Discovery of the heliobacteria

The first photosynthetic bacterium obtained in pure culture wasRhodospirillum rubrum, isolated by Erwin Esmarch in 1887. The organism appeared to be an aerobic heterotroph, and Esmarch was unaware of its photosynthetic capability. The overall general characteristics of a number of major species of photosynthetic bacteria were described by Molisch and van Niel before 1945. Subsequently, our knowledge of the anoxygenic phototrophs increased greatly through the systematic study of numerous new species isolated from enrichment cultures in which capacity for anaerobic (and anoxygenic) growth with light as the energy source was a primary selective factor. A further refinement of the enrichment technique required ability to use N₂ as the sole source of nitrogen for growth under anaerobic photosynthetic conditions, and this led to the isolation of additional new species, including the heliobacteria. The first recognition of the heliobacteria was facilitated by serendipity, which was a significant factor in a number of other researches on photosynthetic bacteria (Gest 1992).     

Protein sequences and redox titrations indicate that the electron acceptors in reaction centers from heliobacteria are similar to Photosystem I

Photosynthetic reaction centers isolated from Heliobacillus mobilis exhibit a single major protein on SDS-PAGE of 47 000 M_r. Attempts to sequence the reaction center polypeptide indicated that the N-terminus is blocked. After enzymatic and chemical cleavage, four peptide fragments were sequenced from the Heliobacillus mobilis apoprotein. Only one of these sequences showed significant specific similarity to any of the protein and deduced protein sequences in the GenBank data base. This fragment is identical with 56% of the residues, including both cysteines, found in the highly conserved region that is proposed to bind iron-sulfur center F_X in the Photosystem I reaction center peptide that is the psaB gene product. The similarity to the psaA gene product in this region is 48%.Redox titrations of laser-flash-induced photobleaching with millisecond decay kinetics on isolated reaction centers from Heliobacterium gestii indicate a midpoint potential of –414 mV with n=2 titration behavior. In membranes, the behavior is intermediate between n=1 and n=2, and the apparent midpoint potential is –444 mV. This is compared to the behavior in Photosystem I, where the intermediate electron acceptor A₁, thought to be a phylloquinone molecule, has been proposed to undergo a double reduction at low redox potentials in the presence of viologen redox mediators.These results strongly suggest that the acceptor side electron transfer system in reaction centers from heliobacteria is indeed analogous to that found in Photosystem I. The sequence similarities indicate that the divergence of the heliobacteria from the Photosystem I line occurred before the gene duplication and subsequent divergence that lead to the heterodimeric protein core of the Photosystem I reaction center.Abbreviations BChl bacteriochlorophyll - %C percent bisacrylamide as a percentage of total acrylamide - DTT dithiothreitol - EPR electron paramagnetic resonance - Fe-S iron-sulfur center - H. Heliobacterium - Hb. Heliobacillus - k one thousand - M_r molecular retention - PS I Photosystem I - PS II Photosystem II - RCs reaction centers - SDS sodium dodecyl sulfate - SDS-PAGE sodium dodecyl sulfate polyacrylamide electrophoresis - %T percent total acrylamide - Tris tris(hydroxymethyl)aminomethane     

<Emphasis Type="Italic">Roseococcus suduntuyensis</Emphasis> sp. nov., a new aerobic bacteriochlorophyll <Emphasis Type="Italic">a</Emphasis>-containing bacterium isolated from a low-mineralized soda lake of Eastern Siberia

A novel strain, SHET, of aerobic bacteriochlorophyll a-containing bacteria was isolated from the surface layer of bottom sediments from the soda lake Shuluutai-Ekhe-Torom (Chita oblast, Eastern Siberia, Russia). The lake water has a total mineralization of 30 g/l and a pH of 9.2. The cells of strain SHET are cocci or short rods, which reproduce by uniform division. The cells are motile by means of flagella. The cell wall structure is of the gram-negative type. Sparse intracytoplasmic membrane vesicles are located close to the cell wall. The new isolate is an obligate aerobe and facultative alkaliphile which grows in a pH range of 7.5–9.5 (with an optimum at pH 8.5–9.0). The best growth of strain SHET occurred at 2.0 g/l NaCl and 23–28°C. Photosynthetic pigments are represented by bacteriochlorophyll a, with the maximum absorption at 865 nm in the in vivo spectrum, and carotenoids (spirilloxanthin derivatives). Analysis of the 16S rRNA gene sequences demonstrated that strain SHET is closely related to Roseococcus thiosulfatophilus of the α-1 subclass of Proteobacteria (98.6 % similarity). The DNA G+C base content is 69.1 mol %. Unlike Rsc. thiosulfatophilus, strain SHET grows well on sugars and glycerol and is not capable of utilizing thiosulfate as an energy source. The new isolate is a facultative alkaliphile and reduces nitrates to nitrites. On the basis of its phenotypic and genetic characteristics, strain SHET was described as a new species of the genus Roseococcus, Rsc. suduntuyensis sp. nov.     

