Carbon - Freshwater Ecology: Concepts and Environmental Applications - page 236

Proteins Carbohydrates Fats

Hydrolysis Hydrolysis

Amino acids Monosaccharides Glycerol | Fatty acids

NH 3

Glycolysis

C 1
compounds

Pyruvate

Glutamate Acetyl-CoA

CO 2 and H 2 O

TCA cycle and respiratory chain

FIGURE 12.3 A general diagram of aerobic breakdown of organic carbon by organisms
(modified from Rheinheimer, 1991).

yield, leading to areas with progressively lower redox (Fig. 12.4). Thus,
NO 3 is used first, followed by Mn 4 , Fe 3 , and SO 4 2 . All these oxida-
tions are more efficient than acetogenesis and methanogenesis, which will
be discussed in the following section.

Fermentation
In addition to oxidizing organic compounds with inorganic electron
acceptors, heterotrophic organisms in anoxic environments can utilize or-
ganic carbon by fermentation, or rearranging the organic molecules to
yield more simple organic and inorganic compounds (e.g., acetate, ethanol,
CH 4 , CO 2 , H 2 , and H 2 O) and energy. A wide variety of these reactions oc-
cur; examples are presented in Table 12.1. Many of these reactions are of
enormous commercial benefit (e.g., fermentation of alcohol), but they are
also central to the carbon flux of anoxic aquatic habits.
A general feature of these fermentation processes is that many yield or-
ganic acids. These acids lower pH and generally decrease the rates of fur-
ther degradation. No individual species of fermenter is able to metabolize
organic polymers (such as cellulose, proteins, and lipids) completely to
CO 2 and H 2 . In contrast, individual species are able to degrade polymers
to CO 2 and H 2 O in the presence of O 2 (Fenchel and Finlay, 1995). Thus,
complex communities of “syntrophic” microorganisms are required to
continue energy cycling in anoxic systems. Individual species from these
complex groups of heterotrophic anoxic microbes cannot grow in isolation
without a supply of very specific metabolic substrates, so they tend to
“cooperate” to break down organic materials.
As complex organic compounds produced by terrestrial plants are de-
graded in the absence of O 2 , humic compounds are formed. The degradation
of the organic compounds leads to lowered pH and creation of phenolic and