16.4B: The Carbon Cycle

The carbon cycle describes the flow of carbon from the atmosphere to the marine and terrestrial biospheres, and the earth’s crust.

Learning Objectives

• Outline the flow of carbon through the biosphere and abiotic matter on earth

Key Points

• Atmospheric carbon is usually in the form of CO₂. Carbon dioxide is converted to organic carbon through photosynthesis by primary producers such as plants, bacteria, and algae.
• Some organic carbon is returned to the atmosphere as CO₂ during respiration. The rest of the organic carbon may cycle from organism to organism through the food chain. When an organism dies, it is decomposed by bacteria and its carbon is released into the atmosphere or the soil.
• Carbon is also found in the earth’s crust, primarily as limestone and kerogens.

Key Terms

• lithosphere: The rigid, mechanically strong, outer layer of the earth; divided into twelve major tectonic plates.
• chemoautotrophic: An organism obtaining its nutrition through the oxidation of non-organic compounds (or other chemical processes); as opposed to the process of photosynthesis.
• carbon cycle: The physical cycle of carbon through the Earth’s biosphere, geosphere, hydrosphere and atmosphere that includes such processes as photosynthesis, decomposition, respiration and carbonification.

The carbon cycle describes the flow of carbon between the biosphere, the geosphere, and the atmosphere, and is essential to maintaining life on earth.

Figure: The Carbon Cycle: The carbon cycle describes the flow of carbon between the atmosphere, the biosphere, and the geosphere.

Atmospheric Carbon Dioxide: Carbon in the earth’s atmosphere exists in two main forms: carbon dioxide and methane. Carbon dioxide leaves the atmosphere through photosynthesis, thus entering the terrestrial and marine biospheres. Carbon dioxide also dissolves directly from the atmosphere into bodies of water (oceans, lakes, etc.), as well as dissolving in precipitation as raindrops fall through the atmosphere. When dissolved in water, carbon dioxide reacts with water molecules and forms carbonic acid, which contributes to ocean acidity. Human activity over the past two centuries has significantly increased the amount of carbon in the atmosphere, mainly in the form of carbon dioxide, both by modifying ecosystems ‘ ability to extract carbon dioxide from the atmosphere and by emitting it directly, e.g. by burning fossil fuels and manufacturing concrete.

Terrestrial Biosphere: The terrestrial biosphere includes the organic carbon in all land-living organisms, both alive and dead, as well as carbon stored in soils. Although people often imagine plants as the most important part of the terrestrial carbon cycle, microorganisms such as single celled algae and chemoautotrophic bacteria are also important in converting atmospheric CO₂ into terrestrial carbon. Carbon is incorporated into living things as part of organic molecules, either through photosynthesis or by animals that consume plants and algae. Some of the carbon in living things is released through respiration, while the rest remains in the tissue. Once organisms die, bacteria break down their tissues, releasing CO₂ back into the atmosphere or into the soil.

Marine Biosphere: The carbon cycle in the marine biosphere is very similar to that in the terrestrial ecosystem. CO₂ dissolves in the water and algae, plants and bacteria convert it into organic carbon. Carbon may transfer between organisms (from producers to consumers). Their tissues are ultimately broken down by bacteria and CO₂ is released back into the ocean or atmosphere.

NASA | A Year in the Life of Earth’s CO2: An ultra-high-resolution NASA computer model has given scientists a stunning new look at how carbon dioxide in the atmosphere travels around the globe. Plumes of carbon dioxide in the simulation swirl and shift as winds disperse the greenhouse gas away from its sources. The simulation also illustrates differences in carbon dioxide levels in the northern and southern hemispheres and distinct swings in global carbon dioxide concentrations as the growth cycle of plants and trees changes with the seasons. The carbon dioxide visualization was produced by a computer model called GEOS-5, created by scientists at NASA Goddard Space Flight Center’s Global Modeling and Assimilation Office. The visualization is a product of a simulation called a “Nature Run.” The Nature Run ingests real data on atmospheric conditions and the emission of greenhouse gases and both natural and man-made particulates. The model is then left to run on its own and simulate the natural behavior of the Earth’s atmosphere. This Nature Run simulates January 2006 through December 2006. While Goddard scientists worked with a “beta” version of the Nature Run internally for several years, they released this updated, improved version to the scientific community for the first time in the fall of 2014.

Geologic Carbon: The earth’s crust also contains carbon. Much of the earth’s carbon is stored in the mantle, and has been there since the earth formed. Much of the carbon on the earth’s lithosphere (about 80%) is stored in limestone, which was formed from the calcium carbonate from the shells of marine animals. The rest of the carbon on the earth’s surface is stored in Kerogens, which were formed through the sedimentation and burial of terrestrial organisms under high heat and pressure.