Hans G. Trüper

The Dissimilatory Sulfate-Reducing Bacteria

The microbial reduction of elemental sulfur to hydrogen sulfide under anaerobic conditions has been observed and described repeatedly during the past 100 years (Beijerinck, 1895; Omelianski, 1904; Pelsh, 1936; Roy and Trudinger, 1970; Starkey, 1937; Woolfolk, 1962). Many different prokaryotic and eukaryotic microorganisms were shown to be able to reduce elemental sulfur in nonspecific, incidental side reactions in the course of their regular fermentative metabolism. Thiosulfate, methylene blue, aldehydes, or other compounds were also reduced. Under these circumstances, it is questionable whether the reduction of elemental sulfur plays any significant role in the normal metabolism of such cells (Roy and Trudinger, 1970).

Rearrangement of the Species and Genera of the Phototrophic “Purple Nonsulfur Bacteria”

A rearrangement of the species of the “purple nonsulfur bacteria” (Rhodospirillaceae) is proposed. The species with vesicular intracytoplasmic membranes are removed from the genus Rhodopseudomonas and are placed in the new genera Rhodobacter and Rhodopila as Rhodobacter capsulatus (type species), Rhodobacter sphaeroides, Rhodobacter sulfidophilus, Rhodobacter adriaticus, and Rhodopila globiformis (type species). Rhodopseudomonas gelatinosa and Rhodospirillum tenue are united with Rhodocyclus purpureus (type species) in the genus Rhodocyclus as Rhodocyclus gelatinosus and Rhodocyclus tenuis. The decisions for this rearrangement were based mainly upon morphological and physiological properties; supporting arguments were based on structural similarities of macromolecular cell constituents.

The Family Chromatiaceae

The Family Chromatiaceae (purple sulfur bacteria) comprises physiologically and genetically closely related species and genera (Fowler et al., 1984) that carry out anoxygenic photosynthesis. The most important and selective environmental factors in their aquatic habitats are anoxic conditions, the presence of hydrogen sulfide, and illumination. The only other groups of phototrophic bacteria that thrive under similar environmental conditions are the Ectothiorhodospiraceae (see Chapter 171) and the Chlorobiaceae (green sulfur bacteria; see Chapter 195). Because they live in the same types of habitats, some discussion of the Chlorobiaceae must be included in this chapter. However, since the Chlorobiaceae are not phylogenetically related to the other anoxygenic phototrophic bacteria (Stackebrandt et al., 1984), they are treated in a separate chapter, Chapter 195.

Isolation of Members of the Families Chromatiaceae and Chlorobiaceae

The green and purple sulfur bacteria (Chlorobiaceae and Chromatiaceae) are two physiological-ecological groups of anaerobic phototrophic bacteria with anoxygenic photosynthesis. The two groups display a competitive advantage over other microorganisms in similar aquatic habitats. The most important environmental factors are anaerobic conditions, the presence of hydrogen sulfide, and illumination. Both families are treated in one chapter because they occur under similar environmental conditions and because the strains of both families are isolated with similar methods and media. Only the genus Ecto-thiorhodospira is treated in a seChapaute chapter (this Handbook, Chapter 15) because of methodological differences.

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 μm wide and, depending on cultural conditions, 2.5–8.0 μm 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.

The Wadi Natrun: Chemical composition and microbial mass developments in alkaline brines of Eutrophic Desert Lakes

Six lakes of the Wadi Natrun, Egypt, were studied with respect to the chemical composition of their brines and the occurrence of microbial mass developments. All investigated lakes showed pH values of approximately 11 and a total salt content of generally more than 30%. The main components were sulfate, carbonate, chloride, sodium, and minor amounts of potassium. Only traces of magnesium and calcium were present, but unusually high concentrations of organic carbon compounds, nitrogen compounds, and phosphate were found. Mass developments of phototrophic sulfur bacteria, halobacteria, cyanobacteria, and green algae were observed. The functions of complete nitrogen and sulfur cycles in the alkaline brines are discussed. The properties of the lakes and their ecology are compared with data on the Dead Sea and Great Salt Lake, Utah.

Degradation of dibenzothiophene by Brevibacterium sp.DO

Dibenzothiophene, a polycyclic aromatic sulfur heterocycle, represents as a model compound the organic sulfur integrated in the macromolecular coal matrix. A pure culture of a Brevibacterium species was isolated, which is able to use dibenzothiophene as sole source of carbon, sulfur and energy for growth. During dibenzothiophene utilization sulfite was released in a stoichiometrical amount and was further oxidized to sulfate. Three metabolites of dibenzothiophene degradation were isolated and identified as dibenzothiophene-5-oxide, dibenzothiophene-5-dioxide and benzoate by cochromatography, UV spectroscopy and gas chromatographymass spectrometry analyses. Based on the identified metabolites a pathway for the degradation of dibenzothiophene by Brevibacterium sp. DO is proposed.

Quantitative speciation of sulfur in bacterial sulfur globules: X-ray absorption spectroscopy reveals at least three different species of sulfur.

X-ray absorption near edge structure (XANES) spectroscopy at the sulfur K-edge was applied to probe the speciation of sulfur of metabolically different sulfur-accumulating bacteria in situ. Fitting the spectra using a least-square fitting routine XANES reveals at least three different forms of sulfur in bacterial sulfur globules. Cyclooctasulfur dominates in the sulfur globules of Beggiatoa alba and the very recently described giant bacterium Thiomargarita namibiensis. A second type of sulfur globules is present in Acidithiobacillus ferrooxidans: here the sulfur occurs as polythionates. In contrast, in purple and green sulfur bacteria the sulfur mainly consists of sulfur chains, irrespective of whether it is accumulated in globules inside or outside the cells. These results indicate that the speciation of sulfur in the sulfur globules reflects the different ecological and physiological properties of different metabolic groups of bacteria.

Thiosulphate oxidation in the phototrophic sulphur bacterium Allochromatium vinosum

Two different pathways for thiosulphate oxidation are present in the purple sulphur bacterium Allochromatium vinosum: oxidation to tetrathionate and complete oxidation to sulphate with obligatory formation of sulphur globules as intermediates. The tetrathionate:sulphate ratio is strongly pH‐dependent with tetrathionate formation being preferred under acidic conditions. Thiosulphate dehydrogenase, a constitutively expressed monomeric 30 kDa c‐type cytochrome with a pH optimum at pH 4.2 catalyses tetrathionate formation. A periplasmic thiosulphate‐oxidizing multienzyme complex (Sox) has been described to be responsible for formation of sulphate from thiosulphate in chemotrophic and phototrophic sulphur oxidizers that do not form sulphur deposits. In the sulphur‐storing A. vinosum we identified five sox genes in two independent loci (soxBXA and soxYZ). For SoxA a thiosulphate‐dependent induction of expression, above a low constitutive level, was observed. Three sox‐encoded proteins were purified: the heterodimeric c‐type cytochrome SoxXA, the monomeric SoxB and the heterodimeric SoxYZ. Gene inactivation and complementation experiments proved these proteins to be indispensable for thiosulphate oxidation to sulphate. The intermediary formation of sulphur globules in A. vinosum appears to be related to the lack of soxCD genes, the products of which are proposed to oxidize SoxY‐bound sulphane sulphur. In their absence the latter is instead transferred to growing sulphur globules.

Dissimilatory sulphite reductase from Archaeoglobus fulgidus: physico-chemical properties of the enzyme and cloning, sequencing and analysis of the reductase genes

A dissimilatory sulphite reductase was isolated from the extremely thermophilic dissimilatory sulphate-reducing archaeon Archaeoglobus fulgidus. In common with other dissimilatory sulphite reductases thus far characterized, the enzyme has an alpha 2 beta 2-structure and contains sirohaem, non-haem iron atoms and acid labile sulphide. The oxidized enzyme exhibited absorption maxima at 281, 394, 545 and 593 nm with a weak band around 715 nm. We have cloned and sequenced the genes for the alpha and beta subunits of this enzyme, which we designate dsrA and dsrB, respectively. They are contiguous in the order dsrA dsrB and probably comprise an operon, since dsrA is preceded by sequences characteristic of promoters in methanogenic archaea, and dsrB is followed by a sequence resembling termination signals in extremely thermophilic sulphur-dependent archaea. dsrA and dsrB encode 47.4 kDa and 41.7 kDa peptides, which have 25.6% amino acid sequence identity, indicating that they may have arisen by duplication of an ancestral gene. Each deduced peptide contains cysteine clusters resembling those postulated to bind sirohaem-[Fe4S4] complexes in sulphite reductases and nitrite reductases from other species. The dsrB encoded peptide lacks a single cysteine residue in one of the two clusters, suggesting that only the alpha subunit binds a sirohaem-[Fe4S4] complex, and chemical analyses showed the presence of only two sirohaems per alpha 2 beta 2 enzyme molecule. Both deduced peptides also contain an arrangement of cysteine residues characteristic of [Fe4S4] ferredoxins, and chemical analyses were consistent with the presence of six [Fe4S4] clusters per alpha 2 beta 2 enzyme molecule, two of which would be expected to be associated with sirohaem while the other four could bind to the ferredoxin-like sites.