<Emphasis Type="Italic">Thiocapsa imhoffii</Emphasis>, sp. nov., an alkaliphilic purple sulfur bacterium of the family <Emphasis Type="Italic">Chromatiaceae</Emphasis> from Soap Lake,Washington (USA)

An alkaliphilic purple sulfur bacterium, strain SC5, was isolated from Soap Lake, a soda lake located in east central Washington state (USA). Cells of strain SC5 were gram-negative, non-motile, and non-gas vesiculate cocci, often observed in pairs or tetrads. In the presence of sulfide, elemental sulfur was deposited internally. Liquid cultures were pink to rose red in color. Cells contained bacteriochlorophyll a and spirilloxanthin as major photosynthetic pigments. Internal photosynthetic membranes were of the vesicular type. Optimal growth of strain SC5 occurred in the absence of NaCl (range 0–4%), pH 8.5 (range pH 7.5–9.5), and 32°C. Photoheterotrophic growth occurred in the presence of sulfide or thiosulfate with only a limited number of organic carbon sources. Growth factors were not required, and cells could fix N₂. Dark, microaerobic growth occurred in the presence of both an organic carbon source and thiosulfate. Sulfide and thiosulfate served as electron donors for photoautotrophy, which required elevated levels of CO₂. Phylogenetic analysis placed strain SC5 basal to the clade of the genus Thiocapsa in the family Chromatiaceae with a 96.7% sequence similarity to its closest relative, Thiocapsa roseopersicina strain 1711^T (DSM217^T). The unique assemblage of physiological and phylogenetic properties of strain SC5 defines it as a new species of the genus Thiocapsa, and we describe strain SC5 herein as Tca. imhoffii, sp. nov.     

A new purple sulfur bacterium from saline littoral sediments,Thiorhodovibrio winogradskyi gen. nov. and sp. nov.

Two strains of a new purple sulfur bacterium were isolated in pure culture from the littoral sediment of a saline lake (Mahoney Lake, Canada) and a marine microbial mat from the North Sea island of Mellum, respectively. Single cells were vibrioid-to spirilloid-shaped and motile by means of single polar flagella. Intracellular photosynthetic membranes were of the vesicular type. As photosynthetic pigments, bacteriochlorophyll a and the carotenoids lycopene, rhodopin, anhydrorhodovibrin, rhodovibrin and spirilloxanthin were present.Hydrogen sulfide and elemental sulfur were used under anoxic conditions for phototrophic growth. In addition one strain (06511) used thiosulfate. Carbon dioxide, acetate and pyruvate were utilized by both strains as carbon sources. Depending on the strain propionate, succinate, fumarate, malate, tartrate, malonate, glycerol or peptone may additionally serve as carbon sources in the light. Optimum growth rates were obtained at pH 7.2, 33 °C, 50 mol m^-2 s^-1 intensity of daylight fluorescent tubes and a salinity of 2.2–3.2% NaCl. During growth on sulfide, up to ten small sulfur globules were formed inside the cells. The strains grew microaerophilic in the dark and exhibited high specific respiration rates. No vitamins were required for growth. The DNA base composition was 61.0–62.4 mol% G+C.The newly isolated bacterium belongs to the family chromatiaceae and is described as a member of a new genus and species, Thiorhodovibrio winogradskyi gen. nov. and sp. nov. with the type strain SSP1, DSM No. 6702.     

Rhabdochromatium marinum gen. nom. rev., sp. nov., a purple sulfur bacterium from a salt marsh microbial mat