Anoxygenic phototrophic bacteria of the high-altitude meromictic Lake Gek-Gel,Azerbaijan

The anoxygenic phototrophic bacterial community of the high-altitude meromictic Lake Gek-Gel (Azerbaijan) was investigated in September 2003. The highest concentration of bacteriochlorophyll e (48 μg/l) was detected at a depth of 30 m; the peak of bacteriochlorophyll a (4.5 μg/l) occurred at 29 m. Phylogenetic analysis revealed that brown-colored green sulfur bacteria Chlorobium phaeobacteroides predominated in the lake. Nonsulfur purple bacteria phylogenetically close to Blastochloris sulfoviridis were found in insignificant amounts; these organisms have not been previously reported in Lake Gek-Gel.     

Heliobacterium sulfidophilum sp. nov. andHeliobacterium undosum sp. nov.: Sulfideoxidizing heliobacteria from thermal sulfidic springs

Two new species of heliobacteria isolated from cyanobacterial mats of two alkaline sulfidic hot springs are formally described. Strains BR4 and BG29 are assigned to anoxygenic phototrophic bacteria of the familyHeliobacteriaceae, since they possess the unique properties of this taxon: strict anaerobiosis, formation of bacteriochlorophyllg, the lack of extensive intracytoplasmic membranes and chlorosomes, an unusual cell wall structure, and phylogenetic relatedness to the low G+C gram-positive eubacteria. Based on the 16S rDNA sequence similarity, strains BR4 and BG29 are assigned to the genusHeliobacterium and described as two new species of this genus:Heliobacterium sulfidophilum sp. nov. andHeliobacterium undosum sp. nov. The G+C content of the DNA is 51.3 mol % inHbt. sulfidophilum and 57.2-57.7 mol % inHbt. undosum. The cells ofHbt. sulfidophilum are rods, and the cells ofHbt. undosum are slightly twisted spirilla or short rods. Both new bacteria are motile by peritrichous flagella.Hbt. sulfidophilum produces endospores. The new bacteria are strict anaerobes growing photoheterotrophically on a limited range of organic compounds. In the dark, they can switch from photosynthesis to the slow fermentation of pyruvate. Biotin is required as a growth factor. Both species are highly tolerant to sulfide (up to 2 mM at pH 7.5) and oxidize it photoheterotrophically to elemental sulfur; photoautotrophic growth was not observed. The temperature optimal for growth ofHbt. sulfidophilum andHbt undosum is 30–35‡C, and the optimal pH is 7–8.     

Growth and adaptation to phototrophic conditions of Rhodospirillum rubrum and Rhodopseudomonas sphaeroides at different temperatures

The influence of temperature on yields of cell protein and bacteriochlorophyll as well as on the rates of growth and bacteriochlorophyll synthesis was studied with Rhodospirillum rubrum and Rhodopseudomonas sphaeroides. Under chemotrophic conditions net cell-protein production increased in cultures of both species along with temperature from 14°C up to the optimum at 33°C. Under phototrophic conditions cell-protein yields were largely constant within the range from 21°C to 33°C. At temperatures below 21°C and above 33°C yields decreased. These results are interpreted in terms of coupling between energy yielding or redox equivalent providing metabolisms and cell biosynthesis. Upon adaptation from chemotrophic to phototrophic conditions a direct relationship between temperature increase and bacteriochlorophyll level was observed. Arrhenius plots of both, specific growth rates and rates of bacteriochlorophyll synthesis, revealed discontinuities at about 20°C. Temperature coefficients either above or below those discontinuities were similar in both species. In R. rubrum temperature coefficients of the synthesis of total bacteriochlorophyll were also representative of the synthesis of photochemical reaction center and light harvesting bacteriochlorophylls. But in R. sphaeroides significant differences were observed between temperature coefficients of the syntheses of bacteriochlorophylls of the costantly composed reaction centerlight harvesting complex on one hand and of both, total and the quantitatively variable light harvesting bacteriochlorophylls on the other. The results are interpreted in light of hypotheses on the regulation (a) of cellular bacteriochlorophyll levels as well as (b) of the ratio of functionally different bacteriochlorophylls in the photosynthetic apparatus.Abbreviation Bchl bacteriochlorophyll     

