Table of Contents
The carbon cycle describes how carbon atoms move and transform between living organisms, the atmosphere, water, and rocks. It links biological processes (like photosynthesis and respiration) with chemical and geological processes (like dissolution in water and rock formation). Because it connects energy flow with matter cycling, it is central to understanding ecosystem function and current environmental change.
Main Carbon Reservoirs (Pools)
Carbon is stored in different “reservoirs” that exchange carbon at different rates:
- Atmosphere
- Mainly as carbon dioxide ($\mathrm{CO_2}$) and, in much smaller amounts, methane ($\mathrm{CH_4}$) and carbon monoxide ($\mathrm{CO}$).
- Reacts quickly with oceans and land vegetation on timescales of years to decades.
- Biosphere (Living and Dead Organic Matter)
- Plants, algae, cyanobacteria: store carbon fixed from $\mathrm{CO_2}$ in organic molecules (e.g. sugars, cellulose, lipids).
- Animals, fungi, protists, bacteria, archaea: store carbon in biomass; obtain it directly or indirectly from primary producers.
- Detritus (dead organic material): leaf litter, dead wood, humus, soil organic matter. Important medium-term carbon store.
- Oceans
- Surface ocean: exchanges $\mathrm{CO_2}$ rapidly with the atmosphere (days–years).
- Deep ocean: holds far more dissolved inorganic carbon than the atmosphere, but mixes with the surface more slowly (hundreds–thousands of years).
- Marine sediments and sedimentary rocks (e.g. limestone): huge long‑term carbon store, exchanging on geological timescales (millions of years).
- Geological Carbon (Rocks and Fossil Fuels)
- Carbonate rocks (limestones, dolomites) contain carbon in the form of carbonate ions ($\mathrm{CO_3^{2-}}$) bound to metals like calcium.
- Fossil fuels (coal, oil, natural gas) store organic carbon from ancient organisms that was not fully decomposed.
- Soil inorganic carbon (e.g. carbonates in arid soils) can also be an important regional store.
The sizes and exchange rates of these reservoirs determine how quickly atmospheric $\mathrm{CO_2}$ can change and how ecosystems respond.
Key Biological and Chemical Processes
Photosynthesis: Entry of Carbon into Biomass
Photosynthetic organisms (plants, algae, cyanobacteria) take up $\mathrm{CO_2}$ and convert inorganic carbon into organic compounds using light energy. In strongly simplified form:
$$
6 \,\mathrm{CO_2} + 6 \,\mathrm{H_2O} \xrightarrow{\text{light, chlorophyll}} \mathrm{C_6H_{12}O_6} + 6 \,\mathrm{O_2}
$$
Here, $\mathrm{C_6H_{12}O_6}$ represents a sugar (e.g. glucose) used to build other organic molecules.
Key aspects specific to the carbon cycle:
- Photosynthesis is the primary biological sink for atmospheric $\mathrm{CO_2}$ on ecological timescales.
- The rate of photosynthetic carbon uptake is often called gross primary production (GPP).
- The net amount of carbon retained in plant biomass after subtracting plant respiration is net primary production (NPP). This is the carbon available to other organisms (herbivores, decomposers, etc.).
Respiration: Return of Carbon to the Atmosphere and Water
Almost all organisms carry out cellular respiration, oxidizing organic molecules and releasing $\mathrm{CO_2}$ (or dissolved inorganic carbon in water):
$$
\mathrm{C_6H_{12}O_6} + 6 \,\mathrm{O_2} \rightarrow 6 \,\mathrm{CO_2} + 6 \,\mathrm{H_2O} + \text{energy}
$$
Relevance to the carbon cycle:
- Autotrophic respiration (by plants, algae, cyanobacteria) returns a significant fraction of the carbon they just fixed back to the atmosphere or water.
- Heterotrophic respiration (by animals, fungi, many bacteria and protists) returns carbon they obtained from their food.
- The sum of all respiratory $\mathrm{CO_2}$ releases from an ecosystem (plants + heterotrophs + microbes) is ecosystem respiration.
Decomposition and Mineralization
When organisms or their parts die, decomposers (bacteria, fungi, detritivores) break down organic matter:
- Fragmentation and digestion
- Soil animals and detritivores physically break organic material into smaller pieces and digest some of it.
- Microbial decomposition
- Microorganisms enzymatically degrade complex molecules (e.g. cellulose, lignin, proteins, lipids) into simpler substances.
- Mineralization
- The inorganic end products, including $\mathrm{CO_2}$ (or $\mathrm{CH_4}$ under anoxic conditions), are released back into the environment.
Not all carbon is decomposed at the same rate:
- Labile (easily degradable) carbon (e.g. sugars, amino acids) is broken down quickly.
