ABSTRACT
The evolution of the Earth’s atmosphere from anaerobic to aerobic resulted in the creation of the following:
• Anaerobic microorganisms (living without oxygen) • Facultative anaerobic microorganisms (living under either anaerobic or
aerobic conditions) • Microaerophilic microorganisms (living at low concentrations of dissolved
oxygen) • Obligate aerobic microorganisms (living only in the presence of oxygen)
Anaerobes produce energy from the following:
• Fermentation (destruction of organic substances without an external acceptor of electrons)
• Anaerobic respiration using electron acceptors such as −NO3, −NO2, Fe3+,
• Anoxygenic (H2S → S) photosynthesis
Microaerophiles and aerobes produce energy from the following:
• Aerobic oxidation of organic matter • Oxygenic photosynthesis
The sequence of increasing production of biological energy per mole of transferred electrons is as follows: fermentation → CO2 respiration (hydrogenotrophic methanogenesis) → dissimilative sulfate reduction → dissimilative iron reduction (iron respiration) → nitrate respiration (denitrification) → aerobic respiration.