A new purple sulfur bacterium was isolated in pure culture (strain 8315) from a laminated microbial mat at Great Sippewissett Salt Marsh, Cape Cod, Mass., USA. Single cells were large rods, 10–20 times longer than wide, and predominantly strainght with slightly conical ends. Cells were motile by polarly inserted flagellar tufts. Intracellular photosynthetic membranes were of the vesicular-type. Photosynthetic pigments were bacteriochlorophylla and the carotenoids lycopene, rhodovibrin, anhydrorhodovibrin, and rhodopin. The new bacterium was strictly anaerobic and obligately phototrophic. Hydrogen, hydrogen sulfide, elemental sulfur, and thiosulfate were used as electron donors for photoautotrophic growth. In sulfide-reduced, bicarbonate-containing media, acetate, propionate, and pyruvate were photoassimilated. Growth factors were not required. Optimum growth rates were obtained at pH 7.3, 30°C, a salinity of 1.5–5.0% NaCl, and a light intensity of about 500 lx (tungsten lamp). The DNA base composition of strain 8315 was 60.4 mol% G+C. Comparison of 16S rDNA oligonucleotide catalogue data showed that the new bacterium must be considered a new genus of the Chromatiaceae. The nameRhabdochromatium is revived, and the new speciesRhabdochromatium marinum sp. nov. is described.     

Haloalkaliphilic spore-forming sulfidogens from soda lake sediments and description of Desulfitispora alkaliphila gen. nov., sp. nov.

An anaerobic enrichment with pyruvate as electron donor and thiosulfate at pH 10 and 0.6 M Na⁺ inoculated with pasteurized soda lake sediments resulted in a sulfidogenic coculture of two morphotypes of obligately anaerobic haloalkaliphilic endospore-forming clostridia, which were further isolated in pure culture. Strain AHT16 was a thin long rod able to ferment sugars and pyruvate and to respire H₂, formate and pyruvate using thiosulfate and fumarate as electron acceptors and growing optimally at pH 9.5. Thiosulfate was reduced incompletely to sulfide and sulfite. The strain was closely related (99% sequence similarity) to a peptolytic alkaliphilic clostridium Natronincola peptidovorans. Strain AHT17 was a short rod with a restricted respiratory metabolism, growing with pyruvate and lactate as electron donor and sulfite, thiosulfate and elemental sulfur as electron acceptors with a pH optimum 9.5. Thiosulfate was reduced completely via sulfite to sulfide. The ability of AHT17 to use sulfite explained the stability of the original coculture of the two clostridia—one member forming sulfite from thiosulfate and another consuming it. Strain AHT17 formed an independent deep phylogenetic lineage within the Clostridiales and is proposed as a new genus and species Desulfitisporum alkaliphilum gen. nov., sp. nov. (=DSM 22410^T = UNIQEM U794^T).     

Thiorhodococcus minus, gen. nov., sp. nov., a new purple sulfur bacterium isolated from coastal lagoon sediments

A new marine phototrophic purple sulfur bacterium (strain CE2203) was isolated in pure culture from a man-made coastal lagoon located on the Atlantic coast (Arcachon Bay, France). Single cells were coccus-shaped, did not contain gas vesicles, and were highly motile. Intracellular photosynthetic membranes were of the vesicular type. Bacteriochlorophyll a and carotenoids of the normal spirilloxanthin series were present as photosynthetic pigments. Hydrogen sulfide, thiosulfate, elemental sulfur, and molecular hydrogen were used as electron donors during photolithotrophic growth under anoxic conditions, while carbon dioxide was utilized as carbon source. Acetate, propionate, lactate, glycolate, pyruvate, fumarate, succinate, fructose, sucrose, ethanol, and propanol were photoassimilated in the presence of hydrogen sulfide. During growth on sulfide, elemental sulfur globules were stored inside the cells. Chemotrophic growth under microoxic conditions in the dark was possible. The DNA base composition was 66.9 mol% G+C. Comparative sequence analysis of the 16S rRNA gene confirmed the membership of strain CE2203 in the family Chromatiaceae. Morphological characteristics of strain CE2203 indicated a close affiliation to the genera Thiocystis and Thiocapsa. However, the phylogenetic treeing revealed no closer relationship to Thiocystis spp. than to Thiocapsa roseopersicina or other known members of the Chromatiaceae. Consequently, strain CE2203 is proposed as the type strain of a new genus and species, Thiorhodococcus minus gen. nov., sp. nov. Received: 23 December 1996 / Accepted: 27 March 1997     

Characterization of Comamonas thiooxidans sp. nov., and Comparison of Thiosulfate Oxidation with Comamonas testosteroni and Comamonas composti

