All living things are made of carbon. Carbon is also a part of the ocean, air, and even rocks. Because the Earth is a dynamic place, carbon does not stay still. It is on the move! In the atmosphere, carbon is attached to some oxygen in a gas called carbon dioxide.
Plants use carbon dioxide and sunlight to make their own food and grow. The carbon becomes part of the plant. Plants that die and are buried may turn into fossil fuels made of carbon like coal and oil over millions of years. When humans burn fossil fuels, most of the carbon quickly enters the atmosphere as carbon dioxide. Carbon dioxide is a greenhouse gas and traps heat in the atmosphere. Without it and other greenhouse gases, Earth would be a frozen world. But humans have burned so much fuel that there is about 30% more carbon dioxide in the air today than there was about 150 years ago, and Earth is becoming a warmer place. In fact, ice cores show us that there is now more carbon dioxide in the atmosphere than there has been in the last 420,000 years.
Carbon dioxide in the atmosphere prevents the sun’s eat from escaping into space, very much like the glass walls of a greenhouse. This isn’t always a bad thing – some carbon dioxide in the atmosphere is good for keeping the Earth warm and its temperature stable. But Earth has experienced catastrophic warming cycles in the past, such as the Permian extinction, which is thought to have been caused by a drastic increase in the atmosphere’s level of greenhouse gases. No one is sure what caused the change that brought about the Permian extinction. Greenhouse gases may have been added to an atmosphere by an asteroid impact, volcanic activity, or even massive forest fires. Whatever the cause, during this warming episode, temperatures rose drastically. Much of the Earth became desert, and over 90% of all species living at that time went extinct. This is a good example of what can happen if our planet’s essential cycles experience a big change. Another important variable effected by the carbon cycle is the acidity of the ocean. Carbon dioxide can react with ocean water to form carbonic acid. This has been an important stabilizing force of of the carbon cycle over the years, since the chemical equilibrium between carbon dioxide and carbonic acid means that the ocean can absorb or release carbon dioxide as atmospheric levels rise and fall.
However, as you might guess, increasing ocean acidity can mean trouble for sea life – and this might eventually pose a problem for other parts of the carbon system. Many forms of sea life that have shells, for example, can take carbon out of the water to create the calcium carbonate that they make their shells out of. If these species suffer, the ocean may lose some of its ability to remove carbon from the atmosphere. Lastly, of course, there is the role of living things in the carbon cycle. The activity of plants and animals has been one of the major forces affecting changes to the carbon cycle in the past several billion years. Photosynthesizers have changed Earth’s atmosphere and climate drastically by taking huge amounts of carbon out of the atmosphere and turning that carbon into cellular materials. Those activities created free oxygen and the ozone layer, and generally set the stage for the evolution of animals that obtain their energy by breaking down the organic materials created by photosynthesizers and extracting the energy that the photosynthesizers used to make those molecules. With one particular species of animals – humans – making big changes, the future of the Earth’s carbon cycle is uncertain. All such cycles in closed systems eventually correct themselves – but sometimes this happens through drastic population reduction of the offending species through starvation.
Steps of Carbon Cycle
- Atmospheric Carbon To become part of the carbon cycle, carbon atoms start out in gaseous form. Carbon dioxide gas – CO2 – can be produced by inorganic processes, or by the metabolisms of living things. Before Earth had life on it, carbon dioxide gas likely came from volcanic activity and asteroid impacts. Today, carbon is also released into the atmosphere through the activities of living things, such the exhalations of animals, the actions of decomposer organisms, and the burning of wood and fossil fuels by humans. However carbon dioxide gets into the atmosphere, CO2 gas is the starting point of the carbon cycle.
- Carbon absorbed by the producers “Producers” – organisms that produce food from sunlight, such as plants – absorb carbon dioxide from the atmosphere and use it to build sugars, lipids, proteins, and other essential building blocks of life. For plants, CO2 is absorbed through pores in their leaves called “stomata.” Carbon dioxide enters the plant through the stomata and is incorporated into containing carbon compounds with the help of energy from sunlight. Plants and other producer organisms such as cyanobacteria are crucial to life on Earth, because they can turn atmospheric carbon into living matter.
- Transfer of Carbon through Producers which are eaten “Consumers” are organisms that eat other living things. Animals are the most visible type of consumer in our ecosystems, though many types of microbes also fall into this category. Consumers incorporate the carbon compounds from plants and other food sources when they eat them. They use some of these carbon compounds from food to build their own bodies – but much of the food they eat is broken down to release energy, in a process that is almost the reverse of what producers do. While producers use energy from sunlight to make bonds between carbon atoms – animals break these bonds to release the energy they contain, ultimately turning sugars, lipids, and other carbon compounds into single-carbon units. These are ultimately released into the atmosphere in the form of CO2. This process of “cellular respiration” – where oxygen gas is inhaled and carbon dioxide is exhaled – is a major source of carbon release back into the atmosphere. But it’s not always the last step of the carbon cycle. What about the carbon compounds that don’t get eaten, or broken down by animals?
