There, it can be taken up by other autotrophs and used again. This
cycle will occur over and over until at last the phosphorus is lost at the bottom of the deepest
parts of the ocean, where it becomes part of the sedimentary rocks forming there. If the rock is
brought to the surface and weathered, this phosphorus will be released. During the natural
process of weathering, the rocks gradually release the phosphorus as phosphate ions, which are
soluble in water and the mineralized phosphate compounds breakdown. Phosphates PO432
formed from this element. Phosphates exist in three forms: orthophosphate, metaphosphate (or
polyphosphate), and organically bound phosphate, each compound containing phosphorus in a
different chemical arrangement. These forms of phosphate occur in living and decaying plant
and animal remains, as free ions or weakly chemically bounded in aqueous systems, chemically
bounded to sediments and soils, or as mineralized compounds in soil, rocks, and sediments.
Orthophosphate forms are produced by natural processes, but major man-influenced sources
include: partially treated and untreated sewage, runoff from agricultural sites, and application of
some lawn fertilizers. Orthophosphate is readily available to the biological community and typically
found in very low concentrations in unpolluted waters. Polyforms are used for treating
boiler waters and in detergents. In water, they are transformed into orthophosphate and become
available for autotrophs uptake. The organic phosphate is the phosphate that is bound or tied up
in autotrophs, waste solids, or other organic materials. After decomposition, this phosphate can
be converted to orthophosphate.
Algae and plants are the key elements to passing on phosphates to other living organisms, but
their importance in phosphorus cycle is connected mainly to the impact of this element on their
growth. As already remarked, both phosphorus and nitrogen are among the nutrients that can
become limiting, hence an overloading of these two elements leads to dramatic changes in the structure
and functioning of an ecosystem.
Phosphorus, in the form of orthophosphate, is generally considered the main limiting nutrient in
freshwater aquatic systems; that is, if all the phosphorus is used, autotroph growth will cease, no
matter how much nitrogen is available. In phosphorus limited systems, excess phosphorus will
trigger eutrophic condition. In these situations the natural cycle of the nutrient becomes overwhelmed
by excessive inputs, which appear to cause an imbalance in the “production versus consumption”
of living material (biomass) in an ecosystem. The system then reacts by producing more
phytoplankton/vegetation than can be consumed by the ecosystem. This overproduction triggers
the series of events determining the aging process of the water body.
Under aerobic conditions, as water plants and algae begin to grow more rapidly than normal,
there is also an excess die off of the plants and algae as sunlight is blocked at lower levels. Bacteria
try to decompose the organic waste, consuming the oxygen and releasing more phosphate, which is
known as “recycling or internal cycling.” Some of the phosphates may be precipitated as iron phosphate
and stored in the sediment where it can then be released if anoxic conditions develop. In
deeper environments, the phosphate may be stored in the sediments and then recycled through
the natural process of lithotrophication, uplift, and erosion of rock formations. In anaerobic conditions,
as conditions worsen as more phosphates and nitrates may be added to the water, all of
the oxygen may be used up by bacteria in trying to decompose all of the waste. Different bacteria
continue to carry on decomposition reactions; however, the products are drastically different. The
carbon is converted to methane gas instead of CO2
; sulfur is converted to hydrogen sulfide gas.
Some of the sulfide may be precipitated as iron sulfide. Under anaerobic conditions the iron phosphate
precipitates in the sediments may be released from the sediments making the phosphate bioavailable.
This is a key component of the growth and decay cycle. The water body may gradually
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
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.