THE 8 MOST IMPORTANT BIOGEOCHEMICAL CYCLES (DESCRIPTION) - BIOLOGY - 2025

The biogeochemical cycles comprise the path followed the different nutrients or elements that are part of organic beings. This transit occurs within biological communities, both in biotic and abiotic entities that compose it.

Nutrients are the building blocks that make up macromolecules, and they are classified according to the amount that the living being needs in macro nutrients and micronutrients.

Source: pixabay.com

Life on planet earth dates back about 3 billion years, where the same nutrient pool has been recycled over and over again. The nutrient reserve is located in the abiotic components of the ecosystem, such as the atmosphere, stones, fossil fuels, the oceans, among others. The cycles describe the pathways of nutrients from these reservoirs, through living things, and back to the reservoirs.

The influence of humans has not gone unnoticed in the transit of nutrients, since anthropogenic activities - particularly industrialization and crops - have altered concentrations and therefore the balance of cycles. These disturbances have important ecological consequences.

Next we will describe the passage and recycling of the most outstanding micro and macronutrients on the planet, namely: water, carbon, oxygen, phosphorus, sulfur, nitrogen, calcium, sodium, potassium, sulfur.

What is a biogeochemical cycle?

Flow of energy and nutrients

The periodic table is made up of 111 elements, of which only 20 are essential for life and, due to their biological role, they are called biogenetic elements. In this way, organisms require these elements and also energy to sustain themselves.

There is a flow of these two components (nutrients and energy) that is transferred gradually through all levels of the food chain.

However, there is a crucial difference between both flows: energy flows only in one direction and enters the ecosystem inexhaustibly; while nutrients are found in limiting quantities and move in cycles - which in addition to living organisms involve abiotic sources. These cycles are the biogeochemicals.

General scheme of a biogeochemical cycle

The term biogeochemical is formed by the union of the Greek roots bio which means life and geo which means earth. For this reason, biogeochemical cycles describe the trajectories of these elements that are part of life, between the biotic and abiotic components of ecosystems.

As these cycles are extremely complex, biologists usually describe their most important stages, which can be summarized as: the location or reservoir of the element in question, its entry into living organisms - generally the primary producers, followed by its continuity through the chain trophic, and finally the reintegration of the element in the reservoir thanks to the decomposing organisms.

This scheme will be used to describe the route of each element for each stage mentioned. In nature, these steps require pertinent modifications depending on each element and the trophic structure of the system.

Microorganisms play a vital role

It is important to highlight the role of microorganisms in these processes, since, thanks to the reduction and oxidation reactions, they allow the nutrients to enter the cycles again.

Study and applications

Studying a cycle is a challenge for ecologists. Although it is an ecosystem whose perimeter is delimited (like a lake, for example) there is a constant flow of material exchange with the environment that surrounds them. That is, in addition to being complex, these cycles are connected to each other.

One methodology used is radioactive isotope labeling and element tracking by abiotic and biotic components of the study system.

Studying how the recycling of nutrients works and in what state is it is a marker of ecological relevance, which tells us about the productivity of the system.

Classifications of biogeochemical cycles

There is no single way to classify biogeochemical cycles. Each author suggests a suitable classification following different criteria. Below we will present three of the classifieds used:

Micro and macronutrient

The cycle can be classified according to the element that is mobilized. Macronutrients are elements used in appreciable quantities by organic beings, namely: carbon, nitrogen, oxygen, phosphorus, sulfur and water.

Other elements are only needed in small amounts, such as phosphorus, sulfur, potassium, among others. In addition, micronutrients are characterized by having a rather low mobility in the systems.

Although these elements are used in small quantities, they are still vital for organisms. If a nutrient is missing, it will limit the growth of living things that inhabit the ecosystem in question. Therefore, the biological components of the habitat are a good marker to determine the efficiency of the movement of the elements.

