Melike Balk

Bio: Melike Balk is an academic researcher from Wageningen University and Research Centre. The author has contributed to research in topics: Moorella thermoacetica & Propionate. The author has an hindex of 13, co-authored 15 publications receiving 1062 citations.

Thermotoga lettingae sp. nov., a novel thermophilic, methanol-degrading bacterium isolated from a thermophilic anaerobic reactor.

Abstract: A novel, anaerobic, non-spore-forming, mobile, Gram-negative, thermophilic bacterium, strain TMOT, was isolated from a thermophilic sulfate-reducing bioreactor operated at 65 C with methanol as the sole substrate. The G+C content of the DNA of strain TMOT was 39.2 mol%. The optimum pH, NaCl concentration, and temperature for growth were 7.0, 1.0%, and 65 degrees C, respectively. Strain TMOT was able to degrade methanol to CO2 and H2 in syntrophic culture with Methanothermobacter thermautotrophicus AH or Thermodesulfovibrio yellowstonii. Thiosulfate, elemental sulfur, Fe(III) and anthraquinone-2,6-disulfonate were able to serve as electron acceptors during methanol degradation. In the presence of thiosulfate or elemental sulfur, methanol was converted to CO2 and partly to alanine. In pure culture, strain TMOT was also able to ferment methanol to acetate, CO2 and H2. However, this degradation occurred slower than in syntrophic cultures or in the presence of electron acceptors. Yeast extract was required for growth. Besides growing on methanol, strain TMOT grew by fermentation on a variety of carbohydrates including monomeric and oligomeric sugars, starch and xylan. Acetate, alanine, CO2, H2, and traces of ethanol, lactate and alpha-aminobutyrate were produced during glucose fermentation. Comparison of 16S rDNA genes revealed that strain TMOT is related to Thermotoga subterranea (98%) and Thermotoga elfii (98%). The type strain is TMOT (= DSM 14385T = ATCC BAA-301T). On the basis of the fact that these organisms differ physiologically from strain TMOT, it is proposed that strain TMOT be classified as a new species, within the genus Thermotoga, as Thermotoga lettingae.

Constraints on the Biological Source(s) of the Orphan Branched Tetraether Membrane Lipids

Abstract: A soil profile from the Saxnas Mosse peat bog, Sweden, has been analysed for glycerol dialkyl glycerol tetraether (GDGT) membrane lipids and 16S rRNA genes in order to constrain the source of the yet ‘orphan,’ but supposedly bacterial, branched GDGTs. Branched GDGT lipids dominate over archaeal membrane lipids. The Acidobacteria comprise the dominant bacterial group, accounting for the majority of total Bacteria, and are generally more abundant than methanogenic archaea. Analysed acidobacterial strains did not contain branched GDGT lipids. Thus, the source organism must likely be searched for in other acidobacterial phyla or in another abundant group within the remaining bacteria.

Desulfotomaculum thermobenzoicum subsp. thermosyntrophicum subsp. nov., a thermophilic, syntrophic, propionate-oxidizing, spore-forming bacterium

Abstract: From granular sludge from a laboratory-scale upflow anaerobic sludge bed reactor operated at 55 degrees C with a mixture of volatile fatty acids as feed, a novel anaerobic, moderately thermophilic, syntrophic, spore-forming bacterium, strain TPO, was enriched on propionate in co-culture with Methanobacterium thermoautotrophicum Z245. The axenic culture was obtained by using pyruvate as the sole source of carbon and energy. The cells were straight rods with pointed ends and became lens-shaped when sporulation started. The cells were slightly motile. The optimum growth temperature was 55 degrees C and growth was possible between 45 and 62 degrees C. The pH range for growth of strain TPO was 6-8, with an optimum at pH 7-7.5. Propionate was converted to acetate, CO2 and CH4 by a co-culture of strain TPO with Methanobacterium thermoautotrophicum Z245. In pure culture, strain TPO could grow fermentatively on benzoate, fumarate, H2/CO2, pyruvate and lactate. Sulphate could serve as inorganic electron acceptor when strain TPO was grown on propionate, lactate, pyruvate and H2/CO2. The G+C content was 53.7 mol%. Comparison of 16S rDNA sequences revealed that strain TPO is related to Desulfotomaculum thermobenzoicum (98%) and Desulfotomaculum thermoacetoxidans (98%). DNA-DNA hybridization revealed 88.2% reassociation between strain TPO and D. thermobenzoicum and 83.8% between strain TPO and D. thermoacetoxidans. However, both organisms differ physiologically from strain TPO and are not capable of syntrophic propionate oxidation. It is proposed that strain TPO should be classified as new subspecies of D. thermobenzoicum as D. thermobenzoicum subsp. thermosyntrophicum.