Comamonas thiooxidans (strain S23^T) capable of oxidizing thiosulfate under a mixotrophic growth condition was isolated from a sulfur spring. DNA–DNA homology study showed 55% similarity with Comamonas testosteroni KCTC2990^T and 52% with Comamonas composti LMG24008^T, the nearest phylogenetic relative (16S rRNA sequence similarity <97%). Comparative genomic fingerprinting by using ERIC and Rep-PCR further delineated species identity of the strain S23^T for which Comamonas thiooxidans sp. nov. is proposed. In addition, thiosulfate oxidation potential of the strain S23^T was compared with Comamonas testosteroni and Comamonas composti.     

Pelodictyon phaeoclathratiforme sp. nov., a new brown-colored member of the Chlorobiaceae forming net-like colonies

A new strain of the green sulfur bacteria was isolated from the monimolimnion of Buchensee (near Radolfzell, Lake Constance region, FRG). Single cells were rod-shaped, nonmotile and contained gas vacuoles. Typical net-like colonies were formed by ternary fission of the cells. As photosynthetic pigments bacteriochlorophylls a, e, isorenieratene and -isorenieratene were present. Sulfide, sulfur and thiosulfate were used as electron donors during anaerobic phototrophic growth. Besides carbon dioxide, acetate and propionate could serve as carbon sources under mixotrophic conditions in the light. Like all other members of the green sulfur bacteria, the new bacterium is strictly anaerobic and obligately phototrophic. The possession of gas vacuoles and the formation of net-like colonies and the guanine plus cytosine content of the DNA (47.9 mol% G+C) are typical characteristics of the genus Pelodictyon. Because of its photosynthetic pigments which differ from those of Pelodictyon clathratiforme, strain BU 1 represents a new species, P. Phaeoclathratiforme sp. nov.     

Fractionation of sulfur isotopes during thiosulfate reduction by Desulfovibrio desulfuricans

Sulfur isotope fractionation during reduction of thiosulfate was investigated with growing batch cultures of Desulfovibrio desulfuricans CSN (DSM 9104) at 30 °C. The sulfide produced was depleted in ³⁴S by 10‰ as compared to total thiosulfate sulfur. The depletion was equal to that during sulfate reduction under similar conditions. The two sulfur atoms of the thiosulfate molecule were affected differently by fractionation. Sulfide produced from sulfonate sulfur was depleted by 15.4‰, sulfide produced from sulfane sulfur by 5.0‰. Received: 29 October 1997 / Accepted: 18 December 1997     

<Emphasis Type="Italic">Ectothiorhodospira magna</Emphasis> sp. nov., a new large alkaliphilic purple sulfur bacterium

Two strains of purple sulfur bacteria of the family Ectothiorhodospiraceae were isolated from moderately saline steppe lakes (with pH above 9.0) of the Transbaikal region (strain B7-7) and Mongolia (strain M10). The cells of the novel strains were spiral-shaped, 2.0–3.2 × 9.6–20.0 μm, motile due to a polar tuft of flagella. Photosynthetic pigments were represented by bacteriochlorophyll a and carotenoids of the spirilloxanthin series. Photosynthetic membranes were represented by long strands of lamellae distributed throughout the whole cell; unlike most Ectothiorhodospiraceae species, the membranes were not packed into regular stacks. Bacteria were capable of weak growth on sulfide and slow grow on hydrogen under photoautotrophic conditions. The best growth was noted on sulfide in the presence of acetate and bicarbonate. Thiosulfate did not stimulate phototrophic growth, even in the presence of organic substrates. The new isolates were alkaliphiles growing at a pH optimum of 9–10. Growth was possible within a salinity range of 0–80 g/l NaCl, with an optimum at 5–15 g/l NaCl. The morphology, the structure of the photosynthetic apparatus (strands of lamellae), and the physiology of the new strains were similar to those of Thiorhodospira sibirica. However, analysis of the 16S rRNA gene sequences demonstrated that the studied isolates were closely related to the type strain Ectothiorhodospira shaposhnikovii (99% similarity) of the family Ectothiorhodospiraceae, whereas the level of similarity between the new strains and Thiorhodospira sibirica was only 94–95%. According to the results of DNA-DNA hybridization, the DNA-DNA homology level between the tested strains was almost 100%; the similarity between the new isolates and the type strain Ectothiorhodospira shaposhnikovii was only 58%. The isolates differed from other representatives of the genus Ectothiorhodospira in the structure of the gene encoding the key enzyme of autotrophic CO₂ fixation, ribulose-1,5-bisphosphate carboxylase (RuBisCo), which was similar to the RuBisCo genes of members of another family of sulfur bacteria, Chromatiaceae. The new isolates of purple bacteria were described as a new species of the genus Ectothiorhodospira, Ect. magna sp. nov. with the type strain B7-7^T (= VKM B-2537 = DSM 22250).     

