Geoscience Reference
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as this, in which cells produce energy by breaking
down compounds in the absence of molecular oxygen
is an
anaerobic respiration
reaction. Anaerobic (“in
the absence of oxygen”) respiration produces energy
more efficiently than does fermentation. Even today,
methanogenic bacteria are anaerobic, generally living
in the intestines of cows, under rice paddies, under land-
fills, and in termite mounds.
In Earth's early atmosphere, methane may have had
an
e
-folding lifetime as long as 10,000 years because
its only loss was photolysis at very short wavelengths.
Today, its lifetime is about 10 years, due to its reaction
with the hydroxyl radical [OH(g)], which was hardly
present in Earth's preoxygen atmosphere. The buildup
of OH(g) following the buildup of oxygen caused a
sudden decrease in atmospheric CH
4
(g), as described
later in the chapter.
Denitrification took over as the main source of molec-
ular nitrogen in the air following the advent of oxygen
and continues to be the main source today.
2.3.3.4. Anoxygenic Photosynthesis
Most early organisms on Earth relied on the conver-
sion of organic or inorganic material to obtain their
energy. During the microbial era, such organisms most
likely lived underground or in water to avoid exposure to
harmful UV radiation hitting the Earth's surface. How-
ever, about 3.5 b.y.a., certain bacteria developed the
ability to obtain their energy from sunlight by a new
process,
photosynthesis
.The first photosynthesizing
bacteria were sulfur cyanobacteria (blue-green algae
or blue-green bacteria), which are photolithotrophic
autotrophs (Table 2.4). A photosynthetic reaction by
blue, green, yellow, or purple sulfur cyanobacteria is
2.3.3.3. Early Molecular Nitrogen
When ammonia accumulated sufficiently in the air due
to outgassing c. 4 b.y.a., it was photolyzed by ultraviolet
sunlight to provide the major source of atmospheric
molecular nitrogen
at the time, by
CO
2
(g)
Carbon
dioxide
+
2H
2
S(g)
Hydrogen
sulfide
+
h
→
CH
2
O(aq)
Carbo
−
hydrate
2 S(g)
Atomic
sulfur
+
H
2
O(aq)
Liquid
water
+
(2.9)
.
(g)
3 H(g)
NH
3
(g)
+
h
→·
+
(2.5)
where CH
2
O(aq) represents a generic carbohydrate dis-
solved in water. Because reduced compounds [e.g.,
H
2
S(g)] were not omnipresent, such bacteria flour-
ished only in limited environments. Early
sulfur-
producing photosynthesis
did not result in the pro-
duction of oxygen; thus, it is referred to as
anoxygenic
photosynthesis
.
Ammonia
Atomic
Atomic
nitrogen
hydrogen
M
→
.
(g)
.
(g)
·
+
N
2
(g)
(2.6)
Atomic
Molecular
nitrogen
nitrogen
However, once atmospheric oxygen levels began to
rise (2.45-0.4 b.y.a.), nitrogen production by ammonia
photolysis became obsolete because oxygen absorbs
the sun's ultraviolet wavelengths capable of pho-
tolyzing ammonia. Long before that, however, about
3.2 b.y.a., some anaerobic chemoorganotrophic het-
erotrophs developed a new mechanism of producing
molecular nitrogen. In this two-step process called
den-
itrification
, one set of denitrifying bacteria reduce the
nitrate ion
(NO
3
−
)tothe
nitrite ion
(NO
2
−
), and
another set reduce the nitrite ion to molecular nitrogen:
2.3.4. The Oxygen Age
Oxygen-producing photosynthesis
may have devel-
oped shortly after anoxygenic photosynthesis, around
3.5 to 2.8 b.y.a. However, until the onset of green
plants [395 to 430 million years ago (m.y.a.)], oxygen-
producing photosynthesis was carried out primarily by
cyanobacteria (Figure 2.9). Prior to 2.45 b.y.a., nearly all
the oxygen produced by photosynthesis was removed
before it could accumulate in the air. The main removal
mechanism was chemical reaction of oxygen with iron
in rocks, producing
iron oxide
[Fe
2
O
3
(s)] in a process
similar to rusting. Most of the oxidation occurred in
ocean water, where dissolved oxygen reacted with the
iron in sediments. Oxygen also reacted with sulfur to
produce the sulfate ion (SO
4
2
−
), which combined fur-
ther to form rock material, such as anhydrite, CaSO
4
(s),
and gypsum, CaSO
4
-2H
2
O(s). Today, about 58 percent
NO
3
−
Nitrate
ion
NO
2
−
Nitrite
ion
Organic compound
+
→
CO
2
(g)
Carbon
dioxide
+
+··
(2.7)
NO
2
−
Nitrite
ion
Organic compound
+
→
CO
2
(g)
Carbon
dioxide
+
N
2
(g)
Molecular
nitrogen
+··
(2.8)
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