Sedimentary and atmospheric

Not all nutrients are in the same amount or are readily available to organisms. And this depends - mainly - on what is its source or abiotic reservoir.

Some authors classify them into two categories, depending on the movement capacity of the element and the reservoir in: sedimentary and atmospheric cycles.

In the former, the element cannot move up to the atmosphere and accumulates in the soil (phosphorus, calcium, potassium); while the latter comprise the gas cycles (carbon, nitrogen, etc.)

In atmospheric cycles the elements are housed in the lower layer of the troposphere, and are available to the individuals that make up the biosphere. In the case of sedimentary cycles, the release of the element from its reservoir requires the action of environmental factors, such as solar radiation, the action of plant roots, rain, among others.

In specific cases, a single ecosystem may not have all the necessary elements for the complete cycle to take place. In these cases, another neighboring ecosystem may be the provider of the missing element, thus connecting multiple regions.

Local and global

A third classification used is the scale at which the site is studied, which can be in a local habitat or globally.

This classification is closely linked to the previous one, since elements with atmospheric reserves have a wide distribution and can be understood globally, while elements are sedimentary reserves and have a limited capacity for movement.

Water cycle

Role of water

Water is a vital component for life on earth. Organic beings are composed of high proportions of water.

This substance is particularly stable, which makes it possible to maintain a suitable temperature inside organisms. In addition, it is the environment where the immense amount of chemical reactions take place inside organisms.

Finally, it is an almost universal solvent (apolar molecules do not dissolve in water), which allows infinities of solutions to be formed with polar solvents.

Reservoir

Logically, the largest reservoir of water on earth are oceans, where we find almost 97% of the total planet and cover more than three-quarters of the planet we live on. The remaining percentage is represented by rivers, lakes and ice.

Motors of the hydrological cycle

There are a series of physical forces that propel the movement of the vital liquid through the planet and allow it to carry out the hydrological cycle. These forces include: solar energy, which allows water to pass from a liquid state to a gaseous state, and gravity that drives water molecules back to earth in the form of rain, snow, or dew.

We will further describe each of the steps mentioned below:

(i) Evaporation: the change of state of water is driven by energy from the sun and occurs mainly in the ocean.

(ii) Precipitation: the water returns to the reservoirs thanks to precipitation in different forms (snow, rain, etc.) and taking different routes, either to the oceans, to the lakes, to the ground, to underground deposits, among others.

In the oceanic component of the cycle, the evaporation process exceeds precipitation, which results in a net gain of water that goes to the atmosphere. The closing of the cycle occurs with the movement of water through the underground routes.

Incorporation of water into living beings

A significant percentage of the body of living beings is made up of water. In us humans, this value is around 70%. For this reason, part of the water cycle occurs inside organisms.

Plants use their roots to obtain water through absorption, while heterotrophic and active organisms can consume it directly from the ecosystem or in food.

Unlike the water cycle, the cycle of the other nutrients includes important modifications in the molecules along their trajectories, while the water remains practically unchanged (only changes in state occur.)

Changes in the water cycle thanks to human presence

Water is one of the most valuable resources for human populations. Today, the shortage of vital fluid grows exponentially and represents a problem of global concern. Although there is a large amount of water, only a small portion corresponds to fresh water.

One of the drawbacks is the reduction in the availability of water for irrigation. The presence of asphalt and concrete surfaces reduces the surface that water could penetrate.

The extensive fields of cultivation also represent a decrease in the root system that maintains an adequate amount of water. In addition, irrigation systems remove enormous amounts of water.

On the other hand, salt to fresh water treatment is a procedure that is carried out in specialized plants. However, treatment is expensive and represents an increase in general contamination levels.

Finally, the consumption of contaminated water is a major problem for developing countries.

Carbon cycle

Carbon role

Life is made of carbon. This atom is the structural framework of all organic molecules that are part of living beings.

Carbon allows the formation of highly variable and very stable structures, thanks to its property of forming single, double and triple covalent bonds with and with other atoms.

