known species of green sulfur bacteria. Since phototrophic consortia have recently become available in enrichment cultures, they can now serve as suitable model systems for the investigation of the molecular mechanisms of cell-cell recognition and signal exchange, and for studies of the coevolution of nonrelated prokaryotes.

Composition and optical properties of reaction centre core complexes from the green sulfur bacteria Prosthecochloris aestuarii and Chlorobium tepidum.

Photosynthetically active reaction centre core (RCC) complexes were isolated from two species of green sulfur bacteria, Prosthecochloris (Ptc.) aestuarii strain 2K and Chlorobium (Chl.) tepidum, using the same isolation procedure. Both complexes contained the main reaction centre protein PscA and the iron-sulfur protein PscB, but were devoid of Fenna-Matthews-Olson (FMO) protein. The Chl. tepidum RCC preparation contained in addition PscC (cytochrome c). In order to allow accurate determination of the pigment content of the RCC complexes, the extinction coefficients of bacteriochlorophyll (BChl) a in several solvents were redetermined with high precision. They varied between 54.8 mM(-1) cm(-1) for methanol and 97.0 mM(-1) cm(-1) for diethylether in the Q(Y) maximum. Both preparations appeared to contain 16 BChls a of which two are probably the 13(2)-epimers, 4 chlorophylls (Chls) a 670 and 2 carotenoids per RCC. The latter were of at least two different types. Quinones were virtually absent. The absorption spectra were similar for the two species, but not identical. Eight bands were present at 6 K in the BChl a Q(Y) region, with positions varying from 777 to 837 nm. The linear dichroism spectra showed that the orientation of the BChl a Q(Y) transitions is roughly parallel to the membrane plane; most nearly parallel were transitions at 800 and 806 nm. For both species, the circular dichroism spectra were dominated by a strong band at 807-809 nm, indicating strong interactions between at least some of the BChls. The absorption, CD and LD spectra of the four Chls a 670 were virtually identical for both RCC complexes, indicating that their binding sites are highly conserved and that they are an essential part of the RCC complexes, possibly as components of the electron transfer chain. Low temperature absorption spectroscopy indicated that typical FMO-RCC complexes of Ptc. aestuarii and Chl. tepidum contain two FMO trimers per reaction centre.

Genes involved in hydrogen and sulfur metabolism in phototrophic sulfur bacteria

The dsr genes and the hydSL operon are present as separate entities in phototrophic sulfur oxidizers of the genera Allochromatium, Marichromatium, Thiocapsa and Thiocystis and are organized similarly as in Allochromatium vinosum and Thiocapsa roseopersicina, respectively. The dsrA gene, encoding the α subunit of ‘reverse’ siroheme sulfite reductase, is also present in two species of green sulfur bacteria pointing to an important and universal role of this enzyme and probably other proteins encoded in the dsr locus in the oxidation of stored sulfur by phototrophic bacteria. The hupSL genes are uniformly present in the members of the Chromatiaceae family tested. The two genes between hydS and hydL encode a membrane-bound b-type cytochrome and a soluble iron-sulfur protein, respectively, resembling subunits of heterodisulfide reductase from methanogenic archaea. These genes are similar but not identical to dsrM and dsrK, indicating that the derived proteins have distinct functions, the former in hydrogen metabolism and the latter in oxidative sulfur metabolism.

Genes involved in hydrogen and sulfur metabolism in phototrophic sulfur bacteria.

The dsr genes and the hydSL operon are present as separate entities in phototrophic sulfur oxidizers of the genera Allochromatium, Marichromatium, Thiocapsa and Thiocystis and are organized similarly as in Allochromatium vinosum and Thiocapsa roseopersicina, respectively. The dsrA gene, encoding the alpha subunit of 'reverse' siroheme sulfite reductase, is also present in two species of green sulfur bacteria pointing to an important and universal role of this enzyme and probably other proteins encoded in the dsr locus in the oxidation of stored sulfur by phototrophic bacteria. The hupSL genes are uniformly present in the members of the Chromatiaceae family tested. The two genes between hydS and hydL encode a membrane-bound b-type cytochrome and a soluble iron-sulfur protein, respectively, resembling subunits of heterodisulfide reductase from methanogenic archaea. These genes are similar but not identical to dsrM and dsrK, indicating that the derived proteins have distinct functions, the former in hydrogen metabolism and the latter in oxidative sulfur metabolism.

Electron transport and triplet formation in membrane fragments of the green sulfur bacterium Prosthecochloris aestuarii

Under conditions where normal electron transport is blocked, at high pH in the presence of dithionite, three triplets could be observed upon flash illumination of membrane fragments of the green sulfur bacterium Prosthecochloris aestuarii. The first triplet decayed in 7.5 μs and is assigned to carotenoid. The second triplet decays in 67 μs and is assigned to bacteriochlorophyll (BChl) a of the Fenna-Matthews-Olson (FMO) complex, since a triplet with the same spectrum and lifetime was also formed in the isolated FMO complex. The third triplet decayed in 165 μs and is assigned to BChl a of the reaction center core complex, based on its main bleaching at 837 nm. There is insufficient evidence to decide whether this triplet is located on the primary electron donor P840 or on a long-wavelength antenna BChl a of the core. A multiple-flash experiment indicated the presence of two photo-oxidizable hemes per reaction center (RC), both having a difference spectrum centered around 553 nm. The oxidation time was 25 μs for both cytochromes. However, a 75- μs delay was observed for the oxidation of the second heme, indicating that another process must take place before this reaction can occur. This result, together with the observed low efficiency of oxidation of the second heme, suggests the presence of a four-heme cytochrome (as observed in some other species of green sulfur bacteria), with only one heme in direct contact with the RC, rather than a model with two cytochromes symmetrically attached to the RC, as proposed by others. The observed delay can then be explained by a relatively slow heme-to-heme electron transfer. The cytochrome oxidation time of both hemes increased with viscosity, suggesting that some molecular motion is involved in the oxidation process.

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.

The nature of the primary electron acceptor in green sulfur bacteria

It was shown previously (Van de Meent, E.J., Kobayashi, M., Erkelens, C., Van Veelen, P.A., Amesz, J. and Watanabe, T. (1991) Biochim. Biophys. Acta 1058, 356–362) by means of HPLC, NMR and optical and mass spectroscopy that the primary electron acceptor of heliobacteria is 8 1-hydroxychlorophyll (Chl) a. In view of the spectral and functional similarities between this pigment and the primary electron acceptor of green sulfur bacteria, we have applied the same methods to various species of green sulfur bacteria ( Prosthecochloris aestuarii, Chlorobium limicola, C. limicola f. thiosulfatophilum, C. vibrioforme and C. phaeovibrioides) in order to study the identity and the occurrence of the latter pigment. It was already shown from flash spectroscopic and reversed phase HPLC experiments on isolated membranes and solubilized membrane fractions of P. aestuarii that the most likely candidate for the primary acceptor is a pigment named bacteriochlorophyll (BChl) 663, which had been tentatively identified as a lipophilic from of BChl c. In this communication we will show by means of optical spectroscopy, 252Cf-plasma desorption mass spectroscopy and 1H-NMR that BChl 663 is an isomer of Chl a. This result again emphasizes the similarities between the reaction centers of green sulfur bacteria, heliobacteria and Photosystem I. By means of normal-phase HPLC analysis of the five species of green sulfur bacteria it is shown that BChl 663 is universally present and in comparable quantities in this group of photosynthetic bacteria. No other pigments with similar spectroscopic properties were detected.