Isolation and characterization of<Emphasis Type="Italic"> Erythrobacter</Emphasis> sp. strains from the upper ocean

Seven strains of marine aerobic anoxygenic phototrophs belonging to the genus Erythrobacter were isolated. The strains were characterized regarding their physiological and biochemical properties, 16S rDNA and pufM gene sequences, morphological features, substrate preference, as well as pigment and lipid composition. All strains had functional type-2 reaction centers containing bacteriochlorophyll, served by small, light-harvesting complex 1, and were photosynthetically competent. In addition, large pools of carotenoids were found, but only some of the accessory pigments transfer energy to the reaction centers. All of the isolates were facultative photoheterotrophs. They required an organic carbon substrate for growth; however, they are able to supplement a significant fraction of their metabolic requirements with photosynthetically derived energy.Abbreviations BChl Bacteriochlorophyll - Chl Chlorophyll - Erb. Erythrobacter - Erm. Erythromicrobium - FAMEs Fatty acid methyl esters - IRFRR Infrared fast repetition rate - LH1, LH2 Light-harvesting complex 1 and 2, respectively - Por. Porphyrobacter - PUFAs Polyunsaturated fatty acids - Rsb. Roseobacter - RubisCO Ribulose-1,5-bisphosphate carboxylase/oxygenase - µ Growth rate - ₄₇₀ Functional cross-section of the photosynthetic unit at 470 nm     

Origin and early evolution of photosynthesis

Photosynthesis was well-established on the earth at least 3.5 thousand million years ago, and it is widely believed that these ancient organisms had similar metabolic capabilities to modern cyanobacteria. This requires that development of two photosystems and the oxygen evolution capability occurred very early in the earth's history, and that a presumed phase of evolution involving non-oxygen evolving photosynthetic organisms took place even earlier. The evolutionary relationships of the reaction center complexes found in all the classes of currently existing organisms have been analyzed using sequence analysis and biophysical measurements. The results indicate that all reaction centers fall into two basic groups, those with pheophytin and a pair of quinones as early acceptors, and those with iron sulfur clusters as early acceptors. No simple linear branching evolutionary scheme can account for the distribution patterns of reaction centers in existing photosynthetic organisms, and lateral transfer of genetic information is considered as a likely possibility. Possible scenarios for the development of primitive reaction centers into the heterodimeric protein structures found in existing reaction centers and for the development of organisms with two linked photosystems are presented.Abbreviation Gyr gigayears     

Some factors controlling the biosynthesis of chlorosome antenna bacteriochlorophylls in green filamentous anoxygenic phototrophic bacteria of the family Oscillochloridaceae

We determined the concentrations of bacteriochlorophylls (BChl) in the light-harvesting antennae of Oscillochloris trichoides (of the family Oscillochloridaceae belonging to green filamentous mesophilic bacteria) cultivated either with gabaculine, an inhibitor of the C-5 pathway of BChl biosynthesis in a number of bacteria, or at various illumination intensities. We determined the BChl c: BChl a molar ratios in intact cells, in chlorosome-membrane complexes, and in isolated chlorosomes. We revealed that BChl c synthesis in Osc. trichoides was more gabaculine-sensitive than BChl a synthesis. Accordingly, an increase in gabaculine concentrations in the medium resulted in a decrease in the BChl c: BChl a ratio in the tested samples. We suggest that BChl synthesis in Osc. trichoides proceeds via the C-5 pathway, similar to representatives of other families of green bacteria (Chlorobium limicola and Chloroflexus aurantiacus). We demonstrated that the BChl c: BChl a ratio in the chlorosomes varied from 55: 1 to 110: 1, depending on light intensity. This ratio is, therefore, closer to that of Chlorobiaceae, and it significantly exceeds the BChl c: BChl a ratio in Chloroflexaceae.     

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     

Irradiance Effects on Growth and Bacteriochlorophyll Content of Phototrophic Heliobacteria,Purple and Green Photosynthetic Bacteria

Two species of heliobacteria along with a purple and green bacterium were tested for their ability to grow phototrophically at irradiances ranging from 0.125 to 50 W m^–2. The heliobacteria were incapable of growth below 0.5 W m^–2, while both the purple and green bacterium grew at significantly lower irradiances. Specific bacteriochlorophyll contents were higher for the purple and green bacteria than for the heliobacteria at all irradiances tested. Thus in distinct contrast to purple and green bacteria, heliobacteria are high-irradiance phototrophs, and this characteristic may influence their distribution in nature.     

Characterization of the chlorosome antenna of the filamentous anoxygenic phototrophic bacterium<Emphasis Type="Italic"> Chloronema</Emphasis> sp. strain UdG9001

The absorption and fluorescence properties of chlorosomes of the filamentous anoxygenic phototrophic bacterium Chloronema sp. strain UdG9001 were analyzed. The chlorosome antenna of Chloronema consists of bacteriochlorophyll (BChl) d and BChl c together with -carotene as the main carotenoid. HPLC analysis combined with APCI LC-MS/MS showed that the chlorosomal BChls comprise a highly diverse array of homologues that differ in both the degree of alkylation of the macrocycle at C-8 and/or C-12 and the alcohol moiety esterified to the propionic acid group at C-17. BChl c and BChl d from Chloronema were mainly esterified with geranylgeraniol (33% of the total), heptadecanol (24%), octadecenol (19%), octadecanol (14%), and hexadecenol (9%). Despite this pigment heterogeneity, fluorescence emission of the chlorosomes showed a single peak centered at 765 nm upon excitation at wavelengths ranging from 710 to 740 nm. This single emission, assigned to BChl c, indicates an energy transfer from BChl d to BChl c within the same chlorosome. Likewise, incubation of chlorosomes under reducing conditions caused a weak increase in fluorescence emission, which indicates a small redox-dependent fluorescence. Finally, protein analysis of Chloronema chlorosomes using SDS-PAGE and MALDI-TOF-MS revealed the presence of a chlorosomal polypeptide with a molecular mass of 5.7 kDa, resembling the CsmA protein found in Chloroflexus aurantiacus and Chlorobium tepidum chlorosomes. Several minor polypeptides were also detected but not identified. These results indicate that, compared with other members of filamentous anoxygenic phototrophic bacteria and green sulfur bacteria, Chloronema possesses an antenna system with novel features that may be of interest for further investigations.Abbreviations APCI LC-MS/MS Atmospheric pressure chemical ionization liquid chromatography mass spectrometry - BChl Bacteriochlorophyll - Chl. Chlorobium - Cfl. Chloroflexus - MALDI-TOF-MS Matrix assisted laser desorption/ionization time-of-flight mass spectrometry - [Et] Ethyl - [i-Bu] Isobutyl - [Me] Methyl - [neo-Pent] Neopentyl - [n-Pr] Propyl - t _R Retention time     

Characterization ofArxula adeninivorans strains from different habitats

SomeArxula adeninivorans strains selected from wood hydrolysates in Siberia, from soil in South Africa and from maize silage and soil in The Netherlands were compared. DNA-fingerprinting, pulse field gel electrophoresis as well as analysis of secretory proteins have been chosen to describe the similarities among the strains. Combination of the three methods allowed identification of each strain. Strains from the same origin show extensive similarities. The results of the DNA-fingerprints indicate that the strain isolated in Siberia belongs to the group of strains originated from South Africa. However, it differed in the molecular weight of the third chromosome and in the pattern of secretory proteins from the South African isolates.     