Thanks to this it can form an almost infinite number of molecules. Today almost 7 million chemical compounds are known. Of this high number, approximately 90% are organic substances, the structural base of which is the carbon atom. The great molecular versatility of the element seems to be the cause of its abundance.

Reservoirs

The carbon cycle involves multiple ecosystems, namely: land regions, bodies of water, and the atmosphere. Of these three carbon reservoirs, the one that stands out as the most important is the ocean. The atmosphere is also an important reservoir even though it is relatively smaller.

In the same way, all the biomass of living organisms represents an important reservoir for this nutrient.

Photosynthesis and respiration: central processes

In both aquatic and terrestrial regions, the central point of carbon recycling is photosynthesis. This process is carried out both by plants and by a series of algae that have the enzymatic machinery required for the process.

That is, carbon enters living beings when they capture it in the form of carbon dioxide and use it as a substrate for photosynthesis.

In the case of photosynthetic aquatic organisms, the uptake of carbon dioxide occurs directly by the integration of the dissolved element in the body of water - which is found in a much greater quantity than in the atmosphere.

During photosynthesis, carbon from the environment is incorporated into the body's tissues. On the contrary, the reactions by which cellular respiration occurs carry out the opposite process: releasing the carbon that has been incorporated into living beings from the atmosphere.

Incorporation of carbon in living beings

Primary consumers or herbivores feed on producers and appropriate the carbon stored in their tissues. At this point, carbon takes two routes: it is stored in the tissues of these animals and another portion is released into the atmosphere through respiration, in the form of carbon dioxide.

Thus the carbon continues its course throughout the entire food chain of the community in question. At some point, the animal will die and its body will be decomposed by the microorganisms. Thus, the carbon dioxide returns to the atmosphere and the cycle can continue.

Alternative routes of the cycle

In all ecosystems - and depending on the organisms that inhabit there - the rhythm of the cycle varies. For example, mollusks and other microscopic organisms that make life in the sea have the ability to extract carbon dioxide dissolved in water and combine it with calcium to yield a molecule called calcium carbonate.

This compound will be part of the shells of organisms. After these organisms die, their shells gradually accumulate in deposits that, as time passes, will transform into limestone.

Depending on the geological context to which the body of water is exposed, the limestone can be exposed and begin to dissolve, which results in the escape of carbon dioxide.

Another long-term path in the carbon cycle is related to the production of fossil fuels. In the next section we will see how the burning of these resources affects the normal or natural course of the cycle.

Changes in the carbon cycle thanks to human presence

Humans have been influencing the natural course of the carbon cycle for thousands of years. All of our activities - such as industrial and deforestation - affect the release and sources of this vital element.

In particular, the use of fossil fuels has affected the cycle. When we burn fuel we are moving huge amounts of carbon that was in an inactive geological reservoir into the atmosphere, which is an active reservoir. Since the last century the increase in carbon release has been dramatic.

The release of carbon dioxide into the atmosphere is a fact that affects us directly, since it increases the temperatures of the planet and is one of the gases known as greenhouse gases.

Cycle of nitrogen

Cycle of nitrogen. Remade by YanLebrel from an image from the Environmental Protection Agency: http://www.epa.gov/maia/html/nitrogen.html, via Wikimedia Commons

Role of nitrogen

In organic beings we find nitrogen in two of its fundamental macromolecules: proteins and nucleic acids.

The former are responsible for a wide variety of functions, from structural to transportation; while the latter are the molecules in charge of storing genetic information and translating it to proteins.

In addition, it is a component of some vitamins that are vital elements for metabolic pathways.

Reservoirs

The main nitrogen reserve is the atmosphere. In this space we find that 78% of the gases present in the air is nitrogen gas (N _2.)

Although it is an essential element for living beings, neither plants nor animals have the ability to extract this gas directly from the atmosphere - as it happens with carbon dioxide, for example.

Assimilable sources of nitrogen

For this reason, nitrogen must be presented as an assimilable molecule. That is, that it is in its reduced or "fixed" form. An example of this are nitrates (NO ₃ ^-) or ammonia (NH _3.)

There are bacteria that establish a symbiotic relationship with some plants (such as legumes) and in exchange for protection and food they share these nitrogen compounds.

Other types of bacteria also produce ammonia using amino acids and other nitrogenous compounds that are stored in corpses and biological waste as substrates.

Nitrogen-fixing organisms

There are two main groups of fixatives. Some bacteria, blue-green algae, and actinomycete fungi can take the nitrogen gas molecule and include it directly as part of their proteins, releasing the excess in the form of ammonia. This process is called ammonification.

Another group of soil-dwelling bacteria are capable of taking up ammonia or the ammonium ion into nitrite. This second process is called nitrification.

Non-biological nitrogen-fixing processes

There are also non-biological processes capable of producing nitrogen oxides, such as electrical storms or fires. In these events, nitrogen combines with oxygen, yielding an assimilable compound.

The nitrogen fixation process is characterized by being slow, being a limiting step for the productivity of ecosystems, both terrestrial and aquatic.

Incorporation of nitrogen in living beings

Once the plants have found the nitrogen reservoir in the assimilable form (ammonia and nitrate) they incorporate them into different biological molecules, namely: amino acids, the building blocks of proteins; nucleic acids; vitamins; etc.

When nitrate is incorporated into plant cells, a reaction occurs and it is reduced back to its ammonium form.

Nitrogen molecules cycle when a primary consumer feeds on plants and incorporates nitrogen into their own tissues. They can also be consumed by debris eaters or by decomposing organisms.

Thus, nitrogen moves through the entire food chain. A significant part of the nitrogen is released together with waste and decomposing corpses.

The bacteria that make life in the soil and in the bodies of water are capable of taking this nitrogen and converting it back into assimilable substances.

It is not a closed cycle

After this description, it seems that the nitrogen cycle is closed and self-perpetuating. However, this is only at a glance. There are various processes that cause nitrogen loss, such as crops, erosion, the presence of fire, water infiltration, etc.

Another cause is called denitrification and it is caused by bacteria that lead the process. When found in an oxygen-free environment, these bacteria take up nitrates and reduce them, releasing it back into the atmosphere as a gas. This event is common in soils whose drainage is not efficient.

Changes in the nitrogen cycle thanks to human presence

Nitrogen compounds used by man dominate the nitrogen cycle. These compounds include synthetic fertilizers that are rich in ammonia and nitrates.

This excess of nitrogen has caused an imbalance in the normal path of the compound, particularly in the alteration of plant communities since they now suffer from excessive fertilization. This phenomenon is called eutrophication. One of the messages of this event is that the increase in nutrients is not always positive.

One of the most serious consequences of this fact is the destruction of the communities of forests, lakes and rivers. As there is not an adequate balance, some species, called dominant species, overgrow and dominate the ecosystem, decreasing diversity.

Phosphorus cycle

Phosphorus role

In biological systems, phosphorus is present in molecules called energy "coins" of the cell, such as ATP, and in other energy transfer molecules, such as NADP. It is also present in the molecules of heredity, both in DNA and RNA, and in the molecules that make up lipid membranes.

It also plays structural roles, as it is present in bone structures of the vertebrate lineage, including both bones and teeth.

Reservoirs

Unlike nitrogen and carbon, phosphorus is not found as a free gas in the atmosphere. Its main reservoir is rocks, linked to oxygen in the form of molecules called phosphates.

As can be expected, this shedding process is slow. Therefore, phosphorus is considered a rare nutrient in nature.

Incorporation of phosphorus in living beings

When the geographical and climatic conditions are suitable, the rocks begin a process of erosion or wear. Thanks to the rain, the phosphates begin to be diluted and can be taken up by the roots of the plants or by another series of primary producing organisms.

This series of photosynthetic organisms are responsible for incorporating phosphorus into their tissues. Starting from these basal organisms, phosphorus begins its transit through trophic levels.

In each link in the chain, part of the phosphorus is excreted by the individuals that compose it. When animals die, a series of special bacteria take up the phosphorus and incorporate it back into the soil as phosphates.

Phosphates can take two paths: be absorbed again by the autotrophs or start their accumulation in the sediments to return to their rocky state.

The phosphorus present in ocean ecosystems also ends up in the sediments of these bodies of water, and part of it can be absorbed by its inhabitants.

Changes in the phosphorus cycle due to human presence

The presence of humans and their agricultural techniques affects the phosphorus cycle in much the same way as it affects the nitrogen cycle. The application of fertilizers produces a disproportionate increase in the nutrient, leading to the eutrophication of the area, causing imbalances in the diversity of their communities.

It is estimated that in the last 75 years, the fertilizer industry has caused phosphorus concentrations to rise almost fourfold.

Sulfur cycle

Role of sulfur

Some amino acids, amines, NADPH, and coenzyme A are biological molecules that serve different functions in metabolism. They all contain sulfur in their structure.

Reservoirs

Sulfur reservoirs are very varied, including bodies of water (fresh and salt), terrestrial environments, atmosphere, rocks and sediments. It is found mainly as sulfur dioxide (SO _2.)

Incorporation of sulfur in living beings

From the reservoirs, the sulfate begins to dissolve and the first links in the food chain can capture it as an ion. After the reduction reactions, the sulfur is ready to be incorporated into the proteins.

Once incorporated, the element can continue its passage through the food chain, until the death of the organisms. Bacteria are responsible for releasing the sulfur that is trapped in corpses and waste, returning it to the environment.

Oxygen cycle

Oxygen cycle. Eme Chicano, from Wikimedia Commons

Role of oxygen

For organisms with aerobic and facultative respiration, oxygen represents the electron acceptor in the metabolic reactions involved in this process. Therefore, it is vital to maintain obtaining energy.

Reservoirs

The most important oxygen reservoir on the planet is represented by the atmosphere. The presence of this molecule gives this region an oxidizing character.

Incorporation of oxygen in living beings

As in the carbon cycle, cellular respiration and photosynthesis are two crucial metabolic pathways that orchestrate the trajectory of oxygen on planet earth.

In the process of respiration, animals take in oxygen and produce carbon dioxide as a waste product. Oxygen comes from the metabolism of plants, which in turn can incorporate carbon dioxide and use it as substrates for future reactions.

Calcium cycle

Reservoirs

Calcium is found in the lithosphere, embedded in sediments and rocks. These rocks may be the product of the fossilization of marine animals whose external structures were rich in calcium. It is also found in caves.

Incorporation of calcium in living beings

The rains and other climatic events cause the erosion of the stones that contain the calcium, causing its release and allowing living organisms to absorb them at any point in the food chain.

This nutrient will be incorporated into the living being, and at the time of its death the bacteria will carry out the pertinent decomposition reactions that achieve the release of this element and the continuity of the cycle.

If calcium is released into a body of water, it can be held at the bottom and rock formation begins again. Groundwater displacement also plays an important role in calcium mobilization.

The same logic applies to the potassium ion cycle, which is found in clay soils.

Sodium cycle

Role of sodium

Sodium is an ion that performs multiple functions in the body of animals, such as nerve impulse and muscle contractions.

Reservoir

The largest reservoir of sodium is found in bad water, where it is dissolved in the form of an ion. Remember that common salt is formed by the union between sodium and chlorine.

Incorporation of sodium in living beings

Sodium is mainly taken up by the organisms that make life in the sea, who absorb it and can transport it to land, either through water or food. The ion can travel dissolved in water, following the path described in the hydrological cycle.

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