PHOSPHORUS CYCLE: CHARACTERISTICS, STAGES, ALTERATIONS, IMPORTANCE - BIOLOGY - 2025

The phosphorus cycle is the storage and circulation of this element through the hydrosphere, lithosphere, living organisms, and the atmosphere. It is a sedimentary-type biogeochemical cycle whose storage phase occurs mainly on the seabed.

The cycle begins with the exposure of phosphate rocks to the erosive action of water, wind and living organisms. When the rock wears, it fragments and carries the particles that carry the phosphates, which are incorporated into the soil or are dragged into bodies of water.

Complete phosphorus cycle. Source: BonniemfIncorporates work by NASA Earth Science Enterprise / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Phosphorus absorbed as phosphates by plants through their roots is integrated into your body and used in metabolism. Thus, it passes from the geological stage of the cycle to the biological phase where it circulates through food or trophic webs.

This stage begins when herbivorous animals consume plants and obtain phosphorus from them. This element then passes to the carnivores that feed on the herbivores and returns to the soil through excrement or when the organisms die and decompose.

On the other hand, phosphorus in the form of phosphates is drawn into lakes and oceans, passing to its hydrological stage. In addition, phosphates dissolved in water pass to the biological stage when they are absorbed by phytoplankton and enter marine food webs.

Subsequently, phosphorus is released through excreta or decomposition of living beings and is integrated again into the hydrological stage. In this phase, it can circulate with ocean currents or settle on the sediments of the ocean floor.

When phosphorus goes to the seabed, the sediment layers accumulate and the lower layers end up buried at great depths. Here high pressures and temperatures are produced that form new rock rich in phosphorus that will be exposed again to continue the cycle.

This cycle can be altered by human activity due to the incorporation of extra amounts of phosphorus polluting the environment that cause eutrophication.

characteristics

The match

This is a non-metallic chemical element that is represented by the symbol P and which is not pure in nature because it oxidizes quickly. When this process occurs, it releases heat energy and produces light, which is why it was given the name of phosphor ("carrier of light" in Greek).

In nature it is found in the form of inorganic phosphorus molecules or as part of living organisms.

Your cycle

The phosphorus cycle is a sedimentary biogeochemical cycle closely related to the cycle of water, carbon, calcium, iron and aluminum. It is called sedimentary because most of its reserves are in marine sediments and in the phosphate rocks of the earth's crust.

Exposition

The phosphate rocks that are formed in the depths of the Earth, are carried to the earth's surface by the movements of the tectonic plates. When this happens, they are exposed to the action of physical agents such as rain and wind, as well as biological ones.

Phosphate rock. Source: David Stanley from Nanaimo, Canada / CC BY (https://creativecommons.org/licenses/by/2.0)

Another way of releasing phosphorus from its underground storage is through volcanic activity, also caused by tectonic movements.

Erosion

As phosphate rocks are exposed to the external environment on the earth's surface, the process of erosion occurs. Thus, the rock is cracked and fragmented by differences in temperature, rain, wind and the action of living beings, and phosphorus becomes part of the soil or moves to other sites.

Transport

The main erosive agent is water, which transfers phosphorus to other points in the lithosphere and finally to the oceans.

Biological absorption and release

Most of the phosphorus in the soil is in an insoluble form and therefore cannot be used by plants. This phosphor is adhered to clay particles, iron and aluminum hydroxides or in the form of calcium, ferric or other phosphate.

Thus, in one hectare of soil there may be between 2 and 10 tons of phosphorus, but plants can only use 3 to 15 kg. Soluble phosphorus is absorbed through the roots and passes into the body of the plant where it is used in various metabolic processes.

Phosphorus is integrated into the structure of plants, forming proteins, ATP, DNA and other molecules. Similarly, when it dissolves in seawater, it is absorbed by phytoplankton in the oceans.

Phosphorus becomes part of the trophic chains when plants and phytoplankton are consumed by herbivores and these by carnivores. Later, it is released from living organisms either through their excreta or when they die.

Sedimentation and accumulation

Phosphorus in the oceans is deposited on the seabed, becoming part of the sediment that will be deposited in successive layers.

Lithification and storage

Rocks are formed when sediments in the deepest parts of the earth's crust are subjected to high pressures and temperatures (lithification). This occurs because the weight of the upper sediments causes the lower ones to compact.

The particles that make up the rocks are held together thanks to the crystallization of oxides, silica and other substances, a process known as cementation. In this way, the so-called sedimentary rocks are formed, among which is phosphorite, which contains up to 20-30% phosphates.

If sedimentary rocks are subjected to processes of higher temperature and pressure, they dissolve, forming part of metamorphic and igneous rocks (18%). There are also phosphates in limestone rocks up to 0.18% and even in sandstone rocks up to 0.27%.

Cycle time

The speed at which a phosphorus molecule completes the cycle depends on the type of reservoir considered. For example, in coastal waters, a phosphorous molecule can be mobilized every 9 months and in deep marine sediments it can take more than 11,000 years.

Stages of the phosphorus cycle

The greatest recycling of phosphorus occurs between living organisms and water or soil depending on the type of ecosystem. It goes through three stages, which are:

- Geological

The most important phosphorus reserves at this stage of the cycle are found in marine sediments and soil. It is also present in the phosphate rocks of the subsoil and in the excrement of seabirds (guano).

Phosphate rocks are formed from marine sediments, which are sedimentary rocks that contain up to 30% phosphates. When they erode, the phosphates become part of the soil.

Similarly, soil and rock erosion drags phosphates into water bodies and ends up reaching the ocean where they are absorbed by marine organisms. On the other hand, a relatively low proportion of phosphates in soil dissolve in water and are absorbed by plants.

