Biomass Degradation in the Presence of Inorganic Electron Acceptors and by an Anaerobic Food Chain

In ecosystems in which molecular oxygen is available, plant and animal biomass is degraded to CO2 and H2O, catalyzed by either single species of aerobic microorganisms or the whole population of the ecosystem, in competition for the substrates. A single organism may be able to hydrolyze the polymers and oxidize the monomers to CO2 and H2O with oxygen. In ecosystems where molecular oxygen is deficient -such as swamps, wet soil, the rumen of animals, the digestive tract of humans, or in river and lake sediments - oxidation of dead biomass proceeds anoxically by reduction of electron acceptors such as nitrate and nitrite or anaerobically by reduction of sulfate, Fe3+, Mn4+, or CO2. The oxidation of the carbon source is either complete or incomplete with acetate excretion. In the absence of inorganic electron acceptors, oxidized metabolites such as pyruvate or acetate are reduced to lactate or ethanol or biotransformed to, e.g., n-butyrate or n-butanol. In permanently anaerobic ecosystems with seasonal overfeeding, periodic accumulation of such metabolites can occur, e.g., in autumn after the non-evergreen plants drop their leaves or decay completely. The biopolymers of the leaves or the plants themselves decompose by extracellular enzymatic hydrolysis. The monomers are fermented, and the fermentation products may be degraded further to biogas by acetogenic and methanogenic bacteria. Whereas single cultures of aerobes can catalyze the whole mineralization process to finally form CO2 and H2O, single cultures of strictly anaerobic bacteria are not capable of degrading biopolymers to CH4 and CO2. Under anaerobic conditions biopolymers must be degraded by a food chain via depolymerization (hydrolysis), fermentation (acidogenesis), oxidation of fatty acids (acetogenesis), and biogas formation (methanogenesis) as the last step (McInerney, 1988). In an initial exoen-zyme-catalyzed reaction the biopolymers are hydrolyzed to soluble mono-, di-, or oligomers. These are taken up by the bacteria and fermented to CO2, H2, formate, acetate, propionate, butyrate, lactate, etc. If fatty acid isomers are produced, they are mainly derived from degradation of amino acids after proteolysis. Fatty acids are further oxidized by acetogenic bacteria, before the cleavage products CO2, H2, and acetate can be taken up by methanogens and be converted to methane and CO2. Lac-tate is oxidized to pyruvate, which is decarboxylated to yield acetate, CO2, and H2. If ethanol is present, it is oxidized to acetate and hydrogen, and the hydrogen is used for CO2 reduction.

Table 1.3 summarizes the reactions that can be catalyzed by methanogens and that can contribute to methane emission in various ecosystems. In sewage digesters about two thirds of the methane is derived from acetate cleavage and one third from CO2 reduction with H2. If hexoses are the substrates and glycolysis is the main degradation pathway, then the 2 mol of pyruvate can be decarboxylated by pyruvate: fer-redoxin oxidoreductase to yield 2 mol acetate and 2 mol CO2. The hydrogens of the 2 mol NADH2 from glycolysis and the 2 mol FdH2 from pyruvate decarboxylation are then released as molecular hydrogen at low H2 partial pressure (Eq. 3). Two

Table 1.3 Reactions catalyzed by methanogens and standard changes in free energy.

Reaction

(kJ per mol of methane)

Substrates (mol)

Products (mol)

Acetate

ch4 + co2

-31.0

4 H2 + CO2

CH4 + 2 H2O

-135.6

4 HCOOH

CH4 + 3 CO2 + 2 H2O

-130.1

4 CO + 2 H2O

CH4 + 3 CO2

-211.0

4 Methanol

3 CH4 + CO2 + 2 H2O

-104.9

Methanol + H2

ch4 + h2o

-112.5

2 Ethanola + CO2

CH4 + 2 acetate

-116.3

4 2-Propanolb + CO2

CH4 + 4 acetone + 2 H2O

-36.5

4 Methylamine + 2 H2O

3 CH4 + CO2 + 4 NH3

-75.0

2 Dimethylamine + 2 H2O

3 CH4 + CO2 + 2 NH3

-73.2

4 Trimethylamine + 6 H2O

9 CH4 + 3 CO2 + 4 NH3

-74.3

2 Dimethylsulfide + 2 H2O

3 CH4 + CO2 + 2 H2S

-73.8

a Other primary alcohols that are used as hydrogen donors for CO2 reduction are 1-propanol and

1-butanol (in a few species). b Other secondary alcohols used as hydrogen donors for CO2 reduction are 2-butanol, 1,3-butanediol, cyclopentanol, and cyclohexanol (in a few species). Compiled from Whitman et al. (1992) and Winter (1984).

a Other primary alcohols that are used as hydrogen donors for CO2 reduction are 1-propanol and

1-butanol (in a few species). b Other secondary alcohols used as hydrogen donors for CO2 reduction are 2-butanol, 1,3-butanediol, cyclopentanol, and cyclohexanol (in a few species). Compiled from Whitman et al. (1992) and Winter (1984).

moles of CH4 are then formed from acetate and 1 mol of CH4 by CO2 reduction (reactions 1 and 2 of Table 1.3).

1 mol glucose ^ 2 mol acetate + 2 mol CO2 + 4 mol H2 (at low pH2) (3)

In complex ecosystems formate is formed if high concentrations of hydrogen accumulate. Syntrophic interactions are usually associated with interspecies hydrogen transfer, but evidence for interspecies formate transfer was also reported (Thiele et al., 1988). The feasibility of the electron carrier depends on its solubility, which is much less for hydrogen than for formate, and on its diffusion coefficient in water, which favors hydrogen 30 times over formate. The efficiency of the appropriate electron transfer depends mainly on the distance between the producing and consuming bacteria. It can be expected, that formate transfer is favored when the distance between communicating bacteria is high and hydrogen transfer when the distance is small (de Bok et al., 2004). Interspecies formate transfer is thought to play a major role in degradation of syntrophic butyrate (Boone et al., 1989) and propionate (Stams, 1994; Schink, 1997). However, at an increased H2 partial pressure formate is also produced by methanogens, either in pure cultures or in a sewage sludge population (Bleicher and Winter, 1994), and this may also contribute to increasing formate concentrations. Other substrates for methanogenic bacteria (Table 1.3), such as methanol (derived, e.g., from methoxy groups of lignin monomers) or methyl-

amines and dimethylsulfide (e.g., from methylsulfonopropionate in algae; Fritsche, 1998) are relevant only in ecosystems where these substances are produced during microbial decay. A few methanogens can also use reduced products such as primary, secondary, and cyclic alcohols as a source of electrons for CO2 reduction (Widdel, 1986; Zellner and Winter, 1987a; Bleicher et al., 1989; Zellner et al., 1989).

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