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At least 19 recordsLinked to original sources

PubMed2005

PpsR: a multifaceted regulator of photosynthesis gene expression in purple bacteria.

Purple bacteria control the level of expression and the composition of their photosystem according to light and redox conditions. This control involves several regulatory systems that have been now well characterized. Among them, the PpsR regulator plays a central role, because it directly or indirectly controls the synthesis of all of the different components of the photosystem. In this review, we report our knowledge of the PpsR protein, highlighting the diversity of its mode of action and focusing on the proteins identified in four model purple bacteria (Rhodobacter capsulatus, Rhodobacter sphaeroides, Rubrivivax gelatinosus, Bradyrhizobium ORS278). This regulator exhibits unique regulatory features in each bacterium: it can activate and/or repress the expression of photosynthesis genes, its activity can be modulated or not by the redox conditions, it can interact with other specific regulators and therefore be involved differently in light and/or redox regulatory circuits.

Bacterial Proteins↗

PubMedDate not supplied

[Possible pathways for acetyl-CoA formation by purple bacteria].

Purple sulfur (Ectothiorhodospira shaposhnikovii, Chromatium minutissimum, Lamprobacter modestohalophilus, Thiocapsa roseopersicina) and nonsulfur (Rhodospirillum rubrum, Rhodopseudomonas palustris, Rhodopseudomonas spheroides) bacteria are capable of forming acetyl-CoA synthetase, phosphotransacetylase and acetokinase independent of the medium composition and growth conditions. In all of the purple sulfur bacteria with an exception of E. shaposhnikovii, the activity of acetokinase is much higher than in purple nonsulfur bacteria. Apart from being involved in the synthesis of acetyl-CoA, such enzymes as phosphotransacetylase, acetokinase and adenylate kinase may play an important role in energy processes of some purple bacteria in the dark.

Acetyl Coenzyme A↗

PubMedDate not supplied

[Use of urea by purple bacteria].

Strains of purple sulfur bacteria (Chromatium minutissimum, Ectothiorhodospira shaposhnikovii, Thiocapsa roseopersicina, Lamprobacter modestohalophilus) and nonsulfur bacteria (Rhodopseudomonas palustris, Rh. spheroides, Rhodospirillum rubrum) grow in media containing urea as a source of nitrogen at concentrations from 0.5 to 5.0%. They can also utilize the carbon of urea and thus grow in the absence of bicarbonate. Urea is decomposed by all the studied purple bacteria with the participation of urease. In a number of strains, the enzyme is inducible and is synthesized only in the presence of urea. However, it is constitutive in certain purple bacteria (L. modestohalophilus, Rh. palustris, Rh. spheroides). The strains of purple bacteria differ in the activity of urease and in its susceptibility to ammonium ions.

Chromatiaceae↗

PubMed1996

Widespread use of the glu-tRNAGln transamidation pathway among bacteria. A member of the alpha purple bacteria lacks glutaminyl-trna synthetase.

The expression of the Rhizobium meliloti glutamyl-tRNA synthetase gene in Escherichia coli under the control of a trc promoter results in a toxic effect upon isopropyl-beta-D-thiogalactopyranoside induction, which is probably caused by a misacylation activity. To further investigate this unexpected result, we looked at the pathway of Gln-tRNAGln formation in R. meliloti. No glutaminyl-tRNA synthetase activity has been found in R. meliloti crude extract, but we detected a specific aminotransferase activity that changes Glu-tRNAGln to Gln-tRNAGln. Our results show that R. meliloti, a member of the alpha-subdivision of the purple bacteria, is the first Gram-negative bacteria reported to use a transamidation pathway for Gln-tRNAGln synthesis. A phylogenetic analysis of the contemporary glutamyl-tRNA synthetase and glutaminyl-tRNA synthetase amino acid sequences reveals that a close evolutionary relationship exists between R. meliloti and yeast mitochondrial glutamyl-tRNA synthetases, which is consistent with an origin of mitochondria in the alpha-subdivision of Gram-negative purple bacteria. A 256-amino acid open reading frame closely related to bacterial glutamyl-tRNA synthetases, which probably originates from a glutamyl-tRNA synthetase gene duplication, was found in the 4-min region of the E. coli chromosome. We suggest that this open reading frame is a relic of an ancient transamidation pathway that occurred in an E. coli ancestor before the horizontal transfer of a eukaryotic glutaminyl-tRNA synthetase (Lamour, V., Quevillon, S., Diriong, S., N'Guyen, V. C., Lipinski, M., and Mirande, M.(1994) Proc. Natl. Acad. Sci. U. S. A. 91, 8670-8674) and that it favored its stable acquisition. From these observations, a revisited model for the evolution of the contemporary glutamyl-tRNA synthetases and glutaminyl-tRNA synthetases that differs from the generally accepted model for the evolution of aminoacyl-tRNA synthetases is proposed.

Amino Acid Sequence↗

PubMed2005

On the role of basic residues in adapting the reaction centre-LH1 complex for growth at elevated temperatures in purple bacteria.

The purple photosynthetic bacterium Thermochromatium tepidum is a moderate thermophile, with a growth optimum of 48-50 degrees C. The X-ray crystal structure of the reaction centre from this organism has been determined, and compared with that from mesophilic bacteria such as Blastochloris viridis and Rhodobacter sphaeroides (Nogi T et al. (2000) Proc Natl Acad Sci USA 97: 13561-13566). Structural features that could contribute to the enhanced thermal stability of the Thermochromatium tepidum reaction centre were discussed, including three arginine residues exposed at the periplasmic side of the membrane that are not present in reaction centres from mesophilic organisms, and potentially could increase the affinity of the complex for the surrounding membrane. In the present report these arginine residues, plus a histidine identified from an extensive sequence alignment, were engineered into structurally homologous positions in the Rhodobacter sphaeroides reaction centre, and the effect on the thermal stability of the Rhodobacter sphaeroides complex was examined. We find that these residues do not enhance the thermal stability of the reaction centre, as assessed by absorbance spectroscopy of the bacteriochlorin cofactors in membrane-bound reaction centres. Possible roles of these residues in the Thermochromatium tepidum reaction centre are discussed, and it is proposed that they facilitate stronger binding of the reaction centre to the encircling LH1 antenna complex, through ionic interactions with acidic residues at the C-terminal end of the LH1 alpha-polypeptide. Such an interaction could enhance the stability of the so-called 'RC-LH1 core' complex that is formed between the reaction centre and the LH1 antenna, and which represents the minimal functional photosynthetic unit in all known purple photosynthetic bacteria. Stronger bonding interactions between the two complexes could also contribute to an increase in the rigidity of the photosynthetic membrane in Thermochromatium tepidum, in accord with the general finding that the cytoplasmic membrane from thermophilic eubacteria is less fluid than its counterpart in mesophilic bacteria.

Cell Proliferation↗

PubMed1987

Oxidation of dimethyl sulfide to dimethyl sulfoxide by phototrophic purple bacteria.

Enrichment cultures of phototrophic purple bacteria rapidly oxidized up to 10 mM dimethyl sulfide (DMS) to dimethyl sulfoxide (DMSO). DMSO was qualitatively identified by proton nuclear magnetic resonance. By using a biological assay, DMSO was always quantitatively recovered from the culture media. DMS oxidation was not detected in cultures incubated in the dark, and it was slow in cultures exposed to full daylight. Under optimal conditions, the second-order rate constant for DMS oxidation was 6 day mg of protein ml. The rate constant was reduced in the presence of high concentration of sulfide (>1 mM), but was not affected by the addition of acetate. DMS was also oxidized to DMSO by a pure strain (tentatively identified as a Thiocystis sp.) isolated from the enrichment cultures. DMS supported growth of the enrichment cultures and of the pure strain by serving as an electron source for photosynthesis. A determination of the amount of protein produced in the cultures and an estimation of the electron balance suggested that the two electrons liberated during the oxidation of DMS to DMSO were quantitatively used to reduce carbon dioxide to biomass. The oxidation of DMS by phototrophic purple bacteria may be an important source of DMSO detected in anaerobic ponds and marshes.

Journal Article↗

PubMedDate not supplied

Taxonomy of phototrophic green and purple bacteria: a review.

