The Sun, the Earth, and the Evolution of the Earth’s Atmosphere - Air Pollution and Global Warming: History, Science, and Solutions - page 38

Table 2.4. Classification and examples of organisms in terms of their energy and carbon sources

Energy Electron Carbon

source donor source Classification Examples

Sunlight Inorganic

compounds

Carbon dioxide Photolithotrophic

autotroph

Green plants; most algae; most cyanobacteria;
purple, blue, and green sulfur bacteria

Organic
compounds

Photolithotrophic
heterotroph

Chlorella, Chlamydomonas

Organic
compounds

Carbon dioxide Photoorganotrophic

autotroph

Some purple nonsulfur bacteria

Organic
compounds

Photoorganotrophic
heterotroph

Some algae, most purple and green nonsulfur
bacteria, some cyanobacteria

Chemicals Inorganic

compounds

Carbon dioxide Chemolithotrophic

autotroph

Hydrogen, colorless sulfur, methanogenic,
nitrifying, iron bacteria

Organic
compounds

Chemolithotrophic
heterotroph

Some colorless sulfur bacteria

Organic
compounds

Carbon dioxide Chemoorganotrophic

autotroph

Pseudomonas oxalaticus

Organic
compounds

Chemoorganotrophic
heterotroph

Animals, most bacteria, fungi, protozoa

algae, purple and green nonsulfur bacteria, and some
cyanobacteria obtain their energy from sunlight and
their electrons and carbon from organic material, and
thus are photoorganotrophic heterotrophs .
Hydrogen, colorless sulfur, methanogenic, nitrify-
ing, and iron bacteria obtain their carbon from carbon
dioxide and their energy and electrons from inorganic
material, and thus are chemolithotrophic autotrophs .
Animals, most bacteria, all fungi, and all protozoa
obtain their energy, electrons, and carbon from organic
material, and thus are all chemoorganotrophic het-
erotrophs .Many early prokaryotes were chemoorgan-
otrophic heterotrophs because they obtained their
energy, electrons, and carbon from organic molecules
produced during abiotic synthesis.

which is exothermic (energy releasing). The energy
source, glucose in this case, is only partially oxidized
to carbon dioxide; thus, the reaction is inefficient. This
reaction is similar to that which occurs during the pro-
duction of bread, whereby fermenting fungi in yeast eat
sugar added to the bread, producing ethanol and carbon
dioxide. The ethanol is burned off in the oven or fire,
and the carbon dioxide causes the bread to rise.

2.3.3.2. Early Methane
About 2.9 b.y.a., a new source of methane gas in
the Earth's early atmosphere was metabolism by a
new strain of bacteria called methanogenic bacteria .
Such bacteria obtain their carbon from carbon dioxide
and their energy and electrons from molecular hydro-
gen; thus, methanogenic bacteria are chemolithotrophic
autotrophs. Their methane-producing reaction is

2.3.3.1. Early Carbon Dioxide
The first major energy-producing process carried out
by newly evolved prokaryotic bacteria was fermenta-
tion .This process developed in chemoorganotrophic
heterotrophic organisms and produced carbon dioxide
gas. One fermentation reaction is

4H 2 (g) + CO 2 (g) → CH 4 (g) + 2H 2 O(aq) (2.4)

Molecular Carbon Methane Liquid

hydrogen dioxide water

Methanogenic bacteria use about 90 to 95 percent of
carbon dioxide available to them for this process. The
rest is used for synthesis of cell carbon. In Reaction
2.4, CO 2 (g) is reduced to CH 4 (g). A reaction such

C 6 H 12 O 6 (aq)
Glucose

→ 2C 2 H 5 OH(aq)

Ethanol

+ 2CO 2 (g)

Carbon
dioxide

(2.3)