Dissimilatory Sulfate- and Sulfur-Reducing Prokaryotes

The composition of sulfur isotopes in sedimentary sulfides and sulfates traces the sulfur cycle throughout Earth’s history. In particular, depletions of sulfur-34 (34S) in sulfide relative to sulfate exceeding 47 per mil (‰) often serve as a proxy for the disproportionation of intermediate sulfur species in addition to sulfate reduction. Here, we demonstrate that a pure, actively growing culture of a marine sulfate-reducing bacterium can deplete 34S by up to 66‰ during sulfate reduction alone and in the absence of an extracellular oxidative sulfur cycle. Therefore, similar magnitudes of sulfur isotope fractionation in sedimentary rocks do not unambiguously record the presence of other sulfur-based metabolisms or the stepwise oxygenation of Earth’s surface environment during the Proterozoic.

https://science.sciencemag.org/content/sci/333/6038/74.full.pdf

Large Sulfur Isotope Fractionation Does Not Require Disproportionation

Growth of Desulfotomaculum orientis, D. ruminis, D. nigrificans and the Desulfotomaculum strains TEP, TWC and TWP, that were newly isolated with sulfate and fatty acids, was studied using defined mineral media. Four of these strains grew with hydrogen plus sulfate as the only energy source. Under these conditions the growth yield of D. orientis in batch culture was 7.5 g cell dry mass per mol sulfate reduced. Growth on methanol with growth yields of about 6 g cell dry mass per mol sulfate was obtained with D. orientis and strain TEP. All strains tested grew slowly with formate as electron donor. Fatty acids from propionate to palmitate were utilized by the strains TEP, TWC and TWP. D. orientis and the strains TEP and TWC were able to utilize the methoxyl groups of trimethoxybenzoate for growth. D. orientis was found to grow chemoautotrophically with hydrogen, carbon dioxide and sulfate; during growth with C1-compounds no additional organic carbon source was required. Furthermore, D. orientis was able to grow slowly in sulfate-free medium with formate, methanol, ethanol lactate, pyruvate or trimethoxybenzoate. Under these conditions acetate was excreted, indicating the function of carbon dioxide as electron acceptor in a homoacetogenic process. A growth-promoting effect of pyrophosphate added to the medium of Desulfotomaculum species was not observed. The results show a high catabolic and anabolic versatility among Desulfotomaculum species, and indicate that electron transport to sulfate can be the sole energy conserving process in this genus.

Growth with hydrogen, and further physiological characteristics of Desulfotomaculum species

A moderately thermophilic, anaerobic, chemolithoheterotrophic, sulfate-reducing bacterium, strain CO-1-SRB(T), was isolated from sludge from an anaerobic bioreactor treating paper mill wastewater. Cells were Gram-positive, motile, spore-forming rods. The temperature range for growth was 30-68 degrees C, with an optimum at 55 degrees C. The NaCl concentration range for growth was 0-17 g l(-1); there was no change in growth rate until the NaCl concentration reached 8 g l(-1). The pH range for growth was 6.0-8.0, with an optimum of 6.8-7.2. The bacterium could grow with 100% CO in the gas phase. With sulfate, CO was converted to H(2) and CO(2) and part of the H(2) was used for sulfate reduction; without sulfate, CO was completely converted to H(2) and CO(2). With sulfate, strain CO-1-SRB(T) utilized H(2)/CO(2), pyruvate, glucose, fructose, maltose, lactate, serine, alanine, ethanol and glycerol. The strain fermented pyruvate, lactate, glucose and fructose. Yeast extract was necessary for growth. Sulfate, thiosulfate and sulfite were used as electron acceptors, whereas elemental sulfur and nitrate were not. A phylogenetic analysis of 16S rRNA gene sequences placed strain CO-1-SRB(T) in the genus Desulfotomaculum, closely resembling Desulfotomaculum nigrificans DSM 574(T) and Desulfotomaculum sp. RHT-3 (99 and 100% similarity, respectively). However, the latter strains were completely inhibited above 20 and 50% CO in the gas phase, respectively, and were unable to ferment CO, lactate or glucose in the absence of sulfate. DNA-DNA hybridization of strain CO-1-SRB(T) with D. nigrificans and Desulfotomaculum sp. RHT-3 showed 53 and 60% relatedness, respectively. On the basis of phylogenetic and physiological features, it is suggested that strain CO-1-SRB(T) represents a novel species within the genus Desulfotomaculum, for which the name Desulfotomaculum carboxydivorans is proposed. This is the first description of a sulfate-reducing micro-organism that is capable of growth under an atmosphere of pure CO with and without sulfate. The type strain is CO-1-SRB(T) (=DSM 14880(T)=VKM B-2319(T)).

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Desulfotomaculum carboxydivorans sp. nov., a novel sulfate-reducing bacterium capable of growth at 100% CO.

A new sulfate-reducing bacterium, strain 86FS1, was isolated from a deep-sea sediment in the western Mediterranean Sea with sodium lactate as electron and carbon source. Cells were ovoid, gram-negative and motile. Strain 86FS1 contained b- and c-type cytochromes. The organism was able to utilize propionate, pyruvate, lactate, succinate, fumarate, malate, alanine, primary alcohols (C(2)-C(5)), and mono- and disaccharides (glucose, fructose, galactose, ribose, sucrose, cellobiose, lactose) as electron donors for the reduction of sulfate, sulfite or thiosulfate. The major products of carbon metabolism were acetate and CO(2), with exception of n-butanol and n-pentanol, which were oxidized only to the corresponding fatty acids. The growth yield with sulfate and glucose or lactate was 8.3 and 15 g dry mass, respectively, per mol sulfate. The temperature limits for growth were 10 degrees C and 30 degrees C with an optimum at 25 degrees C. Growth was observed at salinities ranging from 10 to 70 g NaCl l(-1). Sulfide concentrations above 4 mmol l(-1) inhibited growth. The fatty acid pattern of strain 86FS1 resembled that of Desulfobulbus propionicus with n-14:0, n-16:1omega7, n-16:1 omega5, n-17:1 omega6 and n-18:1 omega7 as dominant fatty acids. On the basis of its phylogenetic position and its phenotypic properties, strain 86FS1 affiliates with the genus Desulfobulbus and is described as a new species, Desulfobulbus mediterraneus sp. nov.

Desulfobulbus mediterraneus sp. nov., a sulfate-reducing bacterium growing on mono- and disaccharides.

Desulfotomaculum nigrificans degraded glucose to acetate, ethyl alcohol, and carbon dioxide. By use of 14C-glucose labeled at different carbon atoms, two pathways of glucose metabolism were detected. They were the Embden-Meyerhof and the Entner-Doudoroff schemes. Because the observed quantities of acetate and carbon dioxide, arising from glucose, were greater than the expected theoretical values, individual fermentations were conducted with 15 uniformly labeled 14C-amino acids. The results indicated that amino acids, supplied by the yeast extract or peptone in the fermentation medium, also contributed to the formation of acetate and carbon dioxide.

Gloria Jackson

Papers

Degradation of Glucose by Proliferating Cells of Desulfotomaculum nigrificans.