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Ecosystems need carbon to be cycled, without it they could not survive. To illustrate this point the coal shown above is formed entirely from partially decayed and fossilised plant and animal remains. Fossil fuels are a valuable sink of carbon. As shown by this and the next topic the balance of the cycle, i.e. where in the cycle the carbon is found, is as critical to ecosystems as the fact that they need carbon to be cycled.

4.3 Carbon Cycling

Continued availability of carbon in ecosystems depends on carbon cycling.

Essential Idea

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Understandings

	Statement
4.3.U1	Autotrophs convert carbon dioxide into carbohydrates and other carbon compounds.
4.3.U2	In aquatic ecosystems carbon is present as dissolved carbon dioxide and hydrogen carbonate ions.
4.3.U3	Carbon dioxide diffuses from the atmosphere or water into autotrophs.
4.3.U4	Carbon dioxide is produced by respiration and diffuses out of organisms into water or the atmosphere.
4.3.U5	Methane is produced from organic matter in anaerobic conditions by methanogenic archaeans and some diffuses into the atmosphere or accumulates in the ground.
4.3.U6	Methane is oxidized to carbon dioxide and water in the atmosphere.
4.3.U7	Peat forms when organic matter is not fully decomposed because of acidic and/or anaerobic conditions in waterlogged soils.
4.3.U8	Partially decomposed organic matter from past geological eras was converted either into coal or into oil and gas that accumulate in porous rocks.
4.3.U9	Carbon dioxide is produced by the combustion of biomass and fossilized organic matter.
4.3.U10	Animals such as reef-building corals and mollusca have hard parts that are composed of calcium carbonate and can become fossilized in limestone.

4.3 Carbon Cycling

Understandings

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	Statement	Guidance
4.3.A1	Estimation of carbon fluxes due to processes in the carbon cycle.	Carbon fluxes should be measured in gigatonnes.
4.3.A2	Analysis of data from air monitoring stations to explain annual fluctuations.
4.3.S1	Construct a diagram of the carbon cycle.

4.3 Carbon Cycling

Applications and Skills

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4.3 Carbon Cycling

Carbon cycle diagrams vary greatly in the detail they contain. This one shows not only the sinks and the flows, but also estimates carbon storage and movement in gigatons/year.

4.3.S1 Construct a diagram of the carbon cycle.

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CO₂ in the atmosphere and hydrosphere (oceans)

Carbon compounds in fossil fuels

Carbon compounds in producers (autotrophs)

Carbon compounds in consumers

Carbon compounds in dead organic matter

Key:

Sink

Flux

n.b. some of the fluxes will need to be used more than once.

Cell respiration

Photosynthesis

Combustion

Feeding

Egestion

Death

Incomplete decomposition & fossilisation

4.3 Carbon Cycling

Carbon cycle diagrams vary greatly in the detail they contain. This one shows not only the sinks and the flows, but also estimates carbon storage and movement in gigatons/year.

4.3.S1 Construct a diagram of the carbon cycle.

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CO₂ in the atmosphere and hydrosphere (e.g. oceans)

Carbon compounds in fossil fuels

Carbon compounds in producers (autotrophs)

Carbon compounds in consumers

Carbon compounds in dead organic matter

Cell respiration

Photosynthesis

Combustion

Feeding

Egestion

Death

Incomplete decomposition & fossilisation

Cell respiration

Combustion

Cell respiration

Feeding

Death

Feeding

4.3 Carbon Cycling

Carbon cycle diagrams vary greatly in the detail they contain. This one shows not only the sinks and the flows, but also estimates carbon storage and movement in gigatons/year.

4.3.S1 Construct a diagram of the carbon cycle.

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4.3 Carbon Cycling

Carbon cycle diagrams vary greatly in the detail they contain. This one shows not only the sinks and the flows, but also estimates carbon storage and movement in gigatons/year.

4.3.S1 Construct a diagram of the carbon cycle.

http://youtu.be/CitOibRcCcI

Use the video to help practise your drawing skills*

*this is a good resource, but there is one mistake in the video – carbon is egested, when not digested by an organism, not excreted.