The presently existing classification for the green and purple bacteria comprises physiological-ecological assemblages of phototrophic bacteria with anoxygenic photosynthesis. The taxonomic units of the different levels were based entirely on common phenotypic traits, including morphological, cytological, physiological and biochemical characteristics. Since degrees of resemblance form the basis of the grouping, this classification cannot reflect the genetic or evolutionary relatedness of these bacteria, neither among themselves nor with other bacteria. The advantage of the artificial system, however, is the use of features which can be established in most laboratories and which allow the comparison and identification of newly isolated strains with those already studied and described. The four existing families correspond to the four major recognized, ecophysiological groups, the Chlorobiaceae and Chloroflexaceae among the green bacteria, and the Chromatiaceae and Rhodospirillaceae among the purple bacteria. Our knowledge of all these groups is incomplete; this is reflected by the fact that seven new species have been described during the past three years (6th Newsletter on phot. bacteria, Trüper and Hansen, 1982). The description of the new genus and species Erythrobacter longus (Shiba and Simidu, 1982) is also interesting, as it comprises aerobic chemoorganotrophic marine bacteria which form bacteriochlorophyll a and carotenoids; however, no strains were able to grow phototrophilcally. Significant success is currently being obtained in the different approaches toward elucidating the genetic relationships within and outside of the purple and green bacteria. Detailed studies of the lipopolysaccharides of several species and genera of the Rhodospirillaceae (Weckesser et al., 1979, and more recent paper) have proven to be very useful for the recognition of relationships or dissimilarities between the species of a genus or between different genera. Amino acid sequence studies of cytochromes c from Rhodospirillaceae, other bacteria and eukaryotic organisms (Dickerson, 1980) have led to the recognition of four different groups of cytochrome c molecules (long, medium and two groups of short protein chains). The subdivision of the Rhodospirillaceae into three species groups, each possessing one of the three types of cytochrome c, proved to be in total agreement with the results of oligonucleotide cataloging of the 16 S ribosomal RNA of these bacteria (Gibson et al., 1979). The latter method also revealed that several chemotropic bacteria, including the nitrifying bacteria, are more closely related to certain purple bacteria than different species of the purple bacteria among themselves (Seewaldt et al., 1982).(ABSTRACT TRUNCATED AT 400 WORDS)

Bacteria↗

PubMed1999

Rhodovulum iodosum sp. nov. and Rhodovulum robiginosum sp. nov., two new marine phototrophic ferrous-iron-oxidizing purple bacteria.

Two new strains of marine purple bacteria, N1T and N2T, were isolated from coastal sediment of the North Sea (Germany) with ferrous iron as the only electron donor for anoxygenic photosynthesis. The isolates are the first salt-dependent, ferrous-iron-oxidizing purple bacteria characterized so far. Analysis of 16S rRNA gene sequences revealed an affiliation with the genus Rhodovulum, which until now comprises only marine species. The sequence similarity of both strains was 95.2%, and their closest relative was Rhodovulum adriaticum. Like all known Rhodovulum species, the new strains had ovoid to rod-shaped cells, contained bacteriochlorophyll a and carotenoids of the spheroidene series, and were able to oxidize sulfide and thiosulfate. Like Rhodovulum adriaticum, both strains were unable to assimilate sulfate; for growth they needed a reduced sulfur source, e.g. thiosulfate. In contrast to the new strains, none of the known Rhodovulum species tested was able to oxidize ferrous iron or iron sulfide. In growth experiments, strains N1T and N2T oxidized 65 and 95%, respectively, of the ferrous iron supplied. Electron diffraction analysis revealed ferrihydrite as the main product of ferrous iron oxidation. In addition, traces of magnetite were formed. Strains N1T (= DSM 12328T) and N2T (= DSM 12329T) are described as Rhodovulum iodosum sp. nov. and Rhodovulum robiginosum sp. nov., respectively.

Bacteria↗

PubMed1956

Delayed light production by blue-green algae, red algae, and purple bacteria.

1. Blue-green algae, red algae, and purple bacteria all show the emission of delayed light. 2. The action spectra for the production of delayed light by three species of blue-green algae have one broad band with a peak at 620 mmicro. 3. The action spectrum for production of delayed light by the red algae has one peak at 550 mmicro with a shoulder from 600 to 660 mmicro. 4. The emission spectra of the delayed light from both the blue-green and red algae were the same as from the green algae, Chlorella. 5. The action spectra for the production of delayed light by the different species of purple bacteria tested consisted of one or more bands not resolved between 800 and 900 mmicro. 6. The emission spectrum of the delayed light from the purple bacteria was largely at wave lengths longer than 900 mmicro.

Bacteria↗

PubMed1996

Formation of the light-harvesting complex I (B870) of anoxygenic phototrophic purple bacteria.

