Fermentation and Sulfur Reduction in the Mat-Building Cyanobacterium Microcoleus chthonoplastes

The mat-building cyanobacterium Microcoleus chthonoplastes carried out a mixed-acid fermentation when incubated under anoxic conditions in the dark. Endogenous storage carbohydrate was fermented to acetate, ethanol, formate, lactate, H(inf2), and CO(inf2). Cells with a low glycogen content (about 0.3 (mu)mol of glucose per mg of protein) produced acetate and ethanol in equimolar amounts. In addition to glycogen, part of the osmoprotectant, glucosyl-glycerol, was degraded. The glucose component of glucosyl-glycerol was fermented, whereas glycerol was released into the medium. Cells with a high content of glycogen (about 2 (mu)mol of glucose per mg of protein) did not utilize glucosyl-glycerol. These cells produced more acetate than ethanol. M. chthonoplastes was also capable of using elemental sulfur as the electron acceptor during fermentation, resulting in the production of sulfide. With sulfur present, acetate production increased whereas ethanol production decreased. Also, less formate was produced and the evolution of hydrogen ceased completely. In general, the carbon recoveries were satisfactory but the oxidation-reduction balances were too high. The latter could be explained by assuming the reduction of ferric iron, which is associated with the cells, mediated by the oxidation of formate. The switch from photoautotrophic to fermentative metabolism did not require de novo protein synthesis, and fermentation started immediately upon transfer to dark anoxic conditions. From the molar ratios of the fermentation products and from measurement of enzyme activities in cell extracts, we concluded that glucose derived from glycogen and glucosyl-glycerol is degraded via the Embden-Meyerhof-Parnas pathway.     

Anaerobic heterotrophic dark metabolism in the cyanobacterium Oscillatoria limnetica: Sulfur respiration and lactate fermentation

The cyanobacterium Oscillatoria limnetica, capable of anoxygenic photosynthesis in the light with sulfide as electron donor can anaerobically break down its intracellular polyglucose in the dark. In the absence of elemental sulfur, the organism carries out lactate fermentation; in its presence, anaerobic respiration occurs in which sulfur is reduced to sulfide. Induction of anoxygenic photosynthesis or synthesis of new proteins is not necessary for either process. Cells adapted in the dark to sulfur reduction are capable of anoxygenic photosynthesis during a subsequent light period, unless protein synthesis has been inhibited during the dark incubation period.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - FCCP Carbonylcyanide p-trifluoromethoxyphenylhydrazone - mgat milligramatom - OD optical density     

Acetylene reduction and hydrogen metabolism by a cyanobacterial/sulfate‐reducing bacterial mat ecosystem

We report a study of nitrogenase activity (acetylene reduction) and hydrogen gas metabolism in intact smooth cyanobacterial mats from Hamelin Pool, Shark Bay, Western Australia. The predominant cyanobacterial population in these mats is Microcoleus chthonoplastes. The mats had a significant capacity for nitrogen fixation, predominantly attributable to the photosyn‐thetic component. By physical and chemical perturbation we revealed an active hydrogen metabolism within the mats. Most of the H₂ formation was attributed to fermentative processes, whereas hydrogen was consumed in light‐dependent, together with oxygen‐ and sulfate‐dependent respiratory processes. It was concluded that H₂ formed by fermentative bacteria in the dark drives a significant proportion of sulfate reduction in the mats, but there was little H₂ transfer from the cyanobacteria to the sulfate‐reducing bacteria. Thus photosynthetically produced H₂ gas is unlikely to significantly alter the previously measured carbon: sulfur ratio relating photosynthesis to sulfate reduction.     

Fermentation metabolism of the unicellular cyanobacterium Cyanothece PCC 7822

The hydrogenase-catalyzed hydrogen production exhibited by the unicellular cyanobacterium Cyanothece 7822 during anoxic incubation in the dark is a result of the fermentative degradation of carbon reserves. Simultaneously with hydrogen production, evolution of carbon dioxide was detected, and excretion of ethanol, lactate, formate and acetate was demonstrated. The fermentation balance indicates that carbohydrates are fermented via a branched pathway, in which both the pentose phosphate pathway and glycolysis appear to be involved. It is proposed that the physiological function of hydrogen production is the introduction of protons as terminal electron acceptors. This removal of reducing equivalents might give rise to continuation of the pyruvate decarboxylation and consequently of the acetate formation, thereby increasing the efficiency of fermentative energy generation.     

<Emphasis Type="Italic">Desulfurispirillum alkaliphilum</Emphasis> gen. nov. sp. nov., a novel obligately anaerobic sulfur- and dissimilatory nitrate-reducing bacterium from a full-scale sulfide-removing bioreactor

Strain SR 1^T was isolated under anaerobic conditions using elemental sulfur as electron acceptor and acetate as carbon and energy source from the Thiopaq bioreactor in Eerbeek (The Netherlands), which is removing H₂S from biogas by oxidation to elemental sulfur under oxygen-limiting and moderately haloalkaline conditions. The bacterium is obligately anaerobic, using elemental sulfur, nitrate and fumarate as electron acceptors. Elemental sulfur is reduced to sulfide through intermediate polysulfide, while nitrate is dissimilatory reduced to ammonium. Furthermore, in the presence of nitrate, strain SR 1^T was able to oxidize limited amounts of sulfide to elemental sulfur during anaerobic growth with acetate. The new isolate is mesophilic and belongs to moderate haloalkaliphiles, with a pH range for growth (on acetate and nitrate) from 7.5 to 10.25 (optimum 9.0), and a salt range from 0.1 to 2.5 M Na⁺ (optimum 0.4 M). According to phylogenetic analysis, SR 1^T is a member of a deep bacterial lineage, distantly related to Chrysiogenes arsenatis (Macy et al. 1996). On the basis of the phenotypic and genetic data, the novel isolate is placed into a new genus and species, Desulfurispirillum alkaliphilum (type strain SR^T = DSM 18275 = UNIQEM U250). Nucleotide sequence accession number: the GenBank/EMBL accession number of the 16S rRNA gene sequence of strain SR 1^T is DQ666683.     

A new purple sulfur bacterium from stratified freshwater lakes,Amoebobacter purpureus sp. nov.

Six strains of a new purple sulfur bacterium were isolated from the chemocline of four different freshwater lakes. Single cells were spherical to oval, nonmotile and contained gas vacuoles in the central part of the cytoplasm. All strains contained bacteriochlorophyll a and okenone as the major carotenoid. The intracytoplasmic membrane system was of vesicular type. All strains resembled each other in growth conditions and utilization of simple organic carbon sources. The strains were able to grow microaerophilic in the dark, used hydrogen sulfide, elemental sulfur or thiosulfate as electron donor, and lacked assimilatory sulfate reduction. On the basis of all characteristics the new bacterium represents a new species of the genus Amoebobacter, A. purpureus sp. nov.     

On the ATP generation by Chromatium in darkness

Summary In Chromatium strain 6412 storage carbohydrate synthesized in the light, gradually disappeared in the dark. Simultaneously, poly--hydroxybutyric acid (PHB) was produced, CO₂ was released and intracellular elemental sulfur was reduced to sulfide. Expressed on a molar base, the ratio between storage carbohydrate monomer (as glucose) consumed, sulfur reduced, PHB monomer produced, and sulfide released was approximately 1 : 3 : 1 : 3. This indicates that sulfur acts electron acceptor in the conversion of storage carbohydrate to PHB. Assuming that in the dark storage carbohydrate is broken down to pyruvate via the Embden-Meyerhoff pathway, whereafter PHB is synthesized from pyruvate via acetyl-CoA and acetoacetyl-CoA, the conversion of storage carbohydrate to PHB results in a net gain of 3 ATP per glycosyl unit converted. Since Thiorhodaceae are photolithotrophs, these processes would provide the organisms with maintenance energy during dark periods. They also explain motility of Chromatium in the dark.     

Utilization of hydrogen and formate by Campylobacter spec. under aerobic and anaerobic conditions

A free-living aspartate-fermenting Campylobacter spec. was shown to utilize hydrogen produced in mixed culture by Clostridium cochlearium from glutamate. Resting cells of Campylobacter were shown to reduce aspartate, fumarate and malate as well as nitrate, nitrite, hydroxylamine, sulphite, thiosulphate and elemental sulphur with molecular hydrogen. Growth of Campylobacter spec. was demonstrated with formate as electron donor and nitrate, thiosulphate, elemental sulphur or oxygen as electron acceptor in the presence of acetate as carbon source.     

