Page 181 - Algae

164 Algae: Anatomy, Biochemistry, and Biotechnology

fill with decaying and partially decomposed plant materials to make a swamp, which is the natural
aging process. The problem is that this process can been significantly accelerated by man’s
activities.
Phosphates were once commonly used in laundry detergents, which contributed to excessive
concentrations in rivers, lakes, and streams. Most detergents no longer contain phosphorus.
Currently, the predominant outside sources of phosphorus are agricultural and lawn fertilizers
and improperly disposed animal wastes.

ALGAE AND THE NITROGEN CYCLE
The growth of all organisms depends on the availability of mineral nutrients, and none is more
important than nitrogen, which is required in large amounts as an essential component of peptides,
proteins, enzymes, chlorophylls, energy-transfer molecules (ATP, ADP), genetic materials (RNA,
DNA), and other cellular constituents.
Nitrogen is present in all the four different spheres of the Earth: the lithosphere contains about
17
98% of the global N (1.7 10 tons), distributed among its different compartments (soils and sedi-
ments of the crust, mantle, and core). The core and the mantle have been estimated to contain a total
17
of over 1.6 10 tons of N. However, this N is not readily available to be cycled in the near-
surface Earth environment. Some periodically enters the atmosphere and hydrosphere through
volcanic eruptions, primarily as ammonia (NH 3 ) and nitrogen (N 2 ) gas. Most of the remainder
( 2%, 4 10 15 tons) is found in the atmosphere, where nitrogen gas (N 2 ) comprises more than
78% of the volume. The hydrosphere and the biosphere together contain relatively little N
compared with the other spheres ( 0.015%, 3 10 12 tons).
Nitrogen has many chemical forms, both organic and inorganic, in the atmosphere, biosphere,
hydrosphere, and lithosphere. It occurs in the gas, liquid (dissolved in water), and solid phases. N
can be associated with carbon (organic species) and with elements other than carbon (inorganic
2
2
species). Important inorganic species include nitrate (NO 3 ), nitrite (NO 2 ), nitric acid (HNO 3 ),
þ
ammonium (NH 4 ), ammonia (NH 3 ), the gas N 2 , nitrous oxide (N 2 O), nitric oxide (NO), and nitro-
gen dioxide (NO 2 ). Most organic N species in the four spheres are biomolecules, such as proteins,
peptides, enzymes, and genetic materials. The presence of these many chemical forms make the N
cycle more complex with respect to the cycle of other nutrients. The key processes linking the major
pathways of the nitrogen cycle are the following:

. N-fixation, that is, reduction of atmospheric N 2 into ammonia NH 3
2
þ
. Assimilation, that is, conversion of NO 3 and NH 4 to organic nitrogen
þ
. Mineralization or ammonification, that is, conversion of organic nitrogen to NH 4
þ 2 2
. Nitrification, that is, conversion of NH 4 to NO 2 and successively NO 3
2
. Denitrification, that is, conversion of NO 3 to gaseous forms of nitrogen (NO, N 2 O, N 2 )
Though complex microbial relationships regulate these processes, we can assume that fixation,
mineralization, nitrification, and denitrification are carried out almost exclusively by bacteria,
whereas algae play a main active role only in nitrogen fixation and assimilation. Greatly simplifying
the overall nitrogen cycle, and from an algal point of view, atmospheric molecular nitrogen is con-
verted by prokaryotic algae (cyanobacteria) to compounds such as ammonia (fixation), which are in
part directly converted into amino acids, proteins, and other nitrogen-containing cell constituents of
the fixators, and in part excreted into the open environment. Eukaryotic algae, unable to perform
fixation, incorporate fixed nitrogen, either ammonium or nitrate, into organic N compounds by
assimilation. When organic matter is degraded, organic compounds are broken down into inorganic
þ
compounds such as NH 3 or NH 4 and CO 2 through the mineralization process. The resultant
ammonium can be nitrified by aerobic chemoautotrophic bacteria that use it as electron donor in
the respiration process. The cycle is completed by denitrification carried out usually by facultative