Carbon - Freshwater Ecology: Concepts and Environmental Applications - page 238

TABLE 12.1 Representative Fermentative, Methanogenic, and Acetogenic Transformations That
Occur in Anoxic Communities a

Reaction Name Comment

Fermentation
Glucose → 2 ethanol 2 CO 2 Ethanol fermentation Formation of alcohol

2 H Glucose → 2 lactate Lactate fermentation

Glucose → ethanol acetate CO 2 H 2 Mixed acid fermentation Produces variable amounts of

products

Glucose → butyrate 2 CO 2 2 H 2 Butyrate fermentation

3 lactate → 2 propionate acetate CO 2 Propionate fermentation Gives Swiss cheese flavor

Alanine 2 glycine → 3 acetate Paired amino acid Important when proteins

3 NH 3 CO 2 fermentation being broken down

Acetogenesis
2 CO 2 8 H → acetate 2 H 2 O Heterotrophic acetogenesis

2 CO 2 4 H 2 → acetate 2 H 2 O Autotrophic acetogenesis

Methanogenesis
CO 2 4 H 2 → CH 4 2 H 2 O Autotrophic methanogenesis Uses CO 2 and H 2 as a source

of energy

Acetate → CH 4 CO 2 Acetoclastic methanogenesis Disproportionation of acetate

a Acetogenic and methanogenic processes require very low redox (Fig. 11.6). The listed processes represent a
small proportion of the fermentative processes that can occur in anoxic communities.

acidic organic acids that are inhibitory to further microbial activity. The
lowered pH encourages precipitation of the humic compounds. As these
build up in the sediments, further degradation slows. For the most part,
humic substances are difficult for microbes to degrade, but these can be
utilized slowly by some microbes (Lovely et al., 1996). Anoxic conditions
lead to slower breakdown of organic materials because of reduced effi-
ciency of carbon oxidation and inhibition of microbial activity by meta-
bolic by-products. The accumulation of organic compounds under anoxic
and acidic conditions has led to formation of peat bogs. These wetlands
are sites of organic accumulation, and meters of material can accumulate
over many years. Such bogs are distributed worldwide, notably in boreal
regions of the Northern Hemisphere. Similar processes lead to the forma-
tion of blackwater swamps. The humic compounds in blackwater swamps
cause the water to appear brown or black in color, and the acidity allows
buildup of organic carbon. Peat bogs are important in archeology and pa-
leolimnology (Sidebar 12.2).

Methanotrophy
Methane and carbon monoxide are commonly produced in fermenta-
tion reactions. These compounds are simple but energetically inefficient for
most microorganisms to metabolize. Some specialized aerobic bacteria, the
methylotrophs, can harvest energy by oxidizing simple compounds con-
taining methyl groups and carbon monoxide. Bacteria that specifically ox-
idize methane are referred to as methanotrophs . These bacteria are central
to the global carbon cycle because their activity is a major reason that
methane does not build to large concentrations in the earth's atmosphere.