6.5.5 The Carbon Cycle

The role of organisms in the Carbon Cycle

• Carbon is constantly being recycled around the biosphere so that the number of carbon atoms in the biosphere is essentially constant; carbon atoms merely swap from one compound to another by the various processes in the carbon cycle
• Carbon is stored in various forms:
  • In the atmosphere (as CO₂)
  • In sedimentary rocks
  • In fossil fuels like coal, oil, and gas; coal is almost pure carbon
  • In soil and other organic matter
  • In vegetation (e.g. as cellulose)
  • Dissolved in the oceans (as CO₂)

Photosynthesis

• Autotrophs use the energy of sunlight to 'fix' carbon dioxide, turning its carbon into sugars and other organic molecules
• This removes carbon from the atmosphere
• The Calvin cycle is where CO₂ is fixed, by the enzyme Rubisco, which carboxylates RuBP
• Terrestrial plants use gaseous CO₂ directly from the air
• Aquatic organisms use CO₂ dissolved in water
• As much CO₂ is fixed from ocean microorganisms, as from terrestrial plants

Sedimentation

• Plants that die are not fully decomposed by saprobionts; their bodies form layers of sediment that can accumulate over millions of years, locking carbon into the ground
• This sediment is a store of energy and can form fossil fuels like peat and coal
• Aquatic organisms that die also form sediments on the sea bed; these can go on to form other fossil fuels like oil and gas
• Shells and other calcium-containing body parts can form sedimentary rocks such as limestone
• The existence of life forms over billions of years has shaped the biosphere, in that their remains are still being recycled

Respiration

• All life forms respire, including autotrophs
• Heterotrophs rely on respiration for all their energy needs
• Respiration puts CO₂ into the atmosphere, in the opposite direction to photosynthesis
• CO₂ is released in the Link Reaction and the Krebs Cycle of aerobic respiration
• Anaerobic respiration also releases CO₂ into the atmosphere, via fermentation by yeast, moulds and bacteria

Feeding

• Carbon is passed from autotroph to heterotroph during feeding
• Carbon is also passed from primary consumer to secondary consumer
• Biomass transfer always includes the transfer of carbon, the main element in biomass

Decay & Decomposition

• Dead plants and animals are fed upon by detritivores and decayed by saprophytes
  • Releasing carbon into the surroundings
  • Supplying carbon to the detritivores
  • Supplying carbon to the saprophytes

• Waste matter such as faeces and urine is used by decaying saprobionts
• Such processes can release CO₂ back into the air

The roles of organisms in the carbon cycle

Burning fossil fuels

• Since the mid 19^th century, humans have extracted and burned increasing amounts of fossil fuels from the Earth
• CO₂ is being returned to the atmosphere faster than it can be absorbed by plants and aquatic producers
• The CO₂ level in the atmosphere is approximately double that of 800,000 years ago
• Warmer temperatures mean that less CO₂ can be dissolved in the oceans, so is released into the air
• This has caused dramatic climate change and affected many other species, mainly through changing habitats