Inhibition of anaerobic digestion process: A review

Biofilms are complex communities of microorganisms attached to surfaces or associated with interfaces. Despite the focus of modern microbiology research on pure culture, planktonic (free-swimming) bacteria, it is now widely recognized that most bacteria found in natural, clinical, and industrial settings persist in association with surfaces. Furthermore, these microbial communities are often composed of multiple species that interact with each other and their environment. The determination of biofilm architecture, particularly the spatial arrangement of microcolonies (clusters of cells) relative to one another, has profound implications for the function of these complex communities. Numerous new experimental approaches and methodologies have been developed in order to explore metabolic interactions, phylogenetic groupings, and competition among members of the biofilm. To complement this broad view of biofilm ecology, individual organisms have been studied using molecular genetics in order to identify the genes required for biofilm development and to dissect the regulatory pathways that control the plankton-to-biofilm transition. These molecular genetic studies have led to the emergence of the concept of biofilm formation as a novel system for the study of bacterial development. The recent explosion in the field of biofilm research has led to exciting progress in the development of new technologies for studying these communities, advanced our understanding of the ecological significance of surface-attached bacteria, and provided new insights into the molecular genetic basis of biofilm development.

Microbial Biofilms: from Ecology to Molecular Genetics

Fatty acids and alcohols are key intermediates in the methanogenic degradation of organic matter, e.g., in anaerobic sewage sludge digestors or freshwater lake sediments. They are produced by classical fermenting bacteria for disposal of electrons derived in simultaneous substrate oxidations. Methanogenic bacteria can degrade primarily only one-carbon compounds. Therefore, acetate, propionate, ethanol, and their higher homologs have to be fermented further to one-carbon compounds. These fermentations are called secondary or syntrophic fermentations. They are endergonic processes under standard conditions and depend on intimate coupling with methanogenesis. The energetic situation of the prokaryotes cooperating in these processes is problematic: the free energy available in the reactions for total conversion of substrate to methane attributes to each partner amounts of energy in the range of the minimum biochemically convertible energy, i.e., 20 to 25 kJ per mol per reaction. This amount corresponds to one-third of an ATP unit and is equivalent to the energy required for a monovalent ion to cross the charged cytoplasmic membrane. Recent studies have revealed that syntrophically fermenting bacteria synthesize ATP by substrate-level phosphorylation and reinvest part of the ATP-bound energy into reversed electron transport processes, to release the electrons at a redox level accessible by the partner bacteria and to balance their energy budget. These findings allow us to understand the energy economy of these bacteria on the basis of concepts derived from the bioenergetics of other microorganisms.

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Energetics of syntrophic cooperation in methanogenic degradation.

https://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_7/b_fdi_57-58/010025232.pdf

Production, oxidation, emission and consumption of methane by soils: A review

Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives

Microbial formate production and consumption during syntrophic conversion of ethanol or lactate to methane was examined in purified flocs and digestor contents obtained from a whey-processing digestor. Formate production by digestor contents or purified digestor flocs was dependent on CO(2) and either ethanol or lactate but not H(2) gas as an electron donor. During syntrophic methanogenesis, flocs were the primary site for formate production via ethanol-dependent CO(2) reduction, with a formate production rate and methanogenic turnover constant of 660 muM/h and 0.044/min, respectively. Floc preparations accumulated fourfold-higher levels of formate (40 muM) than digestor contents, and the free flora was the primary site for formate cleavage to CO(2) and H(2) (90 muM formate per h). Inhibition of methanogenesis by CHCl(3) resulted in formate accumulation and suppression of syntrophic ethanol oxidation. H(2) gas was an insignificant intermediary metabolite of syntrophic ethanol conversion by flocs, and its exogenous addition neither stimulated methanogenesis nor inhibited the initial rate of ethanol oxidation. These results demonstrated that >90% of the syntrophic ethanol conversion to methane by mixed cultures containing primarily Desulfovibrio vulgaris and Methanobacterium formicicum was mediated via interspecies formate transfer and that <10% was mediated via interspecies H(2) transfer. The results are discussed in relation to biochemical thermodynamics. A model is presented which describes the dynamics of a bicarbonate-formate electron shuttle mechanism for control of carbon and electron flow during syntrophic methanogenesis and provides a novel mechanism for energy conservation by syntrophic acetogens.

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Control of Interspecies Electron Flow during Anaerobic Digestion: Significance of Formate Transfer versus Hydrogen Transfer during Syntrophic Methanogenesis in Flocs.

The flora of an anaerobic whey-processing chemostat was separated by anaerobic sedimentation techniques into a free-living bacterial fraction and a bacterial floc fraction. The floc fraction constituted a major part (i.e., 57% total protein) of the total microbial population in the digestor, and it accounted for 87% of the total CO(2)-dependent methanogenic activity and 76% of the total ethanol-consuming acetogenic activity. Lactose was degraded by both cellular fractions, but in the free flora fraction it was associated with higher intermediary levels of H(2), ethanol, butyrate, and propionate production. Electron microscopic analysis of flocs showed bacterial diversity and juxtapositioning of tentative Desulfovibrio and Methanobacterium species without significant microcolony formation. Ethanol, an intermediary product of lactose-hydrolyzing bacteria, was converted to acetate and methane within the flocs by interspecies electron transfer. Ethanol-dependent methane formation was compartmentalized and closely coupled kinetically within the flocs but without significant formation of H(2) gas. Physical disruption of flocs into fragments of 10- to 20-mum diameter initially increased the H(2) partial pressure but did not change the carbon transformation kinetic patterns of ethanol metabolism or demonstrate a significant role for H(2) in CO(2) reduction to methane. The data demonstrate that floc formation in a whey-processing anaerobic digestor functions in juxtapositioning cells for interspecies electron transfer during syntrophic ethanol conversion into acetate and methane but by a mechanism which was independent of the available dissolved H(2) gas pool in the ecosystem.

Control of Interspecies Electron Flow during Anaerobic Digestion: Role of Floc Formation in Syntrophic Methanogenesis.

The microbial species composition of methanogenic granules developed on an acetate-propionate-butyrate mixture was characterized. The granules contained high numbers of adhesive methanogens (1012/g dry weight) and butyrate-, isobutyrate-, and propionate-degrading syntrophic acetogens (1011/g dry weight), but low numbers of hydrolytic-fermentative bacteria (109/g dry weight). Prevalent methanogens in the granules included: Methanobacterium formicicum strain T1N and RF, Methanosarcina mazei strain T18, Methanospirillum hungatei strain BD, and a non-filamentous, bamboo-shaped rod species, Methanothrix/Methanosaeta-like strain M7. Prevalent syntrophic acetogens included: a butyrate-degrading Syntrophospora bryantii-like strain BH, a butyrate-isobutyrate degrading non-spore-forming rod, strain IB, a propionate-degrading sporeforming oval-shaped species, strain PT, and a propionate-degrading none-spore-forming sulfate-reducing rod species, strain PW, which was able to grow syntrophically with an H2-utilizing methanogen. Sulfate-reducing bacteria did not play a significant role in the metabolism of H2, formate, acetate and butyrate but they were involved in propionate degradation.

Microbial composition and characterization of prevalent methanogens and acetogens isolated from syntrophic methanogenic granules

Anaerobic batch digestion of sheep tallow

High performance biomethanation granules with operational specific COD removal rates of 7 kg COD removed/kg SS/d were obtained by ecoengineering conventional, granular, UASB digester sludge using a designed protocol of starvation and selection on a defined volatile fatty acid (VFA) based mineral medium. Addition of low (0.15 mM) sulfate levels to this VFA medium increased the maximum shock-load COD removal rate of the ecoengineered biomethanation granules to 9 kg COD/kg SS/d with specific acetate, propionate, and butyrate removal rates of 111, 28, and 64 mol/g SS/d. Addition of moderate (26 mM) calcium levels inhibited growth and altered the structure of granules. The general cellular, growth, stability, and performance features of these ecoengineered granules are described and discussed in relation to their use as improved biomethanation starter cultures.

Jürgen H. Thiele

Papers

Control of Interspecies Electron Flow during Anaerobic Digestion: Significance of Formate Transfer versus Hydrogen Transfer during Syntrophic Methanogenesis in Flocs.

Control of Interspecies Electron Flow during Anaerobic Digestion: Role of Floc Formation in Syntrophic Methanogenesis.

Microbial composition and characterization of prevalent methanogens and acetogens isolated from syntrophic methanogenic granules

Anaerobic batch digestion of sheep tallow

Ecoengineering high rate anaerobic digestion systems: analysis of improved syntrophic biomethanation catalysts.