Page 182 - Algae

Biogeochemical Role of Algae 165

anaerobic bacteria that reduce nitrate used as electron acceptor in respiration to nitrogen gas. We
must stress that a realistic depiction of the N cycle would have an almost infinite number of inter-
mediary steps along the circumference of a circle, connected by a spider web of internal, crisscross-
ing complex connections.
It is one of nature’s great ironies that though all life forms require nitrogen compounds the most
abundant portion of it (98%) is buried in the rocks, therefore deep and unavailable, and the rest of
nitrogen, the N 2 gas (2%), can be utilized only by very few organisms. This gas cannot be used by
most organisms because the triple bond between the two nitrogen atoms makes the molecule almost
inert. In order for N 2 to be used for growth this gas must be “fixed” in the forms directly accessible
to most organisms, that is, ammonia and nitrate ions.
A relatively small amount of fixed nitrogen is produced by atmospheric fixation (5–8%) by
means of the high temperature and pressure associated with lightning. The enormous energy of
this phenomenon breaks nitrogen molecules and enables their atoms to combine with oxygen in
the air forming nitrogen oxides. These molecules dissolve in rain, forming nitrates that are
carried to the Earth. Another relatively small amount of fixed nitrogen is produced industrially
(industrial fixation) by the Haber-Bosch process, in which atmospheric nitrogen and hydrogen
(usually derived from natural gas or petroleum) can be combined to form ammonia (NH 3 ) using
an iron-based catalyst, very high pressures and a temperature of about 6008C. Ammonia can be
used directly as fertilizer, but most of it is further processed to urea (NH 2 ) 2 CO and ammonium
nitrate (NH 4 NO 3 ).
The major conversion of N 2 into ammonium, and then into proteins, is a biotic process achieved
by microorganisms, which represents one of the most metabolically expensive processes in biology.
Biological nitrogen fixation can be represented by the following equation, in which two moles
of ammonia (but in solution ammonia exists only as ammonium ion, that is, NH 3 þ
2
þ
H 2 O $ NH 4 þ OH ) are produced from one mole of nitrogen gas, at the expense of 16 moles
of ATP and a supply of electrons and protons (hydrogen ions):
þ
N 2 þ 8H þ 8e þ 16ATP ! 2NH 3 þ H 2 þ 16ADP þ 16P i (4:2)

All known nitrogen-fixing organisms (diazotrophs) are prokaryotes, and the ability to fix nitrogen
is widely, though paraphyletically, distributed across both the bacterial and archaeal domains. In
cyanobacteria nitrogen-fixation is an inducible process, triggered by low environmental levels of
fixed nitrogen.
The capacity of nitrogen fixation in diazotrophs relies solely upon an ATP-hydrolyzing, redox
active enzyme complex termed nitrogenase. In many of these organisms nitrogenase comprises
about 10% of total cellular proteins and consists of two highly conserved components, an iron
protein (Fe-protein) and a molybdenum-iron (MoFe-protein). The Fe-protein is a g 2 homodimer
composed of a single Fe 4 S 4 cluster bound between identical 32–40 kDa subunits. The Fe 4 S 4
cluster is redox-active and is similar to those found in small molecular weight electron carrier
proteins such as ferredoxins. It is the only known active agent capable of obtaining more than
two oxidative states and transfers electrons to the MoFe-protein. The MoFe-protein is a a 2 b 2
heterotetramer; the ensemble is approximately 250 kDa. The a subunit contains the active site
for dinitrogen reduction, typically a MoFe 7 S 9 metal cluster (termed FeMo-cofactor), although
some organisms contain nitrogenases wherein Mo is replaced by either Fe or V. These so-called
alternative nitrogenases are found only in a limited subset of diazotrophs and, in all cases
studied so far, are present secondary to the MoFe-nitrogenase. The MoFe-nitrogenase has been
found to be more specific for and more efficient in binding N 2 and reducing it to ammonia
than either of the alternative nitrogenases. The catalytic efficiency of these alternative nitrogenases
is lower than that of the MoFe-nitrogenase. In addition to variations in metal cofactors, the nitro-
genase complex is non-specific and reduces triple and double bond molecules other than N 2 . These