Characterization of Aquamicrobium defluvii gen. nov. sp. nov., a thiophene-2-carboxylate-metabolizing bacterium from activated sludge

A gram-negative bacterium was isolated from activated sewage sludge with thiophene-2-carboxylate as the sole source of carbon and with nitrate as an electron acceptor. The isolate, strain NKK, was a motile, oxidase- and catalase-positive, rod-like bacterium with a G+C content of 61.7 mol%. Besides nitrate, oxygen could serve as a terminal electron acceptor. Among many carbon sources tested, only a few sugars, fatty acids, and thiophene-2-carboxylate supported growth. Other heterocyclic compounds were not used. The sulfur atom of thiophene-2-carboxylate was oxidized to thiosulfate when cells were grown aerobically, or to elemental sulfur when cells were grown anaerobically with nitrate. Nitrate was reduced to nitrite. Growth on thiophene-2-carboxylate was dependent on the addition of molybdate to the medium. Tungstate, a specific antagonist of molybdate, inhibited growth on thiophene-2-carboxylate at concentrations > 10^–7 M. Three inducible enzymes involved in the metabolism of thiophene-2-carboxylate were detected: an ATP-, CoA-, thiophene-2-carboxylate- and Mg²⁺-dependent thiophene-2-carboxyl-CoA ligase (AMP-forming), a molybdenum-containing thiophene-2-carboxyl-CoA dehydrogenase, and a thiophene-2-carboxyl-CoA thioesterase. The sequence of the 16S rRNA gene suggested a classification of strain NKK within the α-subgroup of the Proteobacteria as a new genus and species, Aquamicrobium defluvii gen. nov. sp. nov. (DSM 11603), closely related to Mesorhizobium sp. and Phyllobacterium sp., but representing a distinct lineage equal in depth to those of the two mentioned genera. Aquamicrobium defluvii can be distinguished from both genera by a distinct spectrum of substrates, the maximal growth temperature, and a different salt tolerance. Received: 8 September 1997 / Accepted: 14 November 1997     

Disproportionation of inorganic sulfur compounds by the sulfate-reducing bacterium Desulfocapsa thiozymogenes gen. nov., sp. nov.

A new strictly anaerobic, gram-negative bacterium was isolated from the sediment of a freshwater lake after enrichment with thiosulfate as the energy source. The strain, named Bra2 (DSM 7269), is able to grow by disproportionation of thiosulfate or sulfite to sulfate plus sulfide. Elemental sulfur is also disproportionated to sulfate and sulfide, but this only supports growth if free sulfide is chemically removed from the culture, e.g., by precipitation with amorphous ferric hydroxide. Growth is also possible by coupling the reduction of sulfate to sulfide with the oxidation of ethanol, propanol, or butanol to the corresponding fatty acid. The cells are rod-shaped, motile, and have genomic DNA with a mol% G+C content of 50.7. Cytochromes are present, but desulfoviridin is not. The new strain was shown to be related to, but distinct from members of the genus Desulfobulbus on the basis of physiological characteristics and by comparative sequence analysis of its 16S rDNA. Strain Bra2 is described as the type strain of a new taxon, Desulfocapsa thiozymogenes gen. nov., sp. nov. Received: 29 January 1996 / Accepted: 31 May 1996     