- Carbon released back by the Decomposers Plants and animals that die without being eaten by other animals are broken down by other organisms, called “decomposers.” Decomposers include many bacteria and some fungi. They usually only break down matter that is already dead, rather than catching and eating a living animal or plant. Just like animals, decomposers break down the chemical bonds in their food molecules. They create many chemical products, including in some cases CO2. Carbon that isn’t released back into the atmosphere in this way can also be released by…
- Combustion caused by Humans Humans are the only animals we know of who can create fire on purpose. And we set fire to things a lot. Our cars are driven by burning fossil fuels – oil and gasoline, which are made of dead plant and animal material that spent millions of years buried deep in the Earth. Many of our electrical power plants are powered by burning fossil fuels as well, including coal, which is another form of dead plant matter that was buried underground and transformed by geologic heat. Lastly, humans also burn a lot of wood. We no longer burn wood to power our machines as we did in the 19th century, but now we often burn forests in order to clear land for agriculture, mining, and other purposes. About half of Earth’s forests have been burned or otherwise destroyed by human activity to date. The scientific Community has raised alarms that by making significant changes to the Earth’s carbon cycle, we may end up changing our climate or other important aspects of the ecosystem we rely upon to survive. As a result, many scientists advocate to decrease the amount of carbon burned by humans by reducing car trips and electricity consumption, and investing in non-burning sources of energy such as solar power and wind power.
Examples of the Carbon Cycle
The carbon cycle consists of many parallel systems which can either absorb or release carbon. Together, these systems work to keep Earth’s carbon cycle – and subsequently its climate and biosphere – relatively stable.
One major repository of carbon is the carbon dioxide in the Earth’s atmosphere. Carbon forms a stable, gaseous molecule in combination with two atoms of oxygen. In nature, this gas is released by volcanic activity, and by the respiration of animals who affix carbon molecules from the food they eat to molecules of oxygen before exhaling it. Humans also release carbon dioxide into the atmosphere by burning organic matter such as wood and fossil fuels. Carbon dioxide can be removed from the atmosphere by plants, which take the atmospheric carbon and turn it into sugars, proteins, lipids, and other essential molecules for life.
The Earth’s crust – called the “lithosphere” from the Greek word “litho” for “stone” and “sphere” for globe – can also release carbon dioxide into Earth’s atmosphere. This gas can be created by chemical reactions in the Earth’s crust and mantel. Volcanic activity can result in natural releases of carbon dioxide. Some scientists believe that widespread volcanic activity may be to blame for the warming of the Earth that caused the Permian extinction. While the Earth’s crust can add carbon to the atmosphere, it can also remove it. Movements of the Earth’s crust can bury carbon-containing chemicals such as dead plants and animals deep underground, where their carbon cannot escape back into the atmosphere.
Among living things, some remove carbon from the atmosphere, while others release it back. The most noticeable participants in this system are plants and animals. Plants remove carbon from the atmosphere. They don’t do this as a charitable act; atmospheric carbon is actually the “food” which plants use to make sugars, proteins, lipids, and other essential molecules for life. Plants use the energy of sunlight, harvested through photosynthesis, to build these organic compounds out of carbon dioxide and other trace elements. Indeed, the term “photosynthesis” comes from the Greek words “photo” for “light” and “synthesis” for “to put together.”
The Earth’s oceans have the ability to both absorb and release carbon dioxide. When carbon dioxide from the atmosphere comes into contact with ocean water, it can react with the water molecules to form carbonic acid – a dissolved liquid form of carbon. Like most chemical reactions, the rate of this reaction is determined by the equilibrium between the products and the reactants. When there is more carbonic acid in the ocean compared to carbon dioxide in the atmosphere, some carbonic acid may be released into the atmosphere as carbon dioxide.
All life requires nitrogen-compounds, e.g., proteins and nucleic acids. Air, which is 79% nitrogen gas (N2), is the major reservoir of nitrogen. But most organisms cannot use nitrogen in this form. Plants must secure their nitrogen in “fixed” form, i.e., incorporated in compounds such as: nitrate ions (NO3?) ammonium ions (NH4+) urea (NH2)2CO Animals secure their nitrogen (and all other) compounds from plants (or animals that have fed on plants).
