Certain bacteria in blue green algae also convert nitrogen into ammonia, a process know
as biological nitrogen fixation.
Biological fixation in contrast to industrial fixation achieves this feat at ambient temperatures
and pressures.
Overall, the biological reaction is nitrogen plus eight electrons from eight protons plus
sixteen ATP yields ammonia plus hydrogen gas plus sixteen ADP plus sixteen phosphates.
Notice that the production of one ammonium expends the equivalent of sixteen ATPs at
a high energy compound found in all organisms.
This is an enormous amount of energy, considering that most other biochemical reactions require
energy expenditures of only one, or at most, two ATPs.
Nitrogen fixing microorganisms are usually free living in the soil.
A few of them, however, form associations with higher plants, in which the microorganisms
directly supplies the host plant with organic nitrogen in exchange for other nutrients and
carbohydrates.
The most common type of association is between members of the pea family and soil bacteria
collectively known as rizobia.
Nitrogen, once fixed into ammonia, dissolves into water to form ammonium.
According to the equation, ammonia plus water equals ammonium plus a hydroxide.
This ammonium enters a biochemical cycle, and passes through several organic or inorganic
forms, before it eventually returns to atmospheric nitrogen.
Amino acids, ammonium and nitrate, which are produced through biological fixation or a
release through decomposition of soil organic matter, become the object of intense competition
among plants and microorganisms.
To remain competitive, plants have developed diverse mechanisms for scavenging these nitrogen
forms as quickly as possible.
These mechanisms range from routine to bizarre, from standard reabsorption to carnivory of
small animals.
Carnivorous plants often take center stage on Broadway and Hollywood, and at the local
plant nursery.
Nearly 600 species scattered throughout the plant kingdom are carnivorous.
Native nitrogen-poor sites, these plants kill small animals that have the misfortune to
cross their path, and digest them to acquire their nitrogen.
For example, sun-dews when fed insects labeled with the heavy isotope of nitrogen, N15, developed
a ratio of N15 nitrogen to N14 nitrogen in their tissues, that indicated that they obtain
between 15 and 87 percent of their nitrogen thru carnivory.
Carniverious plants, although intriguing, are relatively rare.
Most plants, even nitrogen fixing and carniverous ones, acquire the majority of their mineral
nitrogen through reabsorption of ammonium and nitrate from the soil solution.
Once roots absorb ammonium and nitrate from the soil the two forms of inorganic nitrogen
suffer distinct fates within a plant.
High levels of ammonium are toxic to all organisms because it interferes with the biochemical
processes that generate energy within cells.
To warn of dangerous levels of ammonium animals have developed a strong aversion to it's smell.
The active ingredient in smelling salts is ammonium carbonate.
And ammonium endows dirty diapers with their characteristic aroma.
Plants also attempt to prevent high accumulations of ammonium in their tissues.
They assimilate ammonium into amino acids near the site of absorption or generation,
and rapidly place any excess into their vacuoles, large storage compartments within plant cells.
The conversion of ammonium into amino acids in roots entails several reactions, that,
in summary, ammonium + an organic acid + two ATP + an electron, yields amino acids + two
ATP + two phosphate + water.