Biomass-derived syngas fermentation into biofuels: opportunities and challenges.

Microbiology of synthesis gas fermentation for biofuel production.

Acetogens utilize the acetyl-CoA Wood-Ljungdahl pathway as a terminal electron-accepting, energy-conserving, CO(2)-fixing process. The decades of research to resolve the enzymology of this pathway (1) preceded studies demonstrating that acetogens not only harbor a novel CO(2)-fixing pathway, but are also ecologically important, and (2) overshadowed the novel microbiological discoveries of acetogens and acetogenesis. The first acetogen to be isolated, Clostridium aceticum, was reported by Klaas Tammo Wieringa in 1936, but was subsequently lost. The second acetogen to be isolated, Clostridium thermoaceticum, was isolated by Francis Ephraim Fontaine and co-workers in 1942. C. thermoaceticum became the most extensively studied acetogen and was used to resolve the enzymology of the acetyl-CoA pathway in the laboratories of Harland Goff Wood and Lars Gerhard Ljungdahl. Although acetogenesis initially intrigued few scientists, this novel process fostered several scientific milestones, including the first (14)C-tracer studies in biology and the discovery that tungsten is a biologically active metal. The acetyl-CoA pathway is now recognized as a fundamental component of the global carbon cycle and essential to the metabolic potentials of many different prokaryotes. The acetyl-CoA pathway and variants thereof appear to be important to primary production in certain habitats and may have been the first autotrophic process on earth and important to the evolution of life. The purpose of this article is to (1) pay tribute to those who discovered acetogens and acetogenesis, and to those who resolved the acetyl-CoA pathway, and (2) highlight the ecology and physiology of acetogens within the framework of their scientific roots.

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Old Acetogens, New Light

2,3-Butanediol (23BD) is a high-value chemical usually produced petrochemically but which can also be synthesized by some bacteria. To date, the best microbial 23BD production rates have been observed using pathogenic bacteria in fermentation systems that depend on sugars as the carbon and energy sources for product synthesis. Here we present evidence of 23BD production by three nonpathogenic acetogenic Clostridium species—Clostridium autoethanogenum, C. ljungdahlii, and C. ragsdalei—using carbon monoxide-containing industrial waste gases or syngas as the sole source of carbon and energy. Through an analysis of the C. ljungdahlii genome, the complete pathway from carbon monoxide to 23BD has been proposed. Homologues of the genes involved in this pathway were also confirmed for the other two species investigated. A gene expression study demonstrates a correlation between mRNA accumulation from 23BD biosynthetic genes and the onset of 23BD production, while a broader expression study of Wood-Ljungdahl pathway genes provides a transcription-level view of one of the oldest existing biochemical pathways.

2,3-butanediol production by acetogenic bacteria, an alternative route to chemical synthesis, using industrial waste gas.

Clostridia: the importance of their exceptional substrate and metabolite diversity for biofuel and biorefinery applications.

A novel solvent-producing, anaerobic clostridium, strain P7T, was isolated from sediment from an agricultural settling lagoon after enrichment with CO as the substrate. The metabolism of this Gram-positive, motile, spore-forming rod was primarily acetogenic. Acetate, ethanol, butyrate and butanol were the end-products of metabolism. Strain P7T grew on CO, H2/CO2, glucose, galactose, fructose, xylose, mannose, cellobiose, trehalose, cellulose, starch, pectin, citrate, glycerol, ethanol, propanol, 2-propanol, butanol, glutamate, aspartate, alanine, histidine, asparagine, serine, betaine, choline and syringate as sole substrates. Growth was not supported by methanol, formate, d-arabinose, fucose, lactose, melibiose, amygdalin, gluconate, lactate, malate, arginine, glutamine or vanillate. Nitrate reduction, production of indole, gelatin hydrolysis and aesculin hydrolysis were not observed. Analysis of the 16S rRNA gene sequence of the isolate showed that it was closely related to Clostridium scatologenes ATCC 25775T (99·7 % sequence similarity) and clostridial strain SL1T (99·8 % sequence similarity). Strain SL1 had been classified as a strain of C. scatologenes. However, DNA–DNA reassociation analysis showed that both strain P7T and strain SL1 represented novel clostridial species. It is proposed that strain P7T (=ATCC BAA-624T=DSM 15243T) be classified as the type strain of Clostridium carboxidivorans sp. nov. and that strain SL1T (=ATCC BAA-623T=DSM 12750T) be reclassified as the type strain of Clostridium drakei sp. nov.

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Clostridium carboxidivorans sp. nov., a solvent-producing clostridium isolated from an agricultural settling lagoon, and reclassification of the acetogen Clostridium scatologenes strain SL1 as Clostridium drakei sp. nov.

Jack S.-C. Liou

Papers

Clostridium carboxidivorans sp. nov., a solvent-producing clostridium isolated from an agricultural settling lagoon, and reclassification of the acetogen Clostridium scatologenes strain SL1 as Clostridium drakei sp. nov.