Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 534

10.7.8. Nitrogen Fixation
Certain microorganisms fix atmospheric nitrogen to form ammonia under reductive or
microaerophilic conditions. Organisms capable of fixing nitrogen under aerobic conditions
include Azotobacter, Azotomonas, Azotococcus, and Biejerinckia. Nitrogen fixation is catalyzed
by the enzyme “nitrogenase,” which is inhibited by oxygen. Typically, these aerobic organ-
isms sequester nitrogenase in compartments that are protected from oxygen.

H þ þ 6 e / 2 N

2 þ 6

NH (10.49)

3

Azotobacter species present in soil provide ammonium for plants by fixing atmospheric
nitrogen and some form associations with plant roots. Some facultative anaerobes such as
Bacillus, Klebsiella, Rhodopseudomonas, and Rhodospirillum fix nitrogen under strict anaerobic
conditions as well as strict anaerobes such as Clostridia can also fix nitrogen under anaerobic
conditions. Certain cyanobacteria, such as Anabaena sp., fix nitrogen under aerobic condi-
tions. The lichens are associations of cyanobacteria and fungi. Cyanobacteria provide nitrogen
to fungi by fixing atmospheric nitrogen. Rhizobium species are heterotrophic organisms
growing in the roots of leguminous plants. Rhizobium fix atmospheric nitrogen under low
oxygen pressure and provide ammonium to plants. Rhizobium and Azospirillum are widely
used for agricultural purposes and are bioprocess products.

10.7.9. Metabolism of Hydrocarbons
The metabolism of aliphatic hydrocarbons is important in some bioprocesses and often
critical in applications such as bioremediation. Such metabolism requires oxygen, and only
few organisms (e.g. Pseudomonas, Mycobacteria, certain yeasts and molds) can metabolize
hydrocarbons. The low solubility of hydrocarbons in water is a barrier to rapid metabolism.
The first step in metabolism of aliphatic hydrocarbons is oxygenation by oxygenases.
Hydrocarbon molecules are converted to an alcohol by incorporation of oxygen into the end
of the carbon skeleton. The alcohol molecule is further oxidized to aldehyde and then to an
organic acid that is further converted to acetyl-CoA, which is metabolized by the TCA cycle.
Oxidation of aromatic hydrocarbons takes place by the action of oxygenases and proceeds
much slower than those of aliphatic hydrocarbons. Cathecol is the key intermediate in this
oxidation sequence and can be further broken down ultimately to acetyl-CoA or TCA cycle
intermediates. Aerobic metabolism of benzene is depicted below:

(10.50)

Anaerobic metabolism of hydrocarbons is more difficult. Only a few organisms can metab-
olize hydrocarbons under anaerobic conditions. They cleave the C e C bonds and saturate with
hydrogen to yield methane.

10.8. OVERVIEW OF BIOSYNTHESIS

The TCA cycle and glycolysis are critical catabolic pathways and also provide important
precursors for the biosynthesis of amino acids, nucleic acids, lipids, and polysaccharides.