Carbon Cycle

 

Carbon Cycle

Carbon cycle represents the movement of carbon in elemental and combined states on earth. Carbon can be present in reduced forms, such as methane (CH4) and organic matter, and in more oxidized forms, such as carbon monoxide (CO) and carbon dioxide (CO2).  Carbon is a principal element in cell systems. Microorganisms have an important role in all aspects of the carbon cycle.  Carbon cycle can be defined as the process where carbon compounds are interchanged among the biosphere, geosphere, pedosphere, hydrosphere, and atmosphere of the earth.

In general, the carbon cycle can be divided (depending on how long it takes to occur) into two types: short term and long term.

Short term carbon cycling involves the annual changes that occur within the atmosphere, terrestrial ecosystems, and the marine ecosystem. This is named as short term since the movement of carbon across reservoirs only takes relatively short time (minutes, hours, days, months, or years).

Long term carbon cycling is slower since it takes thousands to millions of years to occur.  The excess carbons from the short term cycling are stored into the “long term” reservoir until they are removed after a very long time.  Formation of hydrocarbon which we use as fuels are examples.

Carbon Cycle Steps

Following are the main steps that are involved in the process of the carbon cycle

1. Entry of Carbon into the Atmosphere - Carbon enter in the form of carbon dioxide in to atmosphere through the process of respiration (process by which organisms release energy from their food) and combustion (process of burning).

2. Carbon Dioxide Absorption by Producers - Next, carbon dioxide enters the photosynthetic process. Photosynthetic organisms like plants and algae (producers) transform carbon dioxide into sugars for energy use and for their own reproduction.  Nearly all forms of life on Earth depend on the production of sugars from solar energy and carbon dioxide (photosynthesis). During photosynthesis, plants, algae, photosynthetic bacteria, etc absorb carbon dioxide and use sunlight to create carbohydrate for building plant structures.

Carbon enters the biosphere during photosynthesis:

CO₂ + H₂O ---> C₆H₁₂O₆ + O₂ + H₂O

3. Passing of the Carbon Compounds in the Food Chain - Carbon compounds from the plants (producers) through the food chain reach animals when they consume the primary producers. 

4. Return of the Carbon to the Atmosphere - Next is the return of the carbon to the atmosphere.  This occur through respiration and decomposition.
The plants and animals through the process of respiration, the reverse of photosynthesis, releases the energy contained in sugars for use in metabolism and thereby carbohydrate is converted back to carbon dioxide.

Carbon is returned to the atmosphere during cellular respiration:

C₆H₁₂O₆ + O₂ + H₂O ---> CO₂ + H₂O + energy

Carbon is returned to the atmosphere also by the action of decomposers (bacteria and fungi) acting upon the the dead remains of plants and animals.

(www.onlinesciencenotes.com) 

There are various reservoirs of carbon and they are 

1. Atmosphere: The carbon present in the atmosphere of earth in the form of CO₂.  
2. Terrestrial Biosphere:  Carbon incorporated in the living beings like plants and animals living on the earth’s surface.
3. Ocean: the creatures living in the oceans reserves a certain amount of carbon and inorganic carbon is dissolved in the waters and the carbon is present at the surface of the oceans.
4. Geosphere: The excessive carbon is trapped deep within the earth’s crust in the form of various sediments of minerals and fossil fuels. Sediments of limestone store a vast amount of carbon in them.
5. A vast amount of the carbon is also trapped in the inner parts of earth. The mantle and core of earth have carbon.  Carbon from the lower mantle often leaks into the atmosphere by volcanic activities.

Carbon Cycle in the Atmosphere - In the atmosphere of the earth, the major quantity of carbon is present in the form of carbon dioxide and methane. Both are greenhouse gases and cause the atmospheric heating. Carbon dioxide is absorbed by the terrestrial and oceanic biospheres through photosynthesis, Plants in the terrestrial biosphere and cyanobacteria and algaes in the oceanic biospheres. Carbon dioxide, also falls down with the pouring of the rain.

Carbon Cycle on Land, Terrestrial Biosphere - Carbon is present in all the living creatures living on the earth surface. The process of photosynthesis involves the absorption of CO₂ by primary produces to produce carbohydrates and these are passed along the food chain to consumers. Majority of the carbon exists in the body in the form of carbon dioxide through respiration. Then decomposers eat the dead organism and return the carbon from their body back into the atmosphere.

