Biogeochemical Cycles

Elements can neither be created nor destroyed, so instead, they circulate through the biotic and abiotic sections of ecosystems. These elemental circulations are called biogeochemical cycles. If you break down the word itself: ‘bio’ refers to the biosphere (meaning all the living organisms on our planet), while ‘geo’ is a shortened form of geological referring to the Earth’s physical components. Lastly, ‘chemical’ refers to the elements which constantly circulate in the closed system.

The Different Parts of Biogeochemical Cycles

These are the three parts of biogeochemical cycles that you need to understand:

• Reservoirs - Where the major source of the element is situated. Biogeochemical reservoirs are usually slow-moving and abiotic, they store chemicals for long periods at a time (e.g. fossil fuels containing carbon)

• Sources - The organism or processes which return the elements to the reservoir.

• Sinks - The largest site of nutrient movement from the non-living to the living parts of the ecosystem.

Importance of Biogeochemical Cycles

Biogeochemical cycles allow all parts of the ecosystem to thrive at the same time by offering a way of recycling nutrients between the living and non-living parts of the Earth. These non-living parts include the atmosphere (air), lithosphere (soil), and hydrosphere (water). If one section of these biogeochemical processes stopped functioning, the whole ecosystem would collapse as the nutrients would become trapped in one place.

Types of Biogeochemical Cycles

There are two main types of biogeochemical cycles, namely gaseous cycles and sedimentary cycles:

• Gaseous cycles - examples are the carbon, nitrogen, oxygen and water cycles. The reservoirs of these cycles are the atmosphere or hydrosphere.

• Sedimentary cycles - examples are the phosphorus and sulphur cycles. The reservoir of these cycles is in the lithosphere.

Gaseous Cycles

Here we will briefly cover the gaseous cycles of carbon, nitrogen, water, and oxygen.

The Carbon Cycle

Carbon is an essential component of the majority of organisms on this planet. Although cells are made up of mostly water, the rest of their mass is made up of carbon-based compounds (e.g. proteins, lipids, carbohydrates).

The carbon cycle involves the element carbon circulating through Earth’s abiotic and biotic systems. This includes living things (the biosphere), the ocean (the hydrosphere) and the Earth’s crust (the geosphere). Carbon has the form of carbon dioxide in the atmosphere and is taken up by photosynthetic organisms. It is then used to manufacture organic molecules which pass through the food chain. The carbon then returns to the atmosphere as it is released by aerobically respiring organisms.

Photosynthetic Organisms Take up Carbon Dioxide

Carbon dioxide is present in the atmosphere from billions of years of aerobically respiring organisms inhabiting Earth and as a by-product of the burning of fossil fuels. Producers take up atmospheric carbon dioxide via diffusion through the stomata on their leaves. They subsequently manufacture carbon-containing compounds using the energy harnessed from sunlight.

Carbon Passes Through the Food Chain

Producers are eaten by herbivorous consumers, of which are eaten by carnivorous consumers, who may then be eaten by predators themselves. The animals absorb these carbon-containing compounds when they consume another organism. The animals will use the carbon for their own biochemical and metabolic processes. Not all carbon will be absorbed during consumption as the whole organisms may not be eaten, carbon may not be absorbed efficiently into the body, and some is released in faecal matter. Therefore, carbon availability decreases up the trophic levels.

Carbon is Returned to the Atmosphere by Respiration

Consumers are aerobic organisms so when they respire they release carbon dioxide back into the atmosphere, completing the cycle. However, not all carbon

Decomposers Release the Remaining Carbon Dioxide

The rest of the carbon will become trapped in the bodies of the consumers. Aerobic decomposers (e.g fungi, saprobiontic bacteria) will break down the organic matter found in dead organisms and their faeces, releasing carbon dioxide in the process.

The Marine Carbon Cycle

The marine carbon cycle is different because there is no aerobic respiration in the sea; the respiration is referred to as aquatic. Aquatic oxygen is taken up by aquatic organisms (e.g fish, turtles, crabs) and converted into dissolved carbon dioxide. Dissolved carbon dioxide released from marine organisms and absorbed from the atmosphere will form carbonates, for example, calcium carbonate, which are used by calcifying organisms to build their shells and exoskeletons. When these organisms die their matter will sink to the seafloor and be broken down by decomposers in the sediment, releasing carbon dioxide.

Unreleased Carbon and Human Activity

Despite the efforts of decomposing bacteria, not all carbon is released back into the atmosphere as carbon dioxide. Some of it is stored in fossil fuels, like coal and gas, which have formed from millions of years of compression of dead organisms to form a solid mineral. In the past 100 years or so, burning fossil fuels for energy has increased at a rapid rate, releasing carbon dioxide into the atmosphere in the process. So coupled with the fact that deforestation has increased exponentially in recent times, human activity is causing there to be more carbon dioxide in the atmosphere while also reducing the number of photosynthetic organisms on the Earth. Carbon dioxide is a greenhouse gas, which plays a role in trapping heat inside the atmosphere, so more carbon dioxide means a warmer planet.