- Recalcitrant (resistant) carbon (e.g. lignin-rich wood, some soil humus) decomposes slowly, allowing carbon to be stored for years to centuries.
Dissolution and Carbon Chemistry in Water
Carbon dioxide interacts with water to form several inorganic carbon species:
- Dissolution of $\mathrm{CO_2}$
$$
\mathrm{CO_2(g)} \rightleftharpoons \mathrm{CO_2(aq)}
$$ - Formation of carbonic acid and its dissociation
$$
\mathrm{CO_2(aq)} + \mathrm{H_2O} \rightleftharpoons \mathrm{H_2CO_3} \rightleftharpoons \mathrm{H^+} + \mathrm{HCO_3^-} \rightleftharpoons 2\,\mathrm{H^+} + \mathrm{CO_3^{2-}}
$$
These reactions:
- Allow the ocean to act as a buffer, taking up or releasing $\mathrm{CO_2}$ according to temperature, pH, and partial pressure differences between air and water.
- Determine the relative amounts of dissolved $\mathrm{CO_2}$, bicarbonate ($\mathrm{HCO_3^-}$), and carbonate ($\mathrm{CO_3^{2-}}$), which influences marine organisms that form shells and skeletons from calcium carbonate.
Sedimentation and Rock Formation
In aquatic environments, some carbon is transferred from short-term biological cycling into long-term geological storage:
- Biogenic carbonate formation
- Many marine organisms (corals, foraminifera, mollusks) precipitate calcium carbonate:
$$
\mathrm{Ca^{2+}} + \mathrm{CO_3^{2-}} \rightarrow \mathrm{CaCO_3(s)}
$$ - When these organisms die, their shells can accumulate as sediments and eventually form limestone and other carbonate rocks.
- Burial of organic matter
- A small fraction of organic matter escapes complete decomposition and is buried in sediments.
- Over millions of years, high pressure and temperature can transform this organic carbon into fossil fuels (coal, oil, gas).
These processes remove carbon from the short-term biological cycle for long periods (geological sequestration).
Volcanism and Weathering
Over very long timescales, geological processes return stored carbon to the atmosphere or remove it:
- Volcanic and tectonic outgassing
- Subduction of carbon-bearing oceanic crust and sediments leads to the release of $\mathrm{CO_2}$ through volcanoes and mid-ocean ridges.
- Chemical weathering of rocks
- Weathering of silicate and carbonate rocks consumes atmospheric $\mathrm{CO_2}$ and transfers carbon to dissolved forms that eventually reach the ocean, contributing to carbonate formation.
These slow processes help regulate atmospheric $\mathrm{CO_2}$ concentration over millions of years.
Methane Production and Oxidation
In oxygen-poor (anoxic) environments (e.g. wetlands, sediments, ruminant stomachs):
- Methanogenic archaea produce methane from simple substrates:
- Example:
$$
\mathrm{CO_2} + 4\,\mathrm{H_2} \rightarrow \mathrm{CH_4} + 2\,\mathrm{H_2O}
$$ - Methane can be:
- Oxidized back to $\mathrm{CO_2}$ by aerobic or anaerobic methanotrophic microorganisms.
- Released to the atmosphere, where it acts as a potent greenhouse gas before eventually oxidizing to $\mathrm{CO_2}$.
Methane fluxes are relatively small in terms of carbon mass compared with $\mathrm{CO_2}$, but climatically important.
Short-Term vs Long-Term Carbon Cycles
The carbon cycle can be viewed on at least two nested timescales:
Short-Term (Biological) Carbon Cycle
Timescale: hours to centuries.
Main features:
- Photosynthesis–respiration loop: Carbon cycles between atmosphere (or surface ocean) and living organisms via photosynthesis and respiration.
- Decomposition: Returns most organic carbon from dead matter to $\mathrm{CO_2}$ relatively quickly, especially in warm, moist environments.
- Soil and biomass storage: Some carbon persists in biomass (e.g. large trees) and soils for decades to centuries.
This short-term cycle is particularly relevant for ecosystem productivity, food webs, and seasonal patterns of atmospheric $\mathrm{CO_2}$.
Long-Term (Geological) Carbon Cycle
Timescale: thousands to millions of years.
Main processes:
- Burial of organic matter and its conversion into fossil fuels.
- Formation of carbonate rocks from shells and other carbonates.
- Weathering of rocks and volcanic outgassing that slowly move carbon between rocks, oceans, and atmosphere.
The long-term cycle has acted as a planetary thermostat, influencing Earth’s climate over geological history.
Major Carbon Fluxes Between Reservoirs
Fluxes describe the amount of carbon transferred per unit time (often in gigatons of carbon per year, Gt C/yr). The exact values are not crucial here, but the relative magnitudes and directions are.
Important natural fluxes:
- Between atmosphere and terrestrial biosphere
- Photosynthesis: atmosphere → plants.
- Respiration and decomposition: plants and soil organisms → atmosphere.
- Between atmosphere and ocean
- Gas exchange: $\mathrm{CO_2}$ dissolves into surface waters or is released, depending on partial pressure, temperature, and circulation.
- Biological uptake by phytoplankton: converts dissolved inorganic carbon into organic biomass.
- Within the ocean
- Biological pump: sinking of organic matter (dead plankton, fecal pellets) from surface to deep waters, where it is decomposed.
- Carbonate pump: sinking of carbonate shells that may eventually form sediments and rocks.
- Between geological and surface reservoirs
- Volcanic emissions: rocks → atmosphere.
- Weathering and sedimentation: atmosphere → oceans → sediments and rocks.
Terrestrial vs Marine Carbon Cycling
Terrestrial Carbon Cycle
Characteristics:
- Dominance of vegetation and soils
- Forests and grasslands store large amounts of carbon in biomass and soil organic matter.
- Climate dependence
- Temperature and moisture strongly influence photosynthesis, respiration, and decomposition.
- Disturbances
- Fires, insect outbreaks, storms, and land-use changes can rapidly release stored carbon.
Typical pathways:
- $\mathrm{CO_2}$ enters leaves via stomata and is fixed by photosynthesis.
- Carbon is allocated to leaves, stems, roots, and symbiotic organisms (e.g. mycorrhizal fungi).
- Herbivores and predators move carbon through terrestrial food webs.
- Litter fall and root turnover transfer carbon to soil, where decomposers act.
- Some carbon is respired; some becomes stable soil organic matter; a tiny fraction may be transported to rivers and eventually to oceans.
Marine Carbon Cycle
Characteristics:
- Phytoplankton-based primary production
- Microscopic algae and cyanobacteria fix $\mathrm{CO_2}$ in the sunlit surface layer (photic zone).
- Efficient microbial recycling
- A large fraction of organic matter is rapidly respired back to $\mathrm{CO_2}$ by bacteria and other organisms.
- Vertical transport
- The biological and carbonate pumps move carbon from surface waters to the deep ocean and sediments.
Typical pathways:
- $\mathrm{CO_2}$ dissolves in surface water and is converted to organic carbon by phytoplankton.
- Carbon moves through zooplankton, fish, and higher trophic levels.
- Dead organisms and fecal pellets sink; some is decomposed at depth, releasing $\mathrm{CO_2}$ to deep water.
- Only a tiny fraction reaches the seafloor and is buried in sediments.
Human Influences on the Carbon Cycle
Human activities have altered the carbon cycle, especially since the Industrial Revolution, by changing both fluxes and reservoir sizes.
Key anthropogenic processes:
- Fossil fuel combustion
- Burning coal, oil, and gas rapidly transfers long-stored geological carbon to the atmosphere as $\mathrm{CO_2}$.
- Land-use change
- Deforestation, forest degradation, conversion of grasslands and wetlands, and peat drainage reduce biomass and soil carbon stocks.
- Fires associated with land clearing release additional $\mathrm{CO_2}$ and other gases.
- Cement production and industrial processes
- Release $\mathrm{CO_2}$ through chemical reactions (e.g. calcination of limestone).
Consequences for the carbon cycle:
- Increased atmospheric $\mathrm{CO_2}$ and methane concentrations, intensifying the greenhouse effect and driving climate change.
- Enhanced ocean uptake of $\mathrm{CO_2}$, leading to ocean acidification, which affects marine carbonate chemistry and organisms that form shells or skeletons.
- Changes in terrestrial uptake:
- In some regions, regrowing forests and higher $\mathrm{CO_2}$ levels have increased net carbon uptake (carbon sinks).
- In others, deforestation and degradation make ecosystems net carbon sources.
Human activities, therefore, do not create or destroy carbon but redistribute it much faster than many natural geological processes, pushing the carbon cycle out of its former quasi-equilibrium.
The Carbon Cycle and Ecosystem Function
Within ecosystems, the carbon cycle is tightly linked to:
- Energy flow
- Organic carbon compounds carry the chemical energy captured by primary producers. As carbon moves through food webs and is respired, energy flows and is dissipated as heat.
- Nutrient cycles
- Carbon compounds provide the backbone for molecules that contain nitrogen, phosphorus, sulfur, and other elements. Decomposition and mineralization processes simultaneously recycle carbon and these nutrients.
- Climate–biosphere feedbacks
- Changes in temperature and precipitation alter photosynthesis and decomposition, which in turn change $\mathrm{CO_2}$ fluxes and influence climate.
Understanding the carbon cycle thus provides a framework for interpreting ecosystem processes, global environmental change, and the interactions between living organisms and the physical environment.