Oceanic Carbon Cycle - Oceans absorb more carbon than it gives out and thus act as a “carbon sink.” Marine animals convert carbon to carbonate in their hard shells which accumulate on the seafloor and are eventually converted to limestone. When limestone rocks are exposed to air, they get weathered and the carbon is released back into the atmosphere as carbon dioxide.

Role of microbes in Carbon Cycle  

(Microbiology by Prescott, Harley and Klein)

Two fundamental processes driving the cycle are carbon fixation and carbon respiration.

Carbon fixation

Carbon fixation occur through the activities of photoautotrophic and chemoautotrophic microorganisms.   

Photosynthesis - Carbon fixation occurs through the activities of cyanobacteria and green algae, photosynthetic bacteria (e.g., Chromatium and Chlorobium), and aerobic chemolithoautotrophs. Among these the major carbon fixers are the cyanobacteria, green algae and photosynthetic bacteria. 

There are different types of photosynthetic bacteria that are widely distributed in both terrestrial and aquatic environments.  In these habitats, these organisms act as primary producers and play a crucial role in the food chain.  Examples are blue-green algae or Cyanobacteria such as Synechococcus elongates, Microcystis aeruginosis, Nostoc punctiforme, Anabaena variabilis, etc, purple bacteria photosynthetic Proteobacteria such as Rhodobacter sphaeroides, Rhodobacter capsulatus, Chromatium okenii, etc, photosynthetic Heliobacteria, green sulfur bacteria such as Chlorobium tepidum, Chlorobium phaeovibrioides, etc, filamentous green non-sulfur bacteria such as Chloroflexi.

Methanogenesis and methylotrophy- Some methanogenic archaea or methanogens convert carbon dioxide anaerobically to methane.  Methane formation, termed as methanogenesis is carried out by a group of obligately anaerobic archaebacteria called the methanogens such as Methanospirillum, Methannococcus, etc. Methane is released to the atmosphere. 

The methanogens occur in habitats such as saturated soils in wetlands, paddy fields, marshes, landfills and inside the gut of termites and ruminants. In saturated soils as paddy field, the methanogens produce methane autotrophically- through CO₂ fixation, or heterotrophically by utilising C1 and C2 substrates such as acetate, methanol and formate which are produced during decomposition of dead organic matter from plants and animals.

The basic metabolic pathway used by methanogens is:

4H₂ + CO₂   --->  CH₄ + 2H₂O

Methylotrophs are organisms that utilizes simple methyl compounds (such as methane or methanol) as a source of carbon and of energy.

Examples of methylotrophs are Methanosarcina, which can both utilize and produce methane, Methylococcus capsulatus, which requires methane to survive and Pichia pastoris, a biotechnologically important model organism that can use methanol as a carbon and energy source.

Some methylotrophs can degrade the greenhouse gas methane and they are the methanotrophs. Methanotrophs metabolize methane as their only source of carbon and energy. Methylococcus capsulatus is an example.

CH₄ +O₂    --->   CH₃OH     --->     HCHO    --->   HCOOH     --->  CO₂+H₂O

Acetogenesis – a group of facultatively chemoautotrophic anaerobic bacteria reduce CO₂ with H₂ to from acetate.  Examples are Clostridium thermoaceticum and Acetobacterium woodii.  Energetically this is less efficient than methanogenesis. 

Carbon Monoxide cycling – The major source of CO in the atmosphere is the photochemical oxidation of methane and other hydrocarbons.  Trace amounts of CO are also formed biologically during microbial and animal respiration.  An obscure photochemical side reaction during photosynthesis in photosynthetic microbes and plants also release CO. 

Some of the CO formed are photochemically converted to CO₂.  Microbial processes are mainly responsible for the CO conversion both in ocean and in soil.  Both aerobic and anaerobic microbes are responsible for this. 

Aerobically the carboxydobacteria such as Pseudomonas carboxydoflava and Pseudomonas carboxyhydrogena utilize CO as both carbon and energy source. 

CO+H2O    --->   CO2+H2

Anaerobically, some methanogens such as Methnosarcina barkeri reduce CO to CH₄

CO+3H2    --->  CH₄+ H2O

Sometimes CO is reduced to acetate by acetogenes such as Clostridium thermoaceticum

2CO+3H2 --->   CH₃COOH

Carbon respiration and decomposition

Carbon dioxide is released from plants and animals by respiration, great quantities of carbon dioxide result from decomposition of organic matter by microorