Topic 4.3 Carbon Cycling
Carbon Fixation
• Autotrophs convert carbon dioxide into
carbohydrates and other carbon compounds
– Absorb CO2 from the atmosphere and aquatic
envrionments
• The mean CO2 concentration in the
atmosphere is approximately .039% but is
lower above parts of the Earth where
photosynthesis rates are high.
Carbon Dioxide in a solution
• In aquatic habitats carbon
dioxide is present as a
dissolved gas and hydrogen
carbonate ions
• CO2 can be a dissolved gas or
can combine with water to
form carbonic acid(H2CO3)
• This can reduce the pH of
water
• Both of these forms of carbon
can be used in the process of
photosynthesis.
Exchange of Carbon
• Photosynthesis – Autotrophs absorb CO2
through diffusion from the atmosphere or
water
– Reduces the amount of carbon in the atmosphere
or water
• Cellular Respiration – Autotrophs create CO2
and diffuse it to the atmosphere or water
– Increases amount of carbon in the atmosphere or
water
Methanogenesis
• Methane is produced from organic matter in
anaerobic conditions by prokaryotes and diffuses
into the atmosphere.
• 3 groups of anaerobic prokaryotes are involved
– Bacteria that convert organic matter into a mixture of
organic acids, alcohol, H, and CO2
– Bacteria that use the organic acids and alcohol to
produce acetate, CO2, and H
– Archaeans that produce methane from CO2, H, and
acetate
CO2 + 4H2  CH2 + 2H20
CH3COOH  CH4 + CO2
Archaeans
• These archaeans are found in many anaerobic
environments
– Mud along shores and in the beds of lakes
– Swamps, mires, mangrove forests and other wetlands
where soil and peat deposits are waterlogged
– Guts of termites and ruminant mammals such as cattle and
sheep
– Landfill sites where organic matter is in wastes that have
been buried
Oxidation of methane
• When methane is released into the atmosphere they
remain for approximately 12 years
• The methane molecules become oxidized in the
stratosphere and becomes water
• This is why atmospheric concentrations are not high,
despite large amounts of production of methane
• Peat forms when organic matter is not fully
decomposed because of anaerobic conditions in
waterlogged soils
– In many soils all organic matter is digested by saprotorphic
bacetria and fungi.
– In some envrionments water is unable to drain out of soils
– These become very acidic which inhibit saprotrophs and
methanogens from breaking down that matter
Fossilized organic Matter
• This partially decomposed organic matter from past
geological eras was converted into oil and gas in porous
rocks or even coal
• Coal is formed when deposits of peat are buried under
other sediments.
– Peat is heated and compressed over thousands or millions
of years and become coal
– Many coal deposits are ancient swamps
• Oil and Natural Gas
– Formed in the mud at the bottom of seas and lakes.
– Compression and heating leads to chemical changes and
the production of methane and oil.
– Deposits are found where there are porous rocks that can
hold them such as shales and impervious rocks above and
below the porous rocks.
– Fracking
• Combustion –
– Carbon Dioxide is produced by the combustion of
biomass and fossilized organic matter
• Oxidation reaction at high temperatuers
– Some fires are natural and periodic in forests or
grasslands.
• In these areas trees and other organisms are often well
adapted to fires and regenerate communities rapidly
afterwards
• Limestone
– Animals such as reef-building corals and molluscs
have hard parts that are composed of calcium
carbonate(CaCO3) and can become fossilized in
limestone
– When these animals die their soft parts
decompose quickly.
– Limestone makes up approx. 10% of all
sedimentary rock on earth.
Carbon Fluxes
• The amount of carbon in each “pool” cannot
be preciecely measured but they can be
estimated.
• The amount of carbon in our world is so large
the estimates are in gigatonnes (1x1015 grams)