Roles of Molecular Hydrogen and Acetate During Anaerobic Biopolymer Degradation

Molecular hydrogen is produced during different stages of anaerobic degradation. In the fermentative stage, organisms such as Clostridium sp. and Eubacterium sp. produce fatty acids, CO2, and hydrogen from carbohydrates. In the acetogenic stage, acetogens such as Syntrophobacter wolinii and Syntrophomonas wolfei produce acetate, CO2, and hydrogen or acetate and hydrogen by anaerobic oxidation of propionate and n-butyrate (Mclnerney, 1988). Fermentative bacteria release molecular hydrogen even at a high H2 partial pressure and simultaneously excrete reduced products (e.g., clostridia, Ruminococcus, Eubacterium sp.). However, the release of molecular hydrogen during acetogenesis of fatty acids or of other reduced metabolites may occur only when hydrogen does not accumulate, for thermodynamic reasons. Molecular hydrogen is consumed by methanogens (Table 1.4, reaction 1) or, alternatively, by sulfate reducers (Table 1.4, reaction 2) via interspecies hydrogen transfer. In the rumen and in sewage sludge digesters, the hydrogen concentration can be decreased by acetate formation from CO2 and H2 (Table 1.4, reaction 3) by bacteria such as Acetobacterium woodii and Clostridium thermoaceticum. Some additional reactions consuming hydrogen to decrease its concentration are also listed in Table 1.4 (reactions 4-6).

To maintain a low H2 partial pressure, a syntrophism of acetogenic, hydrogen-producing and methanogenic, hydrogen-utilizing bacteria is essential (Ianotti et al., 1973). Complete anaerobic degradation of fatty acids with hydrogen formation by obligate proton-reducing acetogenic bacteria is possible only at H2 partial pressures <10-4 atm (n-butyrate) or 10-5 atm (propionate), which cannot be maintained by

Table 1.4 Hydrogen-consuming reactions in anaerobic ecosystems (Schink, 1997).

Substrates (mol)

(kJ per mol)

(1) 4 H2 + CO2

CH4+ 2 H2O

-131.0

(2) 4 H2 + SO2-

S2- + 4 H2O

-151.0

(3) 4 H2 + 2 CO2

CH3COO-+ H+ + 2 H2O

-0.9

(4) H2 + S0

H2S

-0.9

(5) H2C(NH+)COO- + H2

CH3COO-+ NH+

0.0

(6) COOH-CH-CH-COOH + H2

COOH-CH2-CH2-COOH

0.0

methanogens or sulfate reducers. However, by reversed electron transport electrons can be shifted to a lower redox potential suitable for proton reduction (Schink, 1997). If hydrogen accumulates beyond this threshold concentration, the anaerobic oxidation of fatty acids becomes endergonic and does not proceed (for details, see Chapter 8, this volume). Whereas hydrogen prevents p oxidation of fatty acids by ac-etogens even at very low H2 partial pressure, much higher concentrations of acetate (in the millimolar range) are required for the same effect.

The fermentative metabolism of acidogenic bacteria is exergonic even at H2 partial pressures >10-4 atm. Whereas acetogenic bacteria apparently depend mainly on ATP generation by chemiosmotic phosphorylation, fermentative bacteria produce most of their ATP by substrate chain phosphorylation. This may be why fermentative bacteria do not depend on a syntrophic interaction with electron-consuming bacteria, such as methanogens or sulfate reducers.

In addition to the possibility of anaerobic oxidation of organic compounds via syn-throphic interactions between acetogenic bacteria and acetoclastic + hydrogeno-trophic methanogens (see also Section 1.2.5), other synthrophic associations between acetate-oxidizing bacteria and H2/CO2-utilizing methanogens under thermophilic (Lee and Zinder 1988) and mesophilic (Schnuerer et al. 1996) growth conditions have been observed. Thermodynamic analysis of mesophilic synthrophic acetate oxidation revealed a hydrogen partial pressure of < 0.1-2.6 Pa, which is in the range found in methanogenic ecosystems (Dolfing, 2001). In thermophilic methan-ogenic reactors, acetate is degraded either by synthrophic acetate oxidizers (dominant process at low acetate concentrations) and acetate-degrading methanogens (acetate concentration above the threshold concentration) or by acetate-utilizing methanogens of the genera Methanosaeta or Methanosarcina (Ahring, 2003). Synthrophic acetate oxidation and methane formation from the cleavage products may explain the lack of acetoclastic methanogens (Methanosarcina sp. or Methanosaeta sp.) in anaerobic reactors.

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