Steps: Four processes participate or steps are involved in the cycling of nitrogen through the biosphere:
Microorganisms play major roles in all four of these.
- Nitrogen Fixation The nitrogen molecule (N2) is quite inert. To break it apart so that its atoms can combine with other atoms requires the input of substantial amounts of energy. Three processes are responsible for most of the nitrogen fixation in the biosphere: atmospheric fixation by lightning industrial fixation biological fixation by certain microbes — alone or in a symbiotic relationship with some plants and animals
The enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine with oxygen in the air forming nitrogen oxides. These dissolve in rain, forming nitrates, that are carried to the earth. Atmospheric nitrogen fixation probably contributes some 5– 8% of the total nitrogen fixed.
Under great pressure, at a temperature of 600°C, and with the use of a catalyst, atmospheric nitrogen and hydrogen (usually derived from natural gas or petroleum) can be combined to form ammonia (NH3). Ammonia can be used directly as fertilizer, but most of its is further processed to urea and ammonium nitrate (NH4NO3).
The ability to fix nitrogen is found only in certain bacteria and archaea. Some live in a symbiotic relationship with plants of the legume family (e.g., soybeans, alfalfa). Some establish symbiotic relationships with plants other than legumes (e.g., alders). Some establish symbiotic relationships with animals, e.g., termites and “shipworms” (wood-eating bivalves). Some nitrogen-fixing bacteria live free in the soil. Nitrogen-fixing cyanobacteria are essential to maintaining the fertility of semi-aquatic environments like rice paddies.
Biological nitrogen fixation requires a complex set of enzymes and a huge expenditure of ATP. Although the first stable product of the process is ammonia, this is quickly incorporated into protein and other organic nitrogen compounds.
The proteins made by plants enter and pass through food webs just as carbohydrates do. At each trophic level, their metabolism produces organic nitrogen compounds that return to the environment, chiefly in excretions. The final beneficiaries of these materials are microorganisms of decay. They break down the molecules in excretions and dead organisms into ammonia.
Ammonia can be taken up directly by plants — usually through their roots. However, most of the ammonia produced by decay is converted into nitrates. Until recently this was thought always to be accomplished in two steps: Bacteria of the genus Nitrosomonas oxidize NH3 to nitrites (NO2?). Bacteria of the genus Nitrobacter oxidize the nitrites to nitrates (NO3?). These two groups of autotrophic bacteria are called nitrifying bacteria. Through their activities (which supply them with all their energy needs), nitrogen is made available to the roots of plants.
However, in 2015, two groups reported finding that bacteria in the genus Nitrospira were able to carry out both steps: ammonia to nitrite and nitrite to nitrate. This ability is called “comammox” (for complete ammonia oxidation). In addition, both soil and the ocean contain archaeal microbes, assigned to the Crenarchaeota, that convert ammonia to nitrites. They are more abundant than the nitrifying bacteria and may turn out to play an important role in the nitrogen cycle. Many legumes, in addition to fixing atmospheric nitrogen, also perform nitrification — converting some of their organic nitrogen to nitrites and nitrates. These reach the soil when they shed their leaves.
The three processes above remove nitrogen from the atmosphere and pass it through ecosystems. Denitrification reduces nitrates and nitrites to nitrogen gas, thus replenishing the atmosphere. In the process several intermediates are formed: nitric oxide (NO) nitrous oxide (N2O)(a greenhouse gas 300 times as potent as CO2) nitrous acid (HONO) Once again, bacteria are the agents. They live deep in soil and in aquatic sediments where conditions are anaerobic. They use nitrates as an alternative to oxygen for the final electron acceptor in their respiration.
Anammox (anaerobic ammonia oxidation) Under anaerobic conditions in marine and freshwater sediments, other species of bacteria are able to oxidize ammonia (with NO2?) forming nitrogen gas. NH4+ + NO2? > N2 + 2H2O The anammox reaction may account for as much as 50% of the denitrification occurring in the oceans. All of these processes participate in closing the nitrogen cycle.
Are the denitrifiers keeping up? Agriculture may now be responsible for one-half of the nitrogen fixation on earth through the use of fertilizers produced by industrial fixation the growing of legumes like soybeans and alfalfa. This is a remarkable influence on a natural cycle. Are the denitrifiers keeping up the nitrogen cycle in balance? Probably not. Certainly, there are examples of nitrogen enrichment in ecosystems. One troubling example: the “blooms” of algae in lakes and rivers as nitrogen fertilizers leach from the soil of adjacent farms (and lawns). The accumulation of dissolved nutrients in a body of water is called eutrophication.