(Per)chlorate Reduction by the Thermophilic Bacterium Moorella perchloratireducens sp. nov., Isolated from Underground Gas Storage

Abstract: A thermophilic bacterium, strain An10, was isolated from underground gas storage with methanol as a substrate and perchlorate as an electron acceptor. Cells were gram-positive straight rods, 0.4 to 0.6 μm in diameter and 2 to 8 μm in length, growing as single cells or in pairs. Spores were terminal with a bulged sporangium. The temperature range for growth was 40 to 70°C, with an optimum at 55 to 60°C. The pH optimum was around 7. The salinity range for growth was between 0 and 40 g NaCl liter−1 with an optimum at 10 g liter−1. Strain An10 was able to grow on CO, methanol, pyruvate, glucose, fructose, cellobiose, mannose, xylose, and pectin. The isolate was able to respire with (per)chlorate, nitrate, thiosulfate, neutralized Fe(III) complexes, and anthraquinone-2,6-disulfonate. The G+C content of the DNA was 57.6 mol%. On the basis of 16S rRNA analysis, strain An10 was most closely related to Moorella thermoacetica and Moorella thermoautotrophica. The bacterium reduced perchlorate and chlorate completely to chloride. Key enzymes, perchlorate reductase and chlorite dismutase, were detected in cell extracts. Strain An10 is the first thermophilic and gram-positive bacterium with the ability to use (per)chlorate as a terminal electron acceptor.

The ecology and biotechnology of sulphate-reducing bacteria

Abstract: Sulphate-reducing bacteria (SRB) are anaerobic microorganisms that use sulphate as a terminal electron acceptor in, for example, the degradation of organic compounds. They are ubiquitous in anoxic habitats, where they have an important role in both the sulphur and carbon cycles. SRB can cause a serious problem for industries, such as the offshore oil industry, because of the production of sulphide, which is highly reactive, corrosive and toxic. However, these organisms can also be beneficial by removing sulphate and heavy metals from waste streams. Although SRB have been studied for more than a century, it is only with the recent emergence of new molecular biological and genomic techniques that we have begun to obtain detailed information on their way of life.

Electron transfer in syntrophic communities of anaerobic bacteria and archaea

Abstract: Interspecies electron transfer is a key process in methanogenic and sulphate-reducing environments. Bacteria and archaea that live in syntrophic communities take advantage of the metabolic abilities of their syntrophic partner to overcome energy barriers and break down compounds that they cannot digest by themselves. Here, we review the transfer of hydrogen and formate between bacteria and archaea that helps to sustain growth in syntrophic methanogenic communities. We also describe the process of reverse electron transfer, which is a key requirement in obligately syntrophic interactions. Anaerobic methane oxidation coupled to sulphate reduction is also carried out by syntrophic communities of bacteria and archaea but, as we discuss, the exact mechanism of this syntrophic interaction is not yet understood.

Bacterial membrane lipids: diversity in structures and pathways.

Abstract: For many decades, Escherichia coli was the main model organism for the study of bacterial membrane lipids. The results obtained served as a blueprint for membrane lipid biochemistry, but it is clear now that there is no such thing as a typical bacterial membrane lipid composition. Different bacterial species display different membrane compositions and even the membrane composition of cells belonging to a single species is not constant, but depends on the environmental conditions to which the cells are exposed. Bacterial membranes present a large diversity of amphiphilic lipids, including the common phospholipids phosphatidylglycerol, phosphatidylethanolamine and cardiolipin, the less frequent phospholipids phosphatidylcholine, and phosphatidylinositol and a variety of other membrane lipids, such as for example ornithine lipids, glycolipids, sphingolipids or hopanoids among others. In this review, we give an overview about the membrane lipid structures known in bacteria, the different metabolic pathways involved in their formation, and the distribution of membrane lipids and metabolic pathways across taxonomical groups.