Environmental Engineering Reference
In-Depth Information
the electron donors. On modern Earth, anoxygenic photosynthesis is restricted to environ-
ments such as the oxic-anoxic interface in stratified lakes, where there is both the neces-
sary reduced inorganic compounds (e.g., H
2
or H
2
S) and light. As an example, the
stoichiometry for the photosynthetic sulfur bacteria is:
CO
2
1
2H
2
S
light
! ð
CH
2
O
Þ
1
H
2
O
2S
ð
6
2
Þ
1
1
:
Chemosynthesis
This is another form of CO
2
fixation in which the energy for reducing the CO
2
to organic matter comes not from light but from the oxidation of reduced chemical com-
pounds (e.g., NH
4
,CH
4
,H
2
S, Fe
1
2
) from the environment. Most, but not all, chemosynthe-
sis requires O
2
to oxidize these compounds. Thus, H
2
S is oxidized by O
2
to elemental
sulfur, thiosulfate, or sulfate, and CO
2
is reduced to organic C. There are a few examples
of chemosynthesis in the absence of O
2
, such as Thiobacillus ferrooxidans, which gets energy
under anoxic conditions by oxidizing reduced iron using sulfate as the electron acceptor.
In most cases of chemosynthesis the reduced inorganic compound involved was ulti-
mately produced by photosynthesis, so chemosynthesis is not necessarily independent
from photosynthesis from the point of view of energy. However, some reduced sulfur
and iron compounds are present in minerals and have been produced geothermally.
In deep-sea hot (hydrothermal) vents, entire ecosystems are supported by the oxidation of
geothermal sulfides by chemosynthetic bacteria, some of which are endosymbionts in
invertebrates (
Stewart et al. 2005
). Some examples of chemosynthesis follow. The part of
the equation shown is just the part that generates energy. This energy is used to reduce
CO
2
to carbohydrate in a way similar to the energy generated from photosynthesis; we do
not show the reduction of CO
2
to organic C for simplicity.
Common Name Exemplar Organism Equation
S
o
Sulfide oxidation Thiobacillus
H
2
S
0.5O
2
!
H
2
O
1
1
S
o
Sulfur oxidation Thiobacillus
H
2
SO
4
Nitrification has two parts, ammonium oxidation and nitrite oxidation
1.5 O
2
1
H
2
O
!
1
Ammonium oxidation
NH
4
1
1
NO
2
2
1
2H
1
1
Nitrobacter
1.5 O
2
!
2H
2
O
Nitrite oxidation
NO
2
2
1
NO
3
2
Nitrosomonas
0.5 O
2
!
Oxic Respiration
Oxic respiration is the major energy-generating reaction in aerobic organisms. This is the
most modern form of biological oxidation of organic matter in which O
2
is the oxidant and
organic matter the reductant. Oxic respiration is the form of respiration that we humans
and all metazoans use. CO
2
and H
2
O and energy are the end products. It is oxygenic