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4.3 Carbon Cycling

Partially decomposed organic matter can be compressed to form brown soil-like peat

4.3.U7 Peat forms when organic matter is not fully decomposed because of acidic and/or anaerobic conditions in waterlogged soils.

Peat is a highly effective carbon sink, it is estimated that the world’s peat contains 550 Gt of carbon (International Mire Conservation Group, 2007-01-03)

Once dried peat burns easily and can be used as a fuel.

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4.3 Carbon Cycling

4.3.U7 Peat forms when organic matter is not fully decomposed because of acidic and/or anaerobic conditions in waterlogged soils.

In soils organic matter, e.g. dead leaves, are digested by saprotrophic bacteria and fungi.

Saprotrophs assimilate some carbon for growth and release as carbon dioxide during aerobic respiration.

Aerobic respiration requires oxygen

Waterlogged soils are an anaerobic environment

Partial decomposition causes acidic conditions

saprotrophs and methanogens [4.3.U5] are inhibited

Organic matter is only partially decomposed

Large quantities of (partially decomposed) organic matter build up.

The organic matter is compressed to form peat

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https://www.youtube.com/watch?v=zaXBVYr9Ij0&feature=emb_logo

4.3 Carbon Cycling

Coal is formed when deposits of peat are buried under other sediments.

4.3.U8 Partially decomposed organic matter from past geological eras was converted either into coal or into oil and gas that accumulate in porous rocks.

The peat is compressed and heated over millions years eventually becoming coal.

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Partially decomposed peat when put under extreme weight, pressure and heat from above sediments can be transformed into coal
This transformation takes place over millions of years
The pressure and heat cause lithification (the transformation of sediments into solid rock)
Large deposits were formed during the Carboniferous era
Oil and gas are formed at the bottom of oceans, seas and lakes over millions of years
The dead remains of marine organisms only partial decomposed when they settled at the bottom of the ancient oceans and seas in anaerobic conditions
As more dead remains and sediment accumulated, intense pressure and heat caused this sludge to undergo a chemical transformation into a mixture of carbon compounds or gases.
Lipids which are not easily broken down formed a waxy hydrocarbon called kerogen
Kerogen in porous sedimentary rock becomes crude oil or natural gas; depending on its state

4.3 Carbon Cycling

Partially decomposed organic matter from past geological eras was converted either into coal or into oil and gas that accumulate in porous rocks.

Extra Information

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Extend your understanding:

Between which sinks would you add a flux showing volcanoes and the weathering of rocks?
What additional sink would you add to show the role of corals and shellfish? What additional flux would be needed?
In some environments water is unable to drain out of soils so they become waterlogged and anaerobic. This prevents the decomposition of dead organic matter forming peat deposits [4.3.U7]. Peat can be dried and burnt as a fuel. Suggest how peat could be added to the carbon cycle.
Explain why fossil fuels are classified as non-renewable resources when the carbon cycle indicates they are renewed (hint: refer to the pictorial carbon cycle).
Diffusion is a flux that moves CO2 from the atmosphere to the hydrosphere and back again. Taken together these fluxes are largest in the cycle suggest why.

4.3 Carbon Cycling

Carbon cycle diagrams vary greatly in the detail they contain. This one shows not only the sinks and the flows, but also estimates carbon storage and movement in gigatons/year.

4.3.S1 Construct a diagram of the carbon cycle.

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The reminder of this presentation adds detail and understanding to the carbon cycle you have just learnt to draw.

4.3 Carbon Cycling

Carbon cycle diagrams vary greatly in the detail they contain. This one shows not only the sinks and the flows, but also estimates carbon storage and movement in gigatons/year.

4.3.S1 Construct a diagram of the carbon cycle.

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A few are Chemoautotrophs and fix carbon by utilising the energy in the bonds of inorganic compounds such as hydrogen sulfide.

Plant initially synthesis sugars (e.g. glucose) which are then converted into other organic compounds such as:

complex carbohydrates e.g. starch, cellulose
lipids
amino acids

4.3 Carbon Cycling

All autotrophs however convert carbon dioxide (from the atmosphere or dissolved in water) or into organic compounds.

4.3.U1 Autotrophs convert carbon dioxide into carbohydrates and other carbon compounds.

