Environmental Engineering Reference
In-Depth Information
photosynthesis in reverse. Oxic respiration did not occur until about 2 billion YBP when sig-
nificant oxygen had accumulated in the atmosphere, following the evolution of oxygenic
photosynthesis (
Kennedy et al. 2006
). The basic stoichiometry for oxic respiration is:
C
6
H
12
O
6
6O
2
!
6CO
2
6H
2
O
energy output
ð
6
3
Þ
1
1
1
:
Anaerobic Respiration
This is the oxidation of organic matter using something other than O
2
as the electron
acceptor. With the exception of fermentations and some reports of the use of nitrate as an
electron acceptor in protozoa and diatoms (see
Kamp et al. 2011; Pina-Ochoa et al. 2010
),
these pathways are restricted to prokaryotes. Anaerobic respiration is extremely common
in anoxic environments such as wet soils, sediments, and stratified water bodies. About
half of all modern decomposition goes through anaerobic respiration (
Howarth and Teal
1991
). The most important electron acceptors are SO
4
,NO
3
, and Fe
1
3
. The end products
are inorganic C (HCO
3
2
or CO
2
), energy, and more reduced forms of the electron acceptor
(e.g., sulfide [or elemental S or thiosulfate], nitrite [or nitrous oxide or N
2
], or Fe
2
).
Common Name
Exemplar Organism
Equation
4NO
3
2
1
4H
1
!
Denitrification
Pseudomonas, Bacillus, many others 5[CH
2
O]
5CO
2
1
2N
2
1
7H
2
O
1
CH
3
COO
2
1
2H
1
1
SO
2
2
!
HS
2
Sulfate reduction
Desulfovibrio
2CO
2
1
2H
2
O
1
Methanogensis
(1) acetoclastic
Methanosarcina
CH
3
COOH
!
CO
2
1
CH
4
H
2
/CO
2
Methanococcus
4H
2
1
CO
2
!
CH
4
1
2H
2
O
(2) Glucose fermentation Many yeasts, some bacteria
C
6
H
12
O
6
!
2CH
3
CH
2
OH
2CO
2
1
Fermentative decomposition is familiar in the making of cheese and wine, but it is also
a key decomposition pathway. It is like anaerobic respiration in that oxygen is not
required. Unlike aerobic or anaerobic respiration, there is no external electron acceptor.
Rather, an organic molecule is split such that one portion becomes more oxidized and one
portion more reduced. There are a number of different biochemical variations of the fer-
mentation process using different organic substrates and producing different organic end
products. Glycolysis is a common type of fermentation in which glucose is fermented to
pyruvate and protons. Glycolysis occurs in the cells of most higher organisms (including
yeasts and humans) as a precursor to both aerobic and anaerobic respiration.
Methane (CH
4
) is most often formed by the fermentative oxidation of acetic acid into
CH
4
(more reduced) and CO
2
(more oxidized). This is the acetoclastic (e.g., formation of
methane from the fermentation of acetate) pathway mentioned previously. Methane can
also be formed by the reduction of CO
2
using H
2
as the electron donor (called H
2
/CO
2
methanogenesis previously). This latter type of methanogenesis fits the description of
anaerobic respiration rather than a fermentation. Methanogenesis is restricted to a narrow
taxonomic group of bacteria but is a very common process in anaerobic environments,
particularly in freshwaters and wetlands.