The light-harvesting (LH) complex I (B870) of anoxygenic photosynthetic purple bacteria is the oligomeric form of its subunit B820 consisting of the low-molecular-weight polypeptides alpha, beta, bacteriochlorophyll (BChl), and carotenoids in the stoichiometric ratio [alpha1 beta1 (BChl2) Crt1-2]n. LHI surrounds the photochemical reaction center (RC). The major absorption band of the LHI complex is species-specific and is found at 870-890 nm; those of the subunit and the monomeric BChl a (dissolved in methanol) absorb at 820 and 770 nm, respectively. The isolated LHI complex can be reversibly dissociated to the B820 subunit or to the polypeptides and pigments by addition of detergents. Reconstitution of the B820 or the functional B870 complex is still possible after partial truncation of the N- or C-terminal regions of the alpha- or beta-polypeptide or of the beta-polypeptide only. The minimal structural requirements for reconstitution of a spectrally wild-type form after truncation of the polypeptides and/or modifications of the BChl molecule are described. The insertion of the LHIalpha- and LHIbeta-polypeptides into the membrane and the in vivo assembly of LHI, studied in a cell-free system and in whole cells of Rhodobacter capsulatus, depend on the primary structures of both polypeptides, BChl, the chaperones DnaK and GroEL, membrane-bound proteins, and energized membranes. Exchanges, deletions, or insertions of amino acyl residues, especially in the conserved region of the N-terminus of the LHIalpha-polypeptide, prevent or reduce the efficiency and stability of the LHI assembly. Therefore, reconstitution of LHI in a detergent micelle does not exactly reproduce the formation of the LHI complex in the photosynthetic membrane in vivo. The N-terminal domains play a crucial role in the formation of the oligomeric protein scaffold and of the pigment array. Facultatively phototrophic bacteria such as Rhodospirillum (Rsp.) rubrum or Rhodobacter (Rba.) capsulatus can adjust to changes in oxygen tension, light intensity, temperature, and substrates to grow under chemotrophic or phototrophic conditions. The photosynthetic apparatus (PSA), localized mainly on intracytoplasmic membranes (ICM), is usually synthesized only under low oxygen partial pressure. The cellular amount and composition of the PSA are modified upon changing light intensity in relation to cell growth (Drews and Golecki 1995). The morphogenesis of cellular structures like ICM is quite different from self-assembly. Self-assembly is a reversible process of aggregation of the constituents of a complex structure without protein synthesis and is driven by weak or strong forces in the interactions of the constituents. Morphogenesis results from the interplay of numerous gene products and the cellular organization and is always dependent upon pre-existent structures (Harold 1995). The morphogenesis of the photosynthetic membrane in purple bacteria has been studied in its different steps. The regulation at the transcriptional and post-transcriptional levels in purple bacteria, and the structure and morphogenesis of the ICM have been described recently (Armstrong 1995; Bauer 1995; Biel 1995; Drews and Golecki 1995; Klug 1995). In this mini-review, I will focus on the minimal requirements for the in vitro assembly of light-harvesting (LH) complex I (B870) from its constituents in detergent micelles and compare the results with observations on the complex process of targeting and import of LHI polypeptides into the membrane and assembly of B870.

Amino Acid Sequence↗

PubMed2000

Escape probability and trapping mechanism in purple bacteria: revisited.

Despite intensive research for decades, the trapping mechanism in the core complex of purple bacteria is still under discussion. In this article, it is attempted to derive a conceptionally simple model that is consistent with all basic experimental observations and that allows definite conclusions on the trapping mechanism. Some experimental data reported in the literature are conflicting or incomplete. Therefore we repeated two already published experiments like the time-resolved fluorescence decay in LH1-only purple bacteria Rhodospirillum rubrum and Rhodopseudomonas viridis chromatophores with open and closed (Q(A)(-)) reaction centers. Furthermore, we measured fluorescence excitation spectra for both species under the two redox-conditions. These data, all measured at room temperature, were analyzed by a target analysis based on a three-state model (antenna, primary donor, and radical pair). All states were allowed to react reversibly and their decay channels were taken into consideration. This leads to seven rate constants to be determined. It turns out that a unique set of numerical values of these rate constants can be found, when further experimental constraints are met simultaneously, i.e. the ratio of the fluorescence yields in the open and closed (Q(A)(-)) states F(m)/F(o) approximately 2 and the P(+)H(-)-recombination kinetics of 3-6 ns. The model allows to define and to quantify escape probabilities and the transfer equilibrium. We conclude that trapping in LH1-only purple bacteria is largely transfer-to-the-trap-limited. Furthermore, the model predicts properties of the reaction center (RC) in its native LH1-environment. Within the framework of our model, the predicted P(+)H(-)-recombination kinetics are nearly indistinguishable for a hypothetically isolated RC and an antenna-RC complex, which is in contrast to published experimental data for physically isolated RCs. Therefore RC preparations may display modified kinetic properties.