Complete assimilation of cysteine by a newly isolated non-sulfur purple bacterium resembling Rhodovulum sulfidophilum (Rhodobacter sulfidophilus)

A rod-shaped, motile, phototrophic bacterium, strain SiCys, was enriched and isolated from a marine microbial mat, with cysteine as sole substrate. During phototrophic anaerobic growth with cysteine, sulfide was produced as an intermediate, which was subsequently oxidized to sulfate. The molar growth yield with cysteine was 103 g mol^–1, in accordance with complete assimilation of electrons from the carbon and the sulfur moiety into cell material. Growth yields with alanine and serine were proportionally lower. Thiosulfate, sulfide, hydrogen, and several organic compounds were used as electron donors in the light, whereas cystine, sulfite, or elemental sulfur did not support phototrophic anaerobic growth. Aerobic growth in the dark was possible with fructose as substrate. Cultures of strain SiCys were yellowish-brown in color and contained bacteriochlorophyll a, spheroidene, spheroidenone, and OH-spheroidene as major photosynthetic pigments. Taking the morphology, photosynthetic pigments, aerobic growth in the dark, and utilization of sulfide for phototrophic growth into account, strain SiCys was assigned to the genus Rhodovulum (formerly Rhodobacter) and tentatively classified as a strain of R. sulfidophilum. In cell-free extracts in the presence of pyridoxal phosphate, cysteine was converted to pyruvate and sulfide, which is characteristic for cysteine desulfhydrase activity (l-cystathionine γ-lyase, EC 4.4.1.1). Received: 15 December 1995 / Accepted: 1 April 1996     

Isolation and characterization of Desulfovibrio senezii sp. nov., a halotolerant sulfate reducer from a solar saltern and phylogenetic confirmation of Desulfovibrio fructosovorans as a new species

A new halotolerant Desulfovibrio, strain CVL^T (T = type strain), was isolated from a solar saltern in California. The curved, gram-negative, nonsporeforming cells (0.3 × 1.0–1.3 μm) occurred singly, in pairs, or in chains, were motile by a single polar flagellum and tolerated up to 12.5% NaCl. Strain CVL^T had a generation time of 60 min when grown in lactate-yeast extract medium under optimal conditions (37°C, pH 7.6, 2.5% NaCl). It used lactate, pyruvate, cysteine, or H₂/CO₂ + acetate as electron donors, and sulfate, sulfite, thiosulfate, or fumarate as electron acceptors. Elemental sulfur, nitrate, or oxygen were not used. Sulfite and thiosulfate were disproportionated to sulfate and sulfide. The G+C content of the DNA was 62 mol%. Phylogenetic analysis revealed that Desulfovibrio fructosovorans was the nearest relative. Strain CVL^T is clearly different from other Desulfovibrio species, and is designated Desulfovibrio senezii sp. nov. (DSM 8436). Received: 27 February 1998 / Accepted: 15 June 1998     

Thermotomaculum hydrothermale gen. nov., sp. nov., a novel heterotrophic thermophile within the phylum Acidobacteria from a deep-sea hydrothermal vent chimney in the Southern Okinawa Trough

A novel heterotrophic, thermophilic bacterium, designated strain AC55^T, was isolated from a deep-sea hydrothermal vent chimney at the Hatoma Knoll in the Okinawa Trough, Japan. Cells of strain AC55^T were non-motile, long rods (2.0- to 6.8-μm long and 0.3- to 0.6-μm wide). The strain was an obligatory anaerobic heterotroph capable of fermentative growth on complex proteinaceous substances. Elemental sulfur was reduced to hydrogen sulfide but did not stimulate growth. Growth was observed between 37 and 60°C (optimum 55°C), pH 5.5 and 8.5 (optimum pH 6.6), and in the presence of 1.5–4.5% (w/v) NaCl (optimum 2.5%, w/v). Menaquinone-7 and -8 were the major respiratory quinones. The G + C content of the genomic DNA from strain AC55^T was 51.6 mol%. The 16S rRNA gene sequence analysis revealed that strain AC55^T was the first cultivated representative of Acidobacteria subdivision 10. Based on the physiological and phylogenetic features of the novel isolate, the genus name Thermotomaculum gen. nov. is proposed, with Thermotomaculum hydrothermale sp. nov. as the type species. The type strain is AC55^T (=JCM 17643^T = DSM 24660^T = NBRC 107904^T).     