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4.3 Carbon Cycling

The role of the ocean in the carbon cycle

4.3.U2 In aquatic ecosystems carbon is present as dissolved carbon dioxide and hydrogen carbonate ions.

CO₂strikes water

Makes carbonic acid

2 x H⁺ ions break away

CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃^–

H⁺ ions explains how carbon dioxide reduces the pH of water.

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4.3 Carbon Cycling

Plants must have a constant supply of carbon dioxide (CO₂) to continually photosynthesise

4.3.U3 Carbon dioxide diffuses from the atmosphere or water into autotrophs

CO₂ from outside the leaf diffuses down the concentration gradient into the leaf

High

CO₂ Concentration gradient

Low

CO₂ moves through stomatal pores in the leaves of land plants*

atmosphere or water

Inside the leaf

atmosphere or water

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4.3 Carbon Cycling

Organisms carry out respiration to release energy in the form of ATP. Carbon dioxide is a waste product of cell respiration

4.3.U4 Carbon dioxide is produced by respiration and diffuses out of organisms into water or the atmosphere.

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4.3 Carbon Cycling

Some animals secrete calcium carbonate (CaCO₃) structures to protect themselves:

4.3.U10 Animals such as reef-building corals and mollusca have hard parts that are composed of calcium carbonate and can become fossilized in limestone.

Shells of molluscs
Hard corals exoskeletons

Limestone rock is a huge carbon sink.

The deposits are buried and compressed and eventually form limestone rock.

Imprints of the hard body parts remain in the rock as fossils.

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4.3 Carbon Cycling

If heated dried biomass or fossilized fuels will burn in the presence of oxygen

4.3.U9 Carbon dioxide is produced by the combustion of biomass and fossilized organic matter.

http://youtu.be/zDOq7QZXTdI

Slash & Burn: Why Amazonian farmers use fire

Fossil/Biomass fuel + O₂ → CO₂ + H₂O

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4.3 Carbon Cycling

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4.3 Carbon Cycling

Where would you rather live?

4.3.U5 Methane is produced from organic matter in anaerobic conditions by methanogenic archaeans and some diffuses into the atmosphere or accumulates in the ground.

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4.3 Carbon Cycling

Methanogens are archaean microorganisms that produce methane as a metabolic byproduct in anoxic conditions.

4.3.U5 Methane is produced from organic matter in anaerobic conditions by methanogenic archaeans and some diffuses into the atmosphere or accumulates in the ground.

Methanogens are found in a variety of anoxic environments:

Wetlands (e.g. paddies, swamps and mangroves)
Digestive tracts of animals (e.g. cows, humans and termites)
Marine and freshwater sediments (e.g. mud in the beds of lakes)
Landfill sites (in which organic matter has been buried)
Mining and natural gas leaks

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Methane is widely produced in anaerobic conditions as a waste product of a certain type of anaerobic respiration called methanogenesis

4.3 Carbon Cycling

4.3.U5 Methane is produced from organic matter in anaerobic conditions by methanogenic archaeans and some diffuses into the atmosphere or accumulates in the ground.

1) CO₂ + 4 H₂ → CH₄ + 2H₂O and 2) CH₃COO⁻ + H⁺ → CH₄ + CO₂

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https://www.nrel.gov/research/re-biomass.html

4.3 Carbon Cycling

How is biomass used in combustion?

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4.3 Carbon Cycling

Measurements indicate that the levels of atmospheric methane are increasing

4.3.U6 Methane is oxidized to carbon dioxide and water in the atmosphere.

It is estimated that, on average, methane persists in the atmosphere for 8.4 years.

Methane released into the atmosphere can be removed by a number of mechanisms.

methane+ hydroxyl radical → carbon dioxide + water

The most important process of methane removal is oxidation by hydroxyl radicals.

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It is not possible to measure the size of carbon sinks and the fluxes between them. Estimates are based on many different measurements are often published with large uncertainties as a result.

4.3 Carbon Cycling

Measurements indicate that the levels of atmospheric methane are increasing

4.3.A1 Estimation of carbon fluxes due to processes in the carbon cycle.

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Many field stations globally use the same standardised method. All stations show a clear upward trend with annual cycles.

4.3 Carbon Cycling