Bacterial Chromatophores↗

PubMed2001

Horizontal transfer of the photosynthesis gene cluster and operon rearrangement in purple bacteria.

A 37-kb photosynthesis gene cluster was sequenced in a photosynthetic bacterium belonging to the beta subclass of purple bacteria (Proteobacteria), Rubrivivax gelatinosus. The cluster contained 12 bacteriochlorophyll biosynthesis genes (bch), 7 carotenoid biosynthesis genes (crt), structural genes for photosynthetic apparatuses (puf and puh), and some other related genes. The gene arrangement was markedly different from those of other purple photosynthetic bacteria, while two superoperonal structures, crtEF-bchCXYZ-puf and bchFNBHLM-lhaA-puhA, were conserved. Molecular phylogenetic analyses of these photosynthesis genes showed that the photosynthesis gene cluster of Rvi. gelatinosus was originated from those of the species belonging to the alpha subclass of purple bacteria. It was concluded that a horizontal transfer of the photosynthesis gene cluster from an ancestral species belonging to the alpha subclass to that of the beta subclass of purple bacteria had occurred and was followed by rearrangements of the operons in this cluster.

Amino Acid Sequence↗

PubMed1999

Uphill energy transfer in LH2-containing purple bacteria at room temperature

Uphill energy transfer in the LH2-containing purple bacteria Rhodopseudomonas acidophila, Rhodopseudomonas palustris, Rhodobacter sphaeroides, Chromatium vinosum and Chromatium purpuratum was studied by stationary fluorescence spectroscopy at room temperature upon selective excitation of the B800 pigments of LH2 and the B880 pigments of LH1 at 803 nm and 900 nm, respectively. The resulting fluorescence spectra differed significantly at wavelengths shorter than the fluorescence maximum but agreed at longer wavelengths. The absorption spectra of the species studied were decomposed into five bands at approx. 800, 820, 830, 850 and 880 nm using the shapes of the absorption spectra of the LH1-RC only species Rhodospirillum rubrum and the isolated B800-850 complex from Rps. acidophila strain 10050 as guide spectra. This allowed a quantification of the number of pigments in each pigment group and, consequently, the antenna size of the photosynthetic unit assuming 36 bacteriochlorophyll a molecules in an LH1-RC complex. In most of the LH2-containing purple bacterial strains the number of LH2 rings per LH1-RC was less than the idealized number of eight (Papiz et al., Trends Plant Sci. 1 (1996) 198-206), which was achieved only by C. purpuratum. Uphill energy transfer was assayed by comparing the theoretical fluorescence spectrum obtained from a Boltzmann equilibrium with the measured fluorescence spectrum obtained by 900 nm excitation. The good match of both spectra in all the purple bacteria studied indicates that uphill energy transfer occurs practically up to its thermodynamically maximal possible extent. All strains studied contained a small fraction of either poorly connected or unconnected LH2 complexes as indicated by higher fluorescence yields from the peripheral complexes than predicted by thermal equilibration or kinetic modeling. This impedes generally the quantitative analysis of blue-excited fluorescence spectra.

Journal Article↗

PubMed2003

The structural basis of light-harvesting in purple bacteria.

A typical purple bacterial photosynthetic unit consists of two types of light-harvesting complex (LH1 and LH2) together with a reaction centre. This short review presents a description of the structure of the LH2 complex from Rhodopseudomonas acidophila, which has recently been improved to a resolution of 2.0 A [Papiz et al., J. Mol. Biol. 326 (2003) 1523-1538]. We show how this structure has helped to reveal the details of the various excitation energy transfer events in which it is involved.

Bacterial Proteins↗

PubMedDate not supplied

[Morphological differentiation in non-sulfur purple bacteria].

Cell differentiation in non-sulphur purple bacteria is complicated, as compared to binary fission, during bud formation with production of hyphae and special resting cells of the exospore type, and can be demonstrated in the folowing series of microorganisms: Rhodopseudomonas sulfidophila leads to Rh. capsulata leads to Rh. acidophila leads to Rh. viridis and Rh. palustris leads to Rh. sulfoviridis leads to Rhodom. vannielii. Contrary to phototrophic bacteria multiplying by division, a distinct sequence in appearance of various morphological forms was found in bud-forming bacteria. Formation of specialized cells is regulated by the substrate concentration, accumulation of metabolites, and the population density. Mobile cells are not formed at all or partly, resting cells are formed sometimes at a higher rate, and vegetative cells contain reserve substances in all bud-forming phototrophic bacteria if conditions of the environment are not optimal.

Cell Division↗

PubMedDate not supplied

[Effect of nitrogen-containing compounds on hydrogen light emission and nitrogen fixation by purple bacteria].

The cells of Rhodospirillum rubrum and Thiocapsa roseopersicina grown in media containing glutamate and arginine, respectively, as well as under conditions of nitrogen fixation evolve H2 in the light. If the cultures were grown in media with NH4+, NO3-, urea, glutamine or asparagine, hydrogen photoevolution by the cells and acetylene reduction started after the lag-phase and proceeded at a low rate. Extracts of such cells did not display the activity of nitrogenase which could be assayed by the ATP-dependent evolution of H2 from dithionite. The data obtained confirm the fact that hydrogen photoevolution by purple bacteria involves nitrogenase whose synthesis is regulated (according to the action of glutamine) with the participation of glutamine synthetase. NH4+, glutamine and asparagine inhibit also hydrogen photoproduction by purple bacteria and acetylene photoreduction. However, they have no effect on hydrogen evolution in the dark by the cells of R. rubrum and T. roseopersicina in the presence of formiate or pyruvate, respectively, whereas carbon monoxide inhibits hydrogen production. Therefore, hydrogen production by purple bacteria in the dark must be catalyzed by hydrogenase.

Arginine↗

PubMed1984

X-ray diffraction studies on chromatophore membrane from photosynthetic bacteria. II. Comparison of diffraction patterns of photosynthetic units from various purple bacteria.

Comparative X-ray diffraction studies, in conjunction with infrared absorption spectroscopy, were performed on chromatophores isolated from various purple photosynthetic bacteria in order to achieve a better understanding of the molecular structure of the photosynthetic unit. Purple non-sulfur bacteria used were Rhodospirillum rubrum, Rhodospirillum molischianum, Rhodopseudomonas sphaeroides, and Rhodopseudomonas palustris. Chromatophores of Chromatium vinosum, as a typical example of purple sulfur bacteria, were also investigated. The results were as follows. Distinct equatorial X-ray diffraction patterns were obtained from chromatophores of all the bacteria examined. They showed diffuse, continuous diffraction patterns having several maxima, and the patterns are evidently distinguished from those of either crystalline or amorphous material. The pattern indicates that the photosynthetic unit in the chromatophore has a highly organized molecular structure in the plane of the membrane. Bacteria whose major photosynthetic pigment is bacteriochlorophyll alpha can be categorized in three groups from the viewpoint of near infrared absorption spectra. X-ray diffraction patterns are also grouped accordingly, although the differences are minimal and the patterns display common features. In other words, the bacteriochlorophyll forms, which are bacteriochlorophyll-protein complexes exhibiting different near-infrared absorption spectra, show different X-ray patterns: the molecular structure of photosynthetic units is closely related to the state of pigment in each complex, although the "X-ray" molecular structure is mainly concerned with the arrangement of constituent protein molecules at the present resolution, whereas the "spectroscopic" structure reflects the local environment of pigment.(ABSTRACT TRUNCATED AT 250 WORDS)

Bacterial Chromatophores↗

PubMed2003

Remarkable diversity of phototrophic purple bacteria in a permanently frozen Antarctic lake.

Although anoxygenic photosynthesis is thought to play an important role in the primary productivity of permanently frozen lakes in the Antarctic dry valleys, the bacterial communities responsible for this metabolism remain uncharacterized. Here we report the composition and activity of phototrophic purple bacteria in Lake Fryxell, Antarctica, as determined by analysis of a photosynthesis-specific gene, pufM. The results revealed an extensive diversity and highly stratified distribution of purple nonsulfur bacteria in Lake Fryxell and showed which phylotypes produced pufM transcripts in situ. Enrichment cultures for purple bacteria yielded two morphotypes, each with a pufM signature identical to signatures detected by environmental screening. The isolates also contained gas vesicles, buoyancy structures previously unknown in purple nonsulfur bacteria, that may be necessary for these organisms to position themselves at specific depths within the nearly freezing water column.

Antarctic Regions↗