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.     

Microbial Recovery of Sulfur from Thiosulfate-Bearing Wastewater with Phototrophic and Sulfur-Reducing Bacteria

The spent caustic wastewater from the oxidation of sulfide present in offshore natural gas production mainly comprises thiosulfate and sulfate. A biocatalytic process, employing phototrophic green sulfur bacteria in symbiosis with sulfate-reducing bacteria, is described in this paper for the production of sulfur from the spent caustic wastewater, with synthetic wastewater as the model system. The process entails the conversion of thiosulfate to sulfur and sulfate by photosynthetic green sulfur bacteria Chlorobium vibrioforme f. thiosulfatophilum. Sulfate formed in turn is removed by Desulfovibrio desulfuricans to sulfide, which is further converted to sulfur by Chlorobium limicola through photooxidation. Sulfide is also oxidized to sulfur and sulfate via thiosulfate as an intermediate by Chlorobium vibrioforme f. thiosulfatophilum.     

A sulfate-reducing bacterium from the oxic layer of a microbial mat from Solar Lake (Sinai), Desulfovibrio oxyclinae sp. nov.

In an investigation on the oxygen tolerance of sulfate-reducing bacteria, a strain was isolated from a 10⁷-fold dilution of the upper 3-mm layer of a hypersaline cyanobacterial mat (transferred from Solar Lake, Sinai). The isolate, designated P1B, appeared to be well-adapted to the varying concentrations of oxygen and sulfide that occur in this environment. In the presence of oxygen strain P1B respired aerobically with the highest rates [260 nmol O₂ min^–1 (mg protein)^–1] found so far among marine sulfate-reducing bacteria. Besides H₂ and lactate, even sulfide or sulfite could be oxidized with oxygen. The sulfur compounds were completely oxidized to sulfate. Under anoxic conditions, it grew with sulfate, sulfite, or thiosulfate as the electron acceptor using H₂, lactate, pyruvate, ethanol, propanol, or butanol as the electron donor. Furthermore, in the absence of electron donors the isolate grew by disproportionation of sulfite or thiosulfate to sulfate and sulfide. The highest respiration rates with oxygen were obtained with H₂ at low oxygen concentrations. Aerobic growth of homogeneous suspensions was not obtained. Additions of 1% oxygen to the gas phase of a continuous culture resulted in the formation of cell clumps wherein the cells remained viable for at least 200 h. It is concluded that strain P1B is oxygen-tolerant but does not carry out sulfate reduction in the presence of oxygen under the conditions tested. Analysis of the 16S rDNA sequence indicated that strain P1B belongs to the genus Desulfovibrio, with Desulfovibrio halophilus as its closest relative. Based on physiological properties strain P1B could not be assigned to this species. Therefore, a new species, Desulfovibrio oxyclinae, is proposed. Received: 7 August 1996 / Accepted: 29 January 1997     

Chemolithoautotrophy and mixotrophy in the thiophene-2-carboxylic acid-utilizing Xanthobacter tagetidis

