Anaerobic Degradation of Carbon

In this reaction, the electron donor is hydrogen, and carbon dioxide serves as the electron acceptor. Methane can also be generated from other methyl compounds, including methanol and acetate. The methanogens, such as METHANOBACTERIUM, are strict anaerobes.

Generally, when oxygen is not used as the electron acceptor in respiration, it is likely that the organism:

a. is living in an environment where oxygen is not available,

b. or cannot live in an environment containing oxygen.

Since more energy can be extracted when oxygen is used as the final electron acceptor in respiration, organisms that do not use oxygen could not compete with others that do.

Anaerobic degradation of carbon is strictly done by microorganisms. This is responsible for most of the biological CO₂ and CH₄ released to the atmosphere.

1. Anaerobic respiration involves the complete oxidation of organic substances:

(CH₂O)n + X_ox -----> CO₂ + X_red

Alternative electron acceptors, denoted "X_ox", commonly used are nitrate (NO₃^-), sulfate (SO₄^2-), elemental sulfur (S^o), or ferric iron (Fe³⁺) ions.

2. Anaerobic decomposition of organic substances to carbon dioxide and methane is a collaborative effort involving many different reactions and species of microorganisms. This is also called interspecies hydrogen transfer. These reactions occur in the gut (to a limited extent), sediments, soils, and the rumen.

a. Complex polymers (e.g. cellulose, starch, proteins) are broken down to monomers by fungi and by cellulolytic bacteria such as Bacteroides succinogenes.

b. The monomeric subunits (e.g. sugars, amino acids) thus produced are broken down by enterics and other fermentative bacteria such as Clostridium butyrium, producing organic acids [e.g. butyrate (i.e. CH₃CH₂CH₂COO^-), propionate (i.e. CH₃CH₂COO^-], alcohols, etc.

c. Further fermentations are done by the syntrophic bacteria Syntrophomonas sp. and Syntrophobacter sp. that produce acetate (i.e. CH₃COO^-), carbon dioxide (i.e. CO₂), and molecular hydrogen (i.e. H₂).

d. The carbon dioxide, molecular hydrogen, and acetate are used by methanogens to produce methane (i.e. CH₄).

3. Some important points.

a. Suppose we stop after the reactions of the fermentative bacteria? Organic acids and alcohols would accummulate. The organisms would drown in their own wastes. Many would stop growing or die.

b. What is the rate-limiting step? The rate limiting step in the anaerobic decomposition of carbon is the fermentation of organic acids by the syntrophs. These reactions are thermodynamically unfavorable. They are driven by removal of the hydrogen and acetate products by the methanogens using reactions that are thermodynamically favorable.

c. What do the syntrophs do for the:

(i) Fermentative bacteria? Syntrophs remove the wastes of the fermentative bacteria.

(ii) Methanogens? Syntrophs feed the methanogens.

d. Where do the methanogens get:

(i) Their reducing power? From the H₂ produced primarily by the syntrophs and to a lesser extent by the fermentative bacteria.

(ii) Their carbon? From the CO₂ and acetate produced primarily by the syntrophs and to a lesser extent by the homoacetogens and the fermentative bacteria.

In brief, although these degradative reactions occur in groups of different organisms, they are tightly interrelated in that the waste of one group of bacteria becomes the food for another. Consequently, if any single reaction in the path is deficient or overefficient, many organisms starve to death or are killed by the accumulated wastes.

e. Energetics of syntrophy

(i) 2 butyrate + 4 H2O ----------> 4 acetate + 4 H2 + 2H+ (ii) 4 acetate + 4 H2O ----------> 4 CH4 + 4 HCO3- (iii) 4 H2 + HCO3- + H+ ----------> CH4 + 3 H2O	Delta Gi = 2 x (+48.2 kJ) = 96.4 kJ Delta Gii = 4 x (-31 kJ) = -124 kJ Delta Giii = 1 x (-136 kJ) = -136 kJ
Net reaction: 2 butyrate + 5 H2O ----------> 5 CH4 + 3 HCO3- + H+	Delta Gtotal = -163.6 kJ

Reaction (i) is a thermodynamically unfavorable fermentation done by the syntrophs. The products of this reaction are used by the methanogens as substrates in the favorable reactions (ii) and (iii). The overall free energy change for this series of reactions is negative, indicating that the net conversion of butyrate to methane and bicarbonate is favorable. Note the energetic coupling between the favorable reactions of the methanogens and the unfavorable reaction of the syntrophs.

a. Methane is oxidized, i.e. used as the electron donor. Oxygen serves as the electron acceptor; it is reduced to water. Other substrates may be oxidized by some methylotrophs, of which the methanotrophs are a subset.

b. Where do the methanotrophs live? The problem is that the methanotrophs need the "waste" methane produced by the methanogens as well as oxygen -- but the methanogens are strictly anaerobic. The solution is to live at the "boundaries" of oxic and anoxic environments, where both methane and oxygen are obtainable.

c. The diagnostic enzyme that indicates the presence of a methanotroph is methane monooxygenase; this enzyme is used in the first step of the oxidation of methane wherein methanol is produced.

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