Thiocapsa halophila sp. nov., a new halophilic phototrophic purple sulfur bacterium

A new phototrophic sulfur bacterium has been isolated from a red layer in a laminated mat occurring underneath a gypsum crust in the mediterranean salterns of Salin-de-Giraud (Camargue, France). Single cells were coccus-shaped, non motile, without gas vacuoles and contained sulfur globules. Bacteriochlorophyll a and okenone were present as major photosynthetic pigments. These properties and the G+C content of DNA (65.9–66.6 mol% G+C) are typical characteristics of the genus Thiocapsa. However, the new isolate differs from known species in the genus, particularly in NaCl requirement (optimum, 7% NaCl; range, 3–20% NaCl) and some physiological characteristics. Therefore, a new species is proposed, Thiocapsa halophila, sp. nov.Dedicated to Prof. Dr. Norbert Pfennig in occasion of his 65th birthday     

Complexity of species conservation in managed habitats: interaction betweenMaculinea butterflies and their ant hosts

Europe's five species ofMaculinea butterfly are examples of endangered species adapted to live in traditional, cultural landscapes. All are threatened with extinction in Western Europe because of recent changes in land use. This is illustrated by an historical account of the extinction of the BritishMaculinea arion populations, despite many conservation attempts. It is shown how the failures proved to be due to ignorance of the key factor forM. arion, its specialization on a single ant host,Myrmica sabuleti. A brief account is given of research that shows how each of the five species is similarly dependent upon a separate hostMyrmica ant species and how each has an interesting and rare specific parasitoid. Steps for the practical conservation of existingMaculinea populations including the obligation, under the Bern Convention, to re-establish nationally extinct species are outlined. The procedure and problems involved in re-establishment are illustrated with reference to the successful programme forM. arion in Britain. The best way of ensuring robust populations ofMaculinea butterflies is to manage habitats to optimize the density and distribution of the required species ofMyrmica host and, secondarily, the distribution of the larval food plant. The value of single species conservation in cultural habitats is discussed. It is concluded that this is possible to achieve and that other rare organisms also often benefit, but only when conservation measures are based on the results of detailed autecological research.     

An enzyme and13C-NMR study of carbon metabolism in heliobacteria

Heliobacteria are a group of anoxygenic phototrophs that can grow photoheterotrophically in defined minimal media on only a limited range of organic substrates as carbon sources. In this study the mechanisms which operate to assimilate carbon and the routes employed for the biosynthesis of cellular intermediates were investigated in a newHeliobacterium strain, HY-3. This was achieved using two approaches (1) by measuring the activities of key enzymes in cell-free extracts and (2) by the use of¹³C nuclear magnetic resonance (NMR) spectroscopy to analyze in detail the labelling pattern of amino-acids of cells grown on [¹³C] pyruvate and [¹³C] acetate.Heliobacterium strain HY-3 was unable to grow autotrophically on CO₂/H₂ and neither (ATP)-citrate lyase nor ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBPcase) were detectable in cell-free extracts. The enzyme profile of pyruvate grown cells indicated the presence of a pyruvate:acceptor oxidoreductase at high specific activity which could convert pyruvate to acetyl-Coenzyme A. No pyridine nucleotide dependent pyruvate dehydrogenase complex activity was detected. Of the citric-acid cycle enzymes, malate dehydrogenase, fumarase, fumarate reductase and an NADP-specific isocitrate dehydrogenase were readily detectable but no aconitase or citrate synthase activity was found. However, the labelling pattern of glutamate in long-term 2-[¹³C] acetate incorporation experiments indicated that a mechanism exists for the conversion of carbon from acetyl-CoA into 2-oxoglutarate. A 2-oxoglutarate:acceptor oxidoreductase activity was present which was also assayable by isotope exchange, but no 2-oxoglutarate dehydrogenase complex activity could be detected. Heliobacteria appear to use a type of incomplete reductive carboxylic acid pathway for the conversion of pyruvate to 2-oxoglutarate but are unable to grow autotrophically using this metabolic route due to the absence of ATP-citrate lyase.     

Seeing green bacteria in a new light: genomics-enabled studies of the photosynthetic apparatus in green sulfur bacteria and filamentous anoxygenic phototrophic bacteria

Based upon their photosynthetic nature and the presence of a unique light-harvesting antenna structure, the chlorosome, the photosynthetic green bacteria are defined as a distinctive group in the Bacteria. However, members of the two taxa that comprise this group, the green sulfur bacteria (Chlorobi) and the filamentous anoxygenic phototrophic bacteria (Chloroflexales), are otherwise quite different, both physiologically and phylogenetically. This review summarizes how genome sequence information facilitated studies of the biosynthesis and function of the photosynthetic apparatus and the oxidation of inorganic sulfur compounds in two model organisms that represent these taxa, Chlorobium tepidum and Chloroflexus aurantiacus. The genes involved in bacteriochlorophyll (BChl) c and carotenoid biosynthesis in these two organisms were identified by sequence homology with known BChl a and carotenoid biosynthesis enzymes, gene cluster analysis in Cfx. aurantiacus, and gene inactivation studies in Chl. tepidum. Based on these results, BChl a and BChl c biosynthesis is similar in the two organisms, whereas carotenoid biosynthesis differs significantly. In agreement with its facultative anaerobic nature, Cfx. aurantiacus in some cases apparently produces structurally different enzymes for heme and BChl biosynthesis, in which one enzyme functions under anoxic conditions and the other performs the same reaction under oxic conditions. The Chl. tepidum mutants produced with modified BChl c and carotenoid species also allow the functions of these pigments to be studied in vivo.     

Photosynthetic formation of inorganic pyrophosphate in phototrophic bacteria

In this paper we report studies on photosynthetic formation of inorganic pyrophosphate (PP_i) in three phototrophic bacteria. Formation of PP_i was found in chromatophores from Rhodopseudomonas viridis but not in chromatophores from Rhodopseudomonas blastica and Rhodobacter capsulatus. The maximal rate of PP_i synthesis in Rps. viridis was 0.15 mol PP_i formed/(min*mol Bacteriochlorophyll) at 23°C. The synthesis of PP_i was inhibited by electron transport inhibitors, uncouplers and fluoride, but was insensitive to oligomycin and venturicidin. The steady state rate of PP_i synthesis under continuous illumination was about 15% of the steady-state rate of ATP synthesis. The synthesis of PP_i after short light flashes was also studied. The yield of PP_i after a single 1 ms flash was equivalent to approximately 1 mol PP_i/500 mol Bacteriochlorophyll. In Rps. viridis chromatophores, PP_i was also found to induce a membrane potential, which was sensitive to carbonyl cyanide p-trifluoromethoxyphenylhydrazone and NaF.Abbreviations BChl Bacteriochlorophyll - F₀F₁-ATPase Membrane bound proton translocating ATP synthase - FCCP Carbonyl cyanide p-trifluoromethoxyphenylhydrazone - H⁺-PPase Membrane bound proton translocating PP_i synthase - TPP⁺ Tetraphenyl phosphonium ion - TPB^- Tetraphenyl boron ion - Transmembrane electrical potential difference     

Ectothiorhodospira halochloris sp. nov., a new extremely halophilic phototrophic bacterium containing bacteriochlorophyll b

A new bacteriochlorophyll b containing phototrophic bacterium was isolated from extremely saline and alkaline soda lakes in Egypt. Enrichment and isolation were performed using a synthetic medium with high contents of sodium carbonate, sodium sulfate and sodium chloride. Photoautotrophic growth occurred with hydrogen sulfide as photosynthetic electron donor. During oxidation of sulfide to sulfate extracellular elemental sulfur globules appeared in the medium. Cells were also capable to grow under photoheterotrophic conditions with acetate, propionate, pyruvate, succinate, fumarate or malate as carbon sources and electron donors. Under these conditions sulfate was assimilated. Optimal growth under the applied experimental conditions occurred at a total salinity of 14–27%, a pH-range between 8.1 and 9.1 and a temperature between 47°C and 50°C. The cells were 0.5–0.6 m wide and, depending on cultural conditions, 2.5–8.0 m long; they were spiral shaped, multiplied by binary fission and were motile by means of bipolar flagella. Intercytoplasmic photosynthetic membranes were present as stacks. Bacteriochlorophyll b was the main photosynthetic pigment; small amounts of carotenoids were mainly present as glucosides of rhodopin and its methoxy derivative. The new organism is described as Ectothiorhodospira halochloris.Dedicated to Professor C. B. van Niel on the occasion of his 80th birthday