Inhibition of anaerobic digestion process: A review

[This corrects the article on p. 100 in vol. 41.].

Energy conservation in chemotrophic anaerobic bacteria.

Methanogens: reevaluation of a unique biological group.

Biogenic methane formation in marine and freshwater environments: CO2 reduction vs. acetate fermentation—Isotope evidence

Most methanogenic archaea can reduce CO(2) with H(2) to methane, and it is generally assumed that the reactions and mechanisms of energy conservation that are involved are largely the same in all methanogens. However, this does not take into account the fact that methanogens with cytochromes have considerably higher growth yields and threshold concentrations for H(2) than methanogens without cytochromes. These and other differences can be explained by the proposal outlined in this Review that in methanogens with cytochromes, the first and last steps in methanogenesis from CO(2) are coupled chemiosmotically, whereas in methanogens without cytochromes, these steps are energetically coupled by a cytoplasmic enzyme complex that mediates flavin-based electron bifurcation.

Methanogenic archaea: ecologically relevant differences in energy conservation.

Methanosarcina strain 227 exhibited exponential growth on sodium acetate in the absence of added H2. Under these conditions, rates of methanogenesis were limited by concentrations of acetate below 0.05 M. One mole of methane was formed per mole of acetate consumed. Additional evidence from radioactive labeling studies indicated that sufficient energy for growth was obtained by the decarboxylation of acetate. Diauxic growth and sequential methanogenesis from methanol followed by acetate occurred in the presence of mixtures of methanol and acetate. Detailed studies showed that methanol-grown cells did not metabolize acetate in the presence of methanol, although acetate-grown cells did metabolize methanol and acetate simultaneously before shifting to methanol. Acetate catabolism appeared to be regulated in response to the presence of better metabolizable substrates such as methanol or H2-CO2 by a mechanism resembling catabolite repression. Inhibition of methanogenesis from acetate by 2-bromoethanesulfonate, an analog of coenzyme M, was reversed by addition of coenzyme M. Labeling studies also showed that methanol may lie on the acetate pathway. These results suggested that methanogenesis from acetate, methanol, and H2-CO2 may have some steps in common, as originally proposed by Barker. Studies with various inhibitors, together with molar growth yield data, suggest a role for electron transport mechanisms in energy metabolism during methanogenesis from methanol, acetate, and H2-CO2.

Growth and Methanogenesis by Methanosarcina Strain 227 on Acetate and Methanol

A thermophilic strain of Methanosarcina, designated Methanosarcina strain TM-1, was isolated from a laboratory-scale 55°C anaerobic sludge digestor by the Hungate roll-tube technique. Penicillin and d-cycloserine, inhibitors of peptidoglycan synthesis, were used as selective agents to eliminate contaminating non-methanogens. Methanosarcina strain TM-1 had a temperature optimum for methanogenesis near 50°C and grew at 55°C but not at 60°C. Substrates used for methanogenesis and growth by Methanosarcina strain TM-1 were acetate (12-h doubling time), methanol (7- to 10-h doubling time), methanol-acetate mixtures (5-h doubling time), methylamine, and trimethylamine. When radioactively labeled acetate was the sole methanogenic substrate added to the growth medium, it was predominantly split to methane and carbon dioxide. When methanol was also present in the medium, the metabolism of acetate shifted to its oxidation and incorporation into cell material. Electrons derived from acetate oxidation apparently were used to reduce methanol. H2-CO2 was not used for growth and methanogenesis by Methanosarcina strain TM-1. When presented with both H2-CO2 and methanol, Methanosarcina strain TM-1 was capable of limited hydrogen metabolism during growth on methanol, but hydrogen metabolism ceased once the methanol was depleted. Methanosarcina strain TM-1 required a growth factor (or growth factors) present in the supernatant of anaerobic digestor sludge. Growth factor requirements and the inability to use H2-CO2 are characteristics not found in other described Methanosarcina strains. The high numbers of Methanosarcina-like clumps in sludges from thermophilic digestors and the fast generation times reported here for Methanosarcina TM-1 indicate that Methanosarcina may play an important role in thermophilic methanogenesis.

Isolation and Characterization of a Thermophilic Strain of Methanosarcina Unable to Use H(2)-CO(2) for Methanogenesis.

An acetate-fermenting strain of Methanosarcina was isolated from an acetate enrichment culture inoculated with anaerobic sludge from a waste treatment digestor. In pure culture, this organism fermented acetate in the absence of added hydrogen at rates comparable in magnitude to those found in digestor systems. This rate was significantly higher than previously obtained for pure cultures of this genus. Mineral components of yeast extract were highly stimulatory for cultures growing on methanol. Comparable stimulation was not observed for cultures growing on acetate. Labeling studies indicated that acetate was converted to methane and CO2 as predicted by previous studies on mixed cultures. Total oxidation or reduction of acetate was not the mechanism of conversion of acetate to methane by the pure culture. The ability of this strain to form colonies or to produce methane from acetate was apparently influenced by the choice of substrate and conditions used for growing the inoculum.

Studies on an acetate-fermenting strain of Methanosarcina.

A new anaerobic, mesophilic, spore-forming cellulolytic bacterium is described Cellulose is cleared within 24 to 48 h around colonies formed in cellulose agar roll tubes Cells stain gram negative and are nonmotile rods which form oblong spores either centrally or subterminally in a clostridial swelling Colonies are irregular with an opaque edge and a center devoid of both vegetative cells and spores Cellulose, xylan, pectin, cellubiose, glucose, maltose, galactose, sucrose, lactose, and mannose serve as substrates for growth H/sub 2/, CO/sub 2/, acetate, butyrate, formate, and lactate are produced during fermentation of cellulose or cellobiose The temperature and pH for optimum growth are 37/sup 0/C and 70, respectively The DNA composition is 26 to 27 mol% guanine plus cytosine This bacterium resembles Clostridium lochhheadii in morphological and some biochemical characteristics but is not identical to it The name Clostridium cellulovorans sp nov is proposed The type strain is 743B (ATCC 35296)

https://aem.asm.org/content/aem/48/1/88.full.pdf

Isolation and Characterization of an Anaerobic, Cellulolytic Bacterium, Clostridium cellulovorans sp. nov

Methanohalophilus zhilinae, a new alkaliphilic, halophilic, methylotrophic species of methanogenic bacteria, is described. Strain WeN5T (T = type strain) from Bosa Lake of the Wadi el Natrun in Egypt was designated the type strain and was further characterized. This strain was nonmotile, able to catabolize dimethylsulfide, and able to grow in medium with a methyl group-containing substrate (such as methanol or trimethylamine) as the sole organic compound added. Sulfide (21 mM) inhibited cultures growing on trimethylamine. The antibiotic susceptibility pattern of strain WeN5T was typical of the pattern for archaeobacteria, and the guanine-plus-cytosine content of the deoxyribonucleic acid was 38 mol%. Characterization of the 16S ribosomal ribonucleic acid sequence indicated that strain WeN5T is phylogenetically distinct from members of previously described genera other than Methanohalophilus and supported the partition of halophilic methanogens into their own genus.

Robert A. Mah

Papers

Growth and Methanogenesis by Methanosarcina Strain 227 on Acetate and Methanol

Isolation and Characterization of a Thermophilic Strain of Methanosarcina Unable to Use H(2)-CO(2) for Methanogenesis.

Studies on an acetate-fermenting strain of Methanosarcina.

Isolation and Characterization of an Anaerobic, Cellulolytic Bacterium, Clostridium cellulovorans sp. nov

Methanohalophilus zhilinae sp. nov., an alkaliphilic, halophilic, methylotrophic methanogen.