The Nitrogen Cycle

Nitrogen is the most abundant element in the Earth’s atmosphere, making up about 78% of it, but gaseous nitrogen is inert so is unavailable for organisms to use in this form. This is where the nitrogen cycle comes in. The nitrogen cycle is reliant on various microorganisms:

• Nitrogen-fixing bacteria

• Ammonifying bacteria

• Nitrifying bacteria

• Denitrifying bacteria

We will go over how they contribute to the nitrogen cycle in this section.

There are 5 different steps in the nitrogen cycle:

• Nitrogen-fixation

• Ammonification

• Denitrification

• Assimilation

• Nitrification

Nitrogen Fixation

Nitrogen can be fixed industrially with high temperatures and pressure (e.g. the Haber-Bosch process), or even by lightning strikes, but it is the nitrogen-fixing bacteria in the soil that are an essential component of the nitrogen cycle. These bacteria fix gaseous nitrogen by converting it to ammonia which can be used to build nitrogen-containing compounds. There are two main types of nitrogen-fixing bacteria that you should know:

• Free-living nitrogen-fixing bacteria - these are aerobic bacteria which are present in the soil. They convert nitrogen to ammonia and then to amino acids. When they die, nitrogen-containing compounds are released into the soil which can then be broken down by decomposers.

• Mutualistic nitrogen-fixing bacteria - these bacteria live on the root nodules of many leguminous plants, and have a symbiotic relationship with their host plant. The bacteria will fix the gaseous nitrogen and provide the plant with amino acids while the plant will give the bacteria useful carbohydrates in return.

Ammonification

Ammonification is the process by which nitrogen returns to the non-living part of the ecosystem. Carried out by ammonifying microorganisms, such as bacteria and fungi, nitrogen-rich compounds in the soil are broken down to ammonia which forms ammonium ions. Examples of nitrogen-rich compounds are amino acids, nucleic acids and vitamins; which are all found in decaying organisms and faecal matter.

Nitrification

Nitrification is carried out by aerobic, free-living nitrifying bacteria in the soil. These bacteria harness the energy released from oxidation reactions to survive. The two oxidation reactions that occur are the oxidation of ammonium ions to nitrite ions and the subsequent oxidation of nitrite ions to nitrate ions. These nitrate ions are easily absorbed by the plant and are essential for building up molecules such as chlorophyll, DNA and amino acids.

Assimilation

Assimilation involves the absorption of inorganic ions from the soil into the plant roots by active transport. Plants must have the ability to actively transport ions so that they can still survive even when there is a low concentration of ions in the soil. These ions are translocated throughout the plant and used to manufacture organic compounds essential to the plants growth and function.

Denitrification

Denitrification is the process by which anaerobic denitrifying bacteria in the soil convert nitrogen ions back into gaseous nitrogen, reducing nutrient availability for the plants. These denitrifying bacteria are prevalent when the soil is waterlogged and there is less oxygen available. Denitrification returns nitrogen to the atmosphere completing the nitrogen cycle.

The Oxygen Cycle

2.3 billion years ago, oxygen was first introduced to the atmosphere by the only photosynthetic prokaryote - cyanobacteria. This gave rise to aerobic organisms which were able to rapidly evolve and become the diverse biome that inhabits our planet today. Oxygen is available in the atmosphere as a gaseous molecule and is vital for the survival of aerobic organisms, as it is essential for respiration and the build-up of some molecules like amino acids and nucleic acids. The oxygen cycle is fairly simple compared to some of the other gaseous processes:

Producers Release Oxygen

All photosynthetic organisms take up carbon dioxide and in turn release oxygen into the atmosphere as a by-product. This is why the producer population of the earth is called a reservoir of oxygen, along with the atmosphere and the hydrosphere.

Aerobic Organisms take up Oxygen

All aerobic organisms inhabiting the earth require oxygen to survive. They will all inhale oxygen and exhale carbon dioxide during respiration. Oxygen is necessary for cellular respiration as it is used to release energy from the breakdown of glucose.

The Phosphorus Cycle

Phosphorus is a component of NPK (Nitrogen-Phosphorus-Potassium) fertilisers, which are globally used in agriculture. Phosphorus is required by plants for building up nucleic acids and phospholipid membranes and microorganisms living in the soil also depend on a sufficient level of phosphate ions. The phosphorus cycle is one of the slowest biogeochemical cycles, as weathering of rocks can take thousands of years.

Weathering of Phosphate Rock

Phosphate rocks are rich in phosphorus and phosphate salts are released from these rocks when they are exposed to air and weathered. These phosphate salts are washed away into soils making them more fertile. Therefore, the lithosphere is the reservoir of the phosphorus cycle.

Transfer to the Biosphere

Producers in the soil will absorb these phosphate ions through their roots and use them to make phosphate-containing compounds like DNA and phospholipid bilayers in the plasma membrane. Consumers will then ingest these producers and use their phosphate for their own organic compounds.

Recycling of Phosphate

The producers and consumers who die will be decomposed by microorganisms in the soil which releases inorganic phosphate. This inorganic phosphate will either cycle back into the ecosystem or be recycled back into rocks and sediment which will be weathered starting the process again.