Xanthobacter tagetidis grew as a chemolithotrophic autotroph on thiosulfate and other inorganic sulfur compounds, as a heterotroph on thiophene-2-carboxylic acid, acetic acid and α-ketoglutaric acid, and as a mixotroph on thiosulfate in combination with thiophene-2-carboxylic acid and/or acetic acid. Autotrophic growth on one-carbon organosulfur compounds, and intermediates in their oxidation are also reported. Thiosulfate enhanced the growth yields in mixotrophic cultures, presumably by acting as a supplementary energy source, since ribulose bisphosphate carboxylase was only active in thiosulfate-grown cells and was not detected in mixotrophic cultures using thiosulfate with thiophene-2-carboxylic acid. Bacteria grown on thiophene-2-carboxylic acid also oxidized sulfide, thiosulfate and tetrathionate, indicating these as possible sulfur intermediates in thiophene-2-carboxylic acid degradation. Thiosulfate and tetrathionate were oxidized completely to sulfate and, consequently, did not accumulate as products of thiophene-2-carboxylic acid oxidation in growing cultures. K _m and V _max values for the oxidation of thiosulfate, tetrathionate or sulfide were 13 μM and 83 nmol O₂ min^–1 (mg dry wt.)^–1, respectively; thiosulfate and tetrathionate became autoinhibitory at concentrations above 100 μM. The true growth yield (Y_max) on thiophene-2-carboxylic acid was estimated from chemostat cultures (at dilution rates of 0.034–0.094 h^–1) to be 112.2 g mol^–1, with a maintenance coefficient (m) of 0.3 mmol thiophene-2-carboxylic acid (g dry wt.)^–1 h^–1, and the maximum specific growth rate (μ_max) was 0.116 h^–1. Growth in chemostat culture at a dilution rate of 0.041 h^–1 indicated growth yields [g dry wt. (mol substrate)^–1] of 8.1 g (mol thiosulfate)^–1, 60.9 g (mol thiophene-2-carboxylic acid)^–1, and 17.5 g (mol acetic acid)^–1, with additive yields for growth on mixtures of these substrates. At a dilution rate of 0.034 h^–1, yields of 57.8 g (mol α-ketoglutaric acid)^–1 and 60.7 g (mol thiophene-2-carboxylic acid)^–1 indicated some additional energy conservation from oxidation of the thiophene-sulfur. SDS-PAGE of cell-free preparations indicated a polypeptide (M _r, 21.0 kDa) specific to growth on thiophene-2-carboxylic acid for which no function can yet be ascribed: no metabolism of thiophene-2-carboxylic acid by cell-free extracts was detected. It was shown that X. tagetidis exhibits a remarkable degree of metabolic versatility and is representative of facultatively methylotrophic and chemolithotrophic autotrophs that contribute significantly to the turnover of simple inorganic and organic sulfur compounds (including substituted thiophenes) in the natural environment. Received: 1 July 1997 / Accepted: 3 November 1997     

Caldinitratiruptor microaerophilus, gen. nov., sp. nov. isolated from a French hot spring (Chaudes-Aigues, Massif Central): a novel cultivated facultative microaerophilic anaerobic thermophile pertaining to the Symbiobacterium branch within the Firmicutes

A novel facultative microaerophilic nitrate-reducing bacterium designated CA62N^T was isolated from a thermal spring in France. Cells were non-motile rods (2–3 × 0.2 μm) and showed low cytoplasmic density when observed under a phase-contrast microscope. Strain CA62N^T grew at temperatures between 50 and 75°C (optimum 65°C) and at a pH between 6.3 and 7.9 (optimum 7.0). NaCl was not required for growth but was tolerated up to 10 gl⁻¹. Sulfate, thiosulfate, elemental sulfur, sulfite, and nitrite were not used as electron acceptors. Nitrate was reduced to nitrite. Strain CA62N^T used lactate, pyruvate, glucose, mannose, fructose, and casamino acids and some amino acids as electron donors only in the presence of nitrate as electron acceptor. None of these substrates was fermented. The main end-products of glucose oxidation were acetate, CO₂, and traces of H₂. The G + C content of the genomic DNA was 70.3 mol% (HPLC techniques). Phylogenetic analysis of the small-subunit (SSU) ribosomal RNA (rRNA) gene sequence indicated that strain CA62N^T was affiliated to the Symbiobacterium branch within the Firmicutes and had Symbiobacterium thermophilum and “S. toebii” as its closest phylogenetic relatives. On the basis of phylogenetical and physiological characteristics, strain CA62N^T is proposed to be the type strain for the novel species in the novel genus, Caldinitratiruptor microaerophilus gen. nov., sp. nov. (DSM 22660, JCM 16183).