- Hydrological

The hydrological stage of the phosphorus cycle maintains a permanent exchange with the mainland and with the organisms that inhabit the aquatic world. The greatest amount of phosphorus is found in deep ocean waters as dissolved phosphate.

The phosphorus present in surface waters is absorbed by living organisms and therefore becomes part of the biological stage.

Tickets

It is estimated that about 10 million tons of phosphorus enter water bodies annually. It enters the hydrological stage dragged by water currents, mainly by runoff from rainwater.

In this way it reaches the rivers and from there to the lakes and oceans, as well as a small percentage from atmospheric dust that is deposited in the oceans or other bodies of water.

Circulation

Phosphorus circulates in the oceans especially in the colder lower layers, however in upwelling areas it reaches the surface. Upwellings are areas where deep cold waters rise, carrying phosphates and other nutrients.

In these areas, the availability of phosphates is abundant, which favors the development of phytoplankton that attracts large numbers of fish that feed on it.

Departures

Since phosphorus does not form volatile compounds (gases) in the ocean, it cannot be exchanged directly with the atmosphere. Therefore, the only exits from the hydrological stage are rock formation or fishing (by sea birds or human action).

In the first case, the phosphorus dissolved in the sea or from excreta or dead bodies of living beings, is deposited on the seabed. Over time, these sediments are covered by other layers and consolidate as phosphate rocks that will later be exposed to the earth's surface.

For their part, seabirds consume fish and carry phosphorus to the land through their excreta (guano) or by dying. While the human being extracts large amounts of phosphorus from the oceans through fishing since the bones of fish are composed of 35% of this element.

- Biological

Once phosphorus enters trophic chains or food chains, it is part of the biological stage of the phosphorus cycle. This begins when the phosphates absorbed by plants or by phytoplankton, form proteins and other vital molecules.

The phosphorus then circulates when plants and phytoplankton are consumed by herbivores and these by carnivores. It then moves through excreta and by the decomposition of the bodies of dead organisms by bacteria and fungi.

Although the amount of phosphorus in the biological stage is relatively low, it plays a fundamental role. Thus, about 80% of the total phosphorus in the body of a living being is made up of hydroxyapatite (Ca5 (PO4) 3 OH).

This mineral constitutes around 70% of vertebrate bones and tooth enamel is also made up of a high proportion of this phosphate mineral.

Departures

From this stage, phosphorus continues its cycle towards the hydrological and geological stages, through the death of organisms or their excreta. Likewise, human beings intervene in the phosphorus cycle, extracting it from the land and sea to use it as an industrial raw material or as a fertilizer.

Guano

An important source of phosphorus from the biological stage is the excreta of seabirds, called guano, which contains approximately 4% phosphates.

Guano accumulation. Source: Alex Proimos from Sydney, Australia / CC BY (https://creativecommons.org/licenses/by/2.0)

Seabirds that eat fish form large coastal colonies and their excreta accumulate in enormous quantities in their habitats. These excreta are especially rich in phosphates and have been used by humans as fertilizer.

Alterations

The fundamental alteration of the phosphorus cycle is its acceleration due to human activity. Phosphates are one of the main pollutants in wastewater, causing alterations in the phosphorus cycle by incorporating extra amounts into ecosystems.

Eutrophication

Phosphates are incorporated into sewage due to the use of detergents that incorporate trisodium phosphate. These compounds when combined with water, form phosphates assimilable by living beings.

Eutrophication. Source: F. lamiot (own work) / CC BY-SA (https://creativecommons.org/licenses/by-sa/2.5)

As large amounts of these assimilable phosphates enter, the populations of algae and aquatic plants increase considerably. This ecological imbalance ends up consuming the dissolved oxygen in the water, causing the death of fish and other organisms.

Water quality

The excess of phosphates in the water affects its quality for human consumption, thus reducing the sources of fresh water.

Importance

Essential for life

Phosphorus is a central part of ATP (adenosine triphosphate), which is the molecule through which energy is stored and transmitted in cells. On the other hand, DNA (deoxyribonucleic acid), which is the molecule responsible for the transmission of genetic information, includes a phosphate group.

Diet and health

Phosphorus is an essential element for health, taking into account that it is the second in abundance in the body. It is also a fundamental constituent of teeth and bones, closely related to the B complex vitamins.

In addition, it plays an important role in the functioning of the kidneys, muscles (including the heart) and in the nervous system (nerve signals).

Raw material for industry

In industry phosphates are used for various purposes, for example in the food industry they are used as anticaking agents and stabilizers. Phosphorus is a key ingredient in the making of matches, fireworks, and light signals.

Similarly, it is used in the production of metal alloys, industrial oils and as trisodium phosphate it is used in detergents.

Fertilizers

Phosphorus is an important component in fertilizers and fertilizers used in agriculture, especially useful to induce flowering in crops. The production of fertilizers is responsible for about 90% of the demand for phosphates.

Insecticides

Organic phosphates are in the form of phosphoric acid esters and in most cases have a neurotoxic effect, which is why they are used to make insecticides.

References

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2. Margalef, R. (1974). Ecology. Omega editions.
3. Miller, G. and TYLER, JR (1992). Ecology and Environment. Grupo Editorial Iberoamérica SA de CV
4. Odum, EP and Warrett, GW (2006). Fundamentals of ecology. Fifth edition. Thomson.
5. Ruttenberg, KC (2003). The Global Phosphorus Cycle. Treatise on Geochemistry.
6. Yan, Z., Han, W., Peñuelas, J., Sardans, J., Elser, J., Du, E., Reich, P and Fang, J. (2016). Phosphorus accumulates fester than nitrogen globally in freshwater ecosystems under anthropogenic impacts ”. Ecology Letters.