Phosphorus cycle: understand how it works
The biogeochemical cycle of phosphorus has been increasingly suffering from human interference
To understand how the phosphorus cycle works, you first need to know its main component: phosphorus (P). Phosphorus is a chemical element that reacts very easily with others. For this reason it is not found naturally without being linked to some other element. It's also one of the most essential components in nature - to give you an idea, it boasts second place (right behind calcium) in abundance in human tissues.
Functions in the body
In organisms, it is also an essential component of cells, being part of DNA and RNA molecules. Some of its functions in the body are:
- be part of the structure of bones and teeth (giving them greater solidity);
- participate in reactions with organic molecules formed by hydrogen, oxygen and carbon (called carbohydrates);
- act on muscle contraction.
The simplest
The biogeochemical cycle (so called because it encompasses both the chemical, geological and biological part of the ecosystem) of phosphorus is considered one of the simplest, and this is due to the fact that this element is not found in the atmosphere, but rather is addition, constituent of rocks in the earth's crust. For this reason, its cycle is not classified as atmospheric, as is the case for example with the nitrogen cycle. In this case, it is classified as sedimentary.
Another reason why it is considered the simplest biogeochemical cycle is that the only phosphorus compound that is really important for living beings is phosphate, composed of the union of one atom of phosphorus with three of oxygen (PO43-).
Phosphate groups
In relation to living cells, an important function of phosphate groups is their role as an energy store. This energy is stored in chemical bonds of ATP molecules, adenosine triphosphate, from the metabolism (or breakdown) of carbohydrate molecules; a process that generates energy. This stored energy can then be transferred to carry out any cellular process.
These same phosphate groups are also capable of activating and deactivating cellular enzymes that catalyze various chemical reactions. Furthermore, phosphorus is also important for the formation of molecules called phospholipids, which are the major components of cell membranes; membranes that surround the cells externally with three main functions: coating, protection and selective permeability (selects which substances enter and leave the cell).
The cycle
The main reservoir of phosphorus in nature are rocks, which are only released from them through weathering. Weathering is a set of phenomena (whether physical, chemical or biological) that lead to the breakdown and change in the chemical and mineralogical composition of rocks, transforming them into soil, releasing phosphate.
As it is a soluble compound, it is easily carried to rivers, lakes and oceans by the leaching process (solubilization of chemical constituents of a rock, mineral or soil by the action of a fluid, such as rain) or it is incorporated into organisms alive.
This incorporation takes place, in plants, through the absorption of phosphate through the soil. As such, it is used by organisms to form organic phosphate compounds that are essential to life (and henceforth called organic phosphate). In animal organisms, phosphate is entered through direct water ingestion and biomagnification (a process where the concentration of a compound increases along the food chain).
The decomposition of organic matter by decomposing organisms causes organic phosphate to be returned to soil and water in its inorganic form.
The microorganisms found in the soil, in turn, play important roles in the phosphorus cycle and its availability to plants through the following factors:
- Incorporation of phosphorus into microbial organic matter;
- Inorganic phosphorus solubilization;
- Association between plants and fungi;
- Mineralization of organic phosphorus.
Incorporation of phosphorus into microbial organic matteraatr
When incorporated into living organisms, phosphorus can be immobilized, that is, it becomes “trapped”, and during this period the cycle of these molecules is interrupted. Its release, so that the cycle can be continued, can take place through the following phenomena:- Breakage of microbial cells;
- Climatic variations and soil management;
- Interactions with the microfauna, which, when feeding on microorganisms, releases various nutrients into the soil.
There are some advantages to this incorporation of phosphorus into living organisms. For example, this process prevents its fixation for long periods in soil minerals (from where it would only be removed by weathering), increasing the efficiency of phosphate fertilization.
Inorganic phosphorus solubilization
Bacteria and fungi, including mycorrhizae, excrete organic acids that work by directly dissolving inorganic phosphorus.
- Many soil micro-organisms are described as being able to dissolve different types of rock phosphates;
- The biggest mechanism of solubilization is the action of organic acids synthesized by bacteria.
- These acids produced by organisms are great generators of H+ ions, which are capable of dissolving mineral phosphate and making it available to plants.
Association between plants and fungi
It occurs through mycorrhizae, which are bacteria associated with plant roots that promote mutualism between plant roots and soil fungi, so that the plant provides energy and carbon to the fungi through photosynthesis, and these return the favor by absorbing mineral nutrients and transferring them to plant roots.
Organic Phosphorus Mineralization
In addition to phosphorus from microbial organic matter, the action of phosphate solubilizing microorganisms and fungi associated with roots, the production of enzymes by some microorganisms and plants is responsible for the mineralization of organic phosphorus, which is its transformation into inorganic phosphorus.
Once in lakes and seas, phosphorus can, in addition to being absorbed by organisms, incorporate itself into rocks, closing the cycle.
The phosphorus cycle tends to be long. A single atom can spend up to 100,000 years being cycled, until it settles back into the rocks. With sediments, phosphorus can remain associated for more than 100 million years.
Problems
Human activity has increasingly changed the natural cycle of this macronutrient, whether through activities such as mining or the widespread use of fertilizers.
The excess of phosphorus when leached into watercourses ends up increasing the bioavailability of this nutrient in the aquatic environment and, as a consequence, can intensify the development of algae. An increasing number of algae in a lake, for example, will reduce the amount of light that penetrates this environment (drastically reducing the trophic zone), harming other local organisms. This process is called eutrophication (you can read more about the influence of fertilizer use on the eutrophication process in the article: "What are fertilizers?").
See also some photos of this effect:
