Chapter 4.3
Carbon Cycling
Victoria Barros
Understandings, Applications and Skills
4.3.U1 Autotrophs convert carbon dioxide into carbohydrates and other carbon
compounds
4.3.U2 In aquatic habitats carbon dioxide is present as a dissolved gas 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
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 anaerobic
conditions in waterlogged soils
4.3.U8 Partially decomposed organic matter from past geological eras was converted
into oil and gas in porous rocks or into coal
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 molluscs have hard parts that are
composed of calcium carbonate and can become fossilized in limestone
4.3.A1 Estimation of carbon fluxes due to processes in the carbon cycle
4.3.A2 Analysis of data from atmosphere monitoring stations showing annual
fluctuations
4.3.S1 Construct a diagram of a carbon cycle
Notes from Textbook
Carbon Fixation
 Carbon Fixation - the process by which plants turn inorganic carbon molecules
(i.e. carbon dioxide) into organic compounds (i.e. carbohydrates)
 Autotrophs convert carbon dioxide from the atmosphere to carbohydrates,
lipids and C-compounds that they require
- Thus reducing CO2 in the atmosphere
- Current CO2 in atmosphere is 0.039%
 Tends to be lower where photosynthesis rates are higher
Carbon Dioxide in Solution
 In aquatic habitats carbon dioxide is present as a dissolved gas and hydrogen
carbonate ions
- CO2 is soluble in water
 It may remain in water as a
dissolved gas, or as H2CO3
(Carbonic acid)
 H2CO3 can further
dissociate to form H+ and
HCO3- ions
o This reduces the pH
of the water, thus
resulting in a rise in
ocean acidity and occurrences of acidic rain
 Aquatic autotrophs absorb the dissolved CO2 or H2CO3 and use
them to make different C-compounds
Absorption of Carbon Dioxide
 Carbon dioxide diffuses from the atmosphere or water into autotrophs
 Autotrophs require CO2 for photosynthesis and other life essential processes
- Photosynthesis reduces concentration of CO2 inside the autotrophic
organism, thus setting up a concentration gradient between the cells in
the autotroph and its surrounding
- Diffusion of CO2 happens into autotroph from atmosphere (or water)
around it
- On land the diffusion of CO2 occurs through the stomata of the leaves
- In water the entire surface of the leaves and stem is permeable to CO 2,
thus diffusion happens everywhere
Release of CO2 from Cell Respiration
 Carbon dioxide is produced by respiration and diffuses out of organisms into
water or the atmosphere
- CO2 is a waste product of aerobic respiration by cells that can be
grouped by trophic level
 Non-photosynthetic producer cells (e.g. root cells in plants)
 Animal cells
 Saprotrophs that decompose dead organic matter (e.g. fungi cells)
Methanogenesis
 Methane is produced from organic matter in anaerobic conditions by
methanogenic archaeans and some diffuses into the atmosphere
 Methane history:
- Discovered by Alessandro Volta in 1776 as he collected bubbles of gas
emerging from the mud in a reed bed
 Methanogenesis – the process of making if methane by bacteria or other living
organisms
1. Bacteria convert organic matter into organic acids, alcohol, hydrogen
and carbon dioxide
2. Then bacteria take organic acids and the alcohol to produce acetate,
hydrogen and carbon dioxide
3. Archaeans produce methane from acetate, hydrogen and carbon dioxide
𝐶𝑂2 + 4𝐻2 → 𝐶𝐻4 + 2𝐻2 𝑂
𝐶𝐻3 𝐶𝑂𝑂𝐻 → 𝐶𝐻4 + 𝐶𝑂2
 Archaeans are found in anaerobic environments:

⇒ Mud on shore of lakes, wetlands, swamps, guts of termites
and of other mammals such as sheep, landfills, etc.
Methane produced from archaeans partly diffuses into the atmosphere
- The methane that doesn’t diffuse and methane produced from organic
waste, instead tends to be burned by fuel
Oxidation of Methane
 Methane is oxidized to carbon dioxide and water in the atmosphere
 Oxidation – the process of chemically binding an oxygen to an atom or
molecule
 It takes around 12 years for a methane molecule to naturally oxidize in the
stratosphere due to the reactivity of O atoms and OH+ ion, thus regulation the
amounts of methane in the atmosphere
Peat Formation
 Peat – a brown, soil-like material on acidic grounds consisting of decomposed
vegetable matter
 Peat forms when organic matter is not fully decomposed because of anaerobic
condition in waterlogged soils
 In many soils all organic matter is decomposed by saprotrophic bacteria or
fungi (using oxygen for respiration)
 In some environments water fails to drain out of soils thus becoming
waterlogged and anaerobic
- Saprotrophs and methanogens can’t thrive in such conditions
- Dead organic matter isn’t fully decomposed
 In large quantities, when accumulated it becomes compressed to
form peat
 3% of the Earths land is composed of peat
 Is found in large amounts, in places up to 10m+ deep
- Acidic conditions tend to develop
Fossilized Organic Matter
 Partially decomposed organic matter from past geological eras was converted
into oil and gas in porous rocks or into coal
 C and C-compounds are very chemically stable, thus can resign in rocks for
millions of years
 Deposits of such from past geological occur from incomplete decomposition of
organic matter thus the buried sediments over time become rock or
alternatively oil and gas
 Coal:
- Formed when deposits of peat are buried by other sediments
- Peat when buried is heated, thus gradually turning into coal
 Large coal deposit example happened during the Carboniferous
when sea level rose and fell, forming costal swamps
 Oil and natural gas:
- Formed in mud at the bottom of seas and lakes by anaerobic conditions
thus incomplete decomposition
- As deposit grows it heats up and gets compressed
- Chemical reactions occur producing crude oil and natural gas
- Methane is the largest natural gas deposited
- The deposits rest in shale rock or other porous rock, which prevents
escape
Combustion
 Carbon dioxide is produced by the combustion of biomass and fossilized organic
matter
 In the presence of oxygen, organic matter will set light and burn if heated
- Oxidation reactions occur which are called combustion
- Combustion – when a C-compound gets oxidized and yields heat, light,
water and carbon dioxide or carbon monoxide
- In complete combustion, the products are CO2 and H2O
 Combustion of biomass releases carbon dioxide (e.g. forest fires)
⇒ Natural or man made
 Fossil fuels (coal, oil, gas) are burned as fuels
Limestone
 Animals such as reef-building corals and molluscs have hard parts that are
composed of calcium carbonate and can become fossilized in limestone
 Some animals have CaCO3 in their complexion
- Molluscs shells are composed of CaCO3
- Hard corals that build reefs produce their exoskeletons by secreting
CaCO3
 When these animals die the soft parts decompose quickly
 When these animals die the calcium carbonate an go two ways:
- In acidic conditions it dissolved
- In neutral or alkaline conditions it can get deposited on the sea bed and
be fossilized
 Limestone rock is formed by CaCO3 precipitation in the water, and the
deposited animal parts become fossilized in it
- Around 10% of sedimentary rock on Earth is limestone
- 12% of CaCO2 is C so thus there is a lot of C locked up in the limestone
Carbon Cycle
Diagram: Carbon cycle for terrestrial ecosystems
CO2 in Atmosphere
Cell respiration in
producers
Combustion of Fossil Fuels
Photosynthesis
Cell respiration
in consumers
Cell respiration in
saprotrophs and
detritivores
Death
Egestion
Carbon in dead
organic materials
Coal, Oil and Gas

Incomplete decomposition and fossilization of
organic matter
Carbon Cycle – the series of processes by which carbon compounds are
interconverted in the environment (as shown in diagram above)
-


Pool – a reserve of an element
- Can be organic or inorganic
- CO2 in atmosphere is inorganic pool of C
- Biomass of producers in an ecosystem is an organic pool
- Textboxes represent a pool
Flux – the transfer of an element from one pool to another
- Carbon flux example: the absorption of CO2 from atmosphere and its
conversion by photosynthesis to plant biomass
- Labelled arrows represent fluxes
Carbon Fluxes
 While the carbon cycle shows the movement of pools, it does not give the
quantities of the flux
- No exact quantity can be obtained, only estimates
- Estimates are based on measurements in individual natural ecosystem or
in mesocosms
 Global C fluxes are very large
- Estimates are made in gigatonnes
- 1 gigatonnes = 1x1015 grams
Environmental Monitoring
 CO2 and CH4 concentrations in the atmosphere have very important effects
 CO2 concentration affects photosynthesis and pH of sea water
 Both CO2 and CH4 concentrations affect global temperatures, thus the extent
and amount of ice sheets at the poles
- Indirectly affecting sea levels and coast line positions
- Indirectly affecting heat in oceans thus ocean currents, distribution of
rainfall and weather extremities
 Global Warming Argument
- CO2 concentration of the atmosphere is currently higher than any time in
the past 20 million years
- Human activities have increased the CO2 and CH4 concentrations in the
Earth’s atmosphere
- Therefore human activity will cause CO2 to rise from 397 to 600
micromoles per mole by the end of the century
Tends in Atmospheric CO2
 Data from atmospheric measuring stations is available to the public
 There are long term and annual trends and fluctuations
 Public can analyze data to make evaluation