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     

Reduced sulfur and nitrogen compounds and molecular hydrogen as electron donors for anaerobic CO2 photoreduction in Anacystis nidulans

Photosynthesis by Anacystis nidulans was studied in presence of reduced sulfur or nitrogen compounds, or of hydrogen. O₂ evolution and CO₂ fixation were depressed by sulfide, sulfite, cysteine, thioglycollate, hydroxylamine and hydrazine. Sulfite, cysteine and hydrazine inhibited O₂ evolution much more strongly than CO₂ fixation, indicating ability to supply electrons for CO₂ photoreduction; DCMU suppressed these photoreductions. In contrast, some anoxygenic photosynthetic CO₂ fixation insensitive to DCMU was found with sulfide, thiosulfate and hydrogen. Emerson enhancement studies confirmed that sulfite, cysteine and hydrazine acted on photosystem II, while photoreduction supported by sulfide, thiosulfate and hydrogen needed photosystem I only.Sulfite was photooxidized to sulfate, sulfide to elemental sulfur, and thiosulfate to sulfate plus elemental sulfur; the sulfur accumulated inside the cells. Results on the stoichiometries of the photoreductions were consistent with the photooxidation products determined. Inhibitor studies suggested photosynthetic CO₂ fixation through the Calvin cycle.While photoreduction by all reductants used was found to be constitutive in Anacystis, the process was stimulated by anaerobic preincubation with the reductants only in the cases of hydrogen and thiosulfate; this adaptation was prevented by chloramphenicol and by O₂. Anaerobic photoautotrophic growth of Anacystis was, however, not observed; the increase in dry weight with H₂ and thiosulfate was not accompanied by cell multiplication or by an increase in chlorophyll content. Parallel short-term experiments with Chlorella did not reveal any constitutive photoreduction in this eukaryotic alga.Abbreviations CAP chloramphenicol - CCCP carbonyl cyanide m-chlorophenylhydrazone - DBMIB dibromothymoquinone - DCMU dichlorophenyl dimethyl urea - DSPD disalicylidenepropane diamine-(1,3) - EDAC 1-ethyl-3(3-dimethylaminopropyl-) carbodiimide     

Production of elemental sulfur by green and purple sulfur bacteria

The utilization of sulfide by phototrophic sulfur bacteria temporarily results in the accumulation of elemental sulfur. In the green sulfur bacteria (Chlorobiaceae), the sulfur is deposited outside the cells, whereas in the purple sulfur bacteria (Chromatiaceae) sulfur is found intracellularly. Consequently, in the latter case, sulfur is unattainable for other individuals. Attempts were made to analyze the impact of the formation of extracellular elemental sulfur compared to the deposition of intracellular sulfur.According to the theory of the continuous cultivation of microorganisms, the steady-state concentration of the limiting substrate is unaffected by the reservoir concentration (S _R).It was observed in sulfide-limited continuous cultures ofChlorobium limicola f.thiosulfatophilum that higherS _R values not only resulted in higher steady-state population densities, but also in increased steady-state concentrations of elemental sulfur. Similar phenomena were observed in sulfide-limited cultures ofChromatium vinosum.It was concluded that the elemental sulfur produced byChlorobium, althouth being deposited extracellularly, is not easily available for other individuals, and apparently remains (in part) attached to the cells. The ecological significance of the data is discussed.Non-standard abbreviations RP reducing power - BChl bacteriochlorophyll - N_cell cell material - specific growth rate - {ie52-1} maximal specific growth rate - D dilution rate - K _s saturation constant - s concentration of limiting substrate - S _R same ass but in reservoir bottle - Y yield factor - iS^o intracellular elemental sulfur - eS^o extracellular elemental sulfur - PHB poly-beta-hydroxybutyric acid     

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     

Elemental sulfur as an intermediate of sulfide oxidation with oxygen byDesulfobulbus propionicus

The sulfate-reducing bacteriumDesulfobulbus propionicus oxidized sulfide, elemental sulfur, and sulfite to sulfate with oxygen as electron acceptor. Thiosulfate was reduced and disproportionated exclusively under anoxic conditions. When small pulses of oxygen were added to washed cells in sulfide-containing assays, up to 3 sulfide molecules per O₂ disappeared transiently. After complete oxygen consumption, part of the sulfide reappeared. The intermediate formed was identified as elemental sulfur by chemical analysis and turbidity measurements. When excess sulfide was present, sulfur dissolved as polysulfide. This process was faster in the presence of cells than in their absence. The formation of sulfide after complete oxygen consumption was due to a disproportionation of elemental sulfur (or polysulfide) to sulfide and sulfate. The uncoupler tetrachlorosalicylanilide (TCS) and the electron transport inhibitor myxothiazol inhibited sulfide oxidation to sulfate and caused accumulation of sulfur. In the presence of the electron transport inhibitor 2-n-heptyl-4-hydroxyquinoline-N-oxide (HQNO), sulfite and thiosulfate were formed. During sulfur oxidation at low oxygen concentrations, intermediary formation of sulfide was observed, indicating disproportionation of sulfur also under these conditions. It is concluded that sulfide oxidation inD. propionicus proceeds via oxidation to elemental sulfur, followed by sulfur disproportionation to sulfide and sulfate. Dedicated to Prof. Dr. Dr. h.c. Norbert Pfennig on the occasion of his 70th birthday     

Inhibition of microbiological sulfide oxidation by methanethiol and dimethyl polysulfides at natron-alkaline conditions

To avoid problems related to the discharge of sulfidic spent caustics, a biotechnological process is developed for the treatment of gases containing both hydrogen sulfide and methanethiol. The process operates at natron-alkaline conditions (>1 mol L⁻¹ of sodium- and potassium carbonates and a pH of 8.5–10) to enable the treatment of gases with a high partial CO₂ pressure. In the process, methanethiol reacts with biologically produced sulfur particles to form a complex mixture predominantly consisting of inorganic polysulfides, dimethyl disulfide (DMDS), and dimethyl trisulfide (DMTS). The effect of these organic sulfur compounds on the biological oxidation of sulfide to elemental sulfur was studied with natron-alkaliphilic bacteria belonging to the genus Thioalkalivibrio. Biological oxidation rates were reduced by 50% at 0.05 mM methanethiol, while for DMDS and DMTS, this was estimated to occur at 1.5 and 1.0 mM, respectively. The inhibiting effect of methanethiol on biological sulfide oxidation diminished due to its reaction with biologically produced sulfur particles. This reaction increases the feasibility of biotechnological treatment of gases containing both hydrogen sulfide and methanethiol at natron-alkaline conditions.     

Bacterial sulfur reduction in hot vents

Abstract: Elemental sulfur can be reduced through different microbial processes, including catabolically significant sulfur respiration and reduction of sulfur in the course of fermentation. Both of these processes are found in thermophilic microorganisms inhabiting continental and submarine hot vents, where elemental sulfur is one of the most common sulfur species. Among extreme thermophiles, respresented mainly by Archaea, sulfur-respiring bacteria include hydrogen-utilizing lithoautotrophs and heterotrophs, oxidizing complex organic substrates. Some marine heterotrophic sulfur-reducing Archaea were found to ferment peptides and polysaccharides, using elemental sulfur as an electron sink and thus avoiding the formation of molecular hydrogen which is highly inhibiting. Moderately thermophilic communities contain eubacterial sulfur reducers capable of lithotrophic and heterotrophic growth. Total mineralization of organic matter is carried out by a complex microbial system consisting of fermentative heterotrophs, which use elemental sulfur as an electron sink, and sulfur-respiring bacteria of the genus Desulfurella , which oxidize other fermentation products, yielding only CO_f2 and H_f2S. The most remarkable thermophilic microbial community is the thermophilic cyanobacterial mat found in the Uzon caldera, Kamchatka, which contains elemental sulfur among the layers. Organic matter produced by the thermophilic Oscillatoria is completely and rapidly mineralized by means of sulfur reduction.     

The production and utilization of intermediary elemental sulfur during the oxidation of reduced sulfur compounds by Thiobacillus ferrooxidans

The intermediary production of elemental sulfur during the microbial oxidation of reduced sulfur compounds has frequently been reported. Thiobacillus ferrooxidans, an acidophilic chemolithoautotroph, was found to produce an insoluble sulfur compound, primarily elemental sulfur, during the oxidation of thiosulfate, trithionate, tetrathionate and sulfide. This was confirmed by light and electron microscopy. Sulfur was produced from sulfide by an oxidative step, while the production from tetrathionate was initiated by a hydrolytic step, probably followed by a series of chemical reactions. The oxidation of intermediary sulfur was severely inhibited by sulfhydryl-binding reagents such as N-ethylmaleimide, by the addition of uncouplers or after freezing and thawing of the cells, which probably damaged the cell membrane. The mechanisms behind these inhibitions have not yet been clarified. Finally, it was observed that elemental sulfur oxidation by whole cells depended on the medium composition. The absence of sulfate or selenate reduced the sulfur oxidation rate.Non-standard abbreviations NEM N-ethylmaleimide - CCCP carbonyl cyanide m-chlorophenyl hydrazone     

Rhodopseudomonas adriatica sp. nov., a new species of the Rhodospirillaceae,dependent on reduced sulfur compounds

A new purple nonsulfur bacterium was isolated from enrichment cultures of a sulfide-containing marine lagoon. The bacterium is similar to Rhodopseudomonas capsulata and is described as a new species of the genus Rhodopseudomonas: Rhodopseudomonas adriatica. Cells are non-motile, 0.5–0.8 m by 1.3–1.8 m, and multiply by binary fission. Intracytoplasmic membranes are of the vesicular type. The photosynthetic pigments are bacteriochlorophyll a and carotenoids of the spheroidene group. Growth is possible anaerobically in the light and at low pO₂ in the dark. Biotin and thiamine are required as growth factors. A wide variety of organic compounds, as well as sulfide and thiosulfate, are used as photosynthetic electron donors. Sulfide is oxidized to elemental sulfur, which is subsequently converted to sulfate, whereas thiosulfate oxidation occurs without measurable intermediate. Rhodopseudomonas adriatica is unable to assimilate sulfate, growth is only possible in the presence of a reduced sulfur compound.     

The transformation of inorganic sulfur compounds and the assimilation of organic and inorganic carbon by the sulfur disproportionating bacterium Desulfocapsa sulfoexigens

The physiology of the sulfur disproportionator Desulfocapsa sulfoexigens was investigated in batch cultures and in a pH-regulated continuously flushed fermentor system. It was shown that a sulphide scavanger in the form of ferric iron was not obligatory and that the control of pH allowed production of more biomass than was possible in carbonate buffered but unregulated batch cultures. Small amounts of sulphite were produced during disproportionation of elemental sulfur and thiosulphate. In addition, it was shown that in the presence of hydrogen, a respiratory type of process is favored before the disproportionation of sulphite, thiosulphate and elemental sulfur. Sulphate reduction was not observed. D. sulfoexigens assimilated inorganic carbon even in the presence of organic carbon sources. Inorganic carbon assimilation was probably catalyzed by the reverse CO-dehydrogenase pathway, which was supported by the constitutive expression of the gene encoding CO-dehydrogenase in cultures grown in the presence of acetate and by the high carbon fractionation values that are indicative of this pathway.     

Inhibition of respiration and nitrate assimilation enhances photohydrogen evolution under low oxygen concentrations in Synechocystis sp. PCC 6803

In cyanobacterial membranes photosynthetic light reaction and respiration are intertwined. It was shown that the single hydrogenase of Synechocystis sp. PCC 6803 is connected to the light reaction. We conducted measurements of hydrogenase activity, fermentative hydrogen evolution and photohydrogen production of deletion mutants of respiratory electron transport complexes. All single, double and triple mutants of the three terminal respiratory oxidases and the ndhB-mutant without a functional complex I were studied. After activating the hydrogenase by applying anaerobic conditions in the dark hydrogen production was measured at the onset of light. Under these conditions respiratory capacity and amount of photohydrogen produced were found to be inversely correlated. Especially the absence of the quinol oxidase induced an increased hydrogenase activity and an increased production of hydrogen in the light compared to wild type cells. Our results support that the hydrogenase as well as the quinol oxidase function as electron valves under low oxygen concentrations. When the activities of photosystem II and I (PSII and PSI) are not in equilibrium or in case that the light reaction is working at a higher pace than the dark reaction, the hydrogenase is necessary to prevent an acceptor side limitation of PSI, and the quinol oxidase to prevent an overreduction of the plastoquinone pool (acceptor side of PSII). Besides oxygen, nitrate assimilation was found to be an important electron sink. Inhibition of nitrate reductase resulted in an increased fermentative hydrogen production as well as higher amounts of photohydrogen.     

Characterization of a new platelet-forming purple sulfur bacterium,Amoebobacter pedioformis sp. nov.

The dominant purple sulfur bacterium of a reddish-colored waste water pond near Taichung, Taiwan, was isolated in pure culture, strain CML2. Individual cells were nearly spherical, nonmotile, and contained in their peripheral parts was vacuoles that appeared like elongated, curved tubes. Four to sixteen cells formed platelet-like aggregates reminiscent of Thiopedia rosea. The intracellular photosynthetic membrane system of the cells was of vesicular type; the photosynthetic pigments consisted of bacteriochlorophyll a and spirilloxanthin as the major carotenoid. The color of cell suspensions was pink to rosered. Under anaerobic conditions photolithoautotrophic growth occurred with sulfide, elemental sulfur or thiosulfate; sulfur globules were stored as an intermediary oxidation product. In the presence of sulfide, acetate, lactate and pyruvate were photoassimilated; strain CML2 lacked assimilatory sulfate reduction. Fastest photoautotrophic growth (11 h doubling time) was obtained at pH 7.5, 35°C and a light intensity of about 1000 lux (tungsten lamp). Chemolithoautotrophic growth in the dark was possible under reduced oxygen partial pressure with reduced sulfur compounds as respiratory substrates. The DNA base composition of strain CML2 was 65.5 mol% G+C. Strain CML2 is described as type strain of a new species, Amoebobacter pedioformis sp. nov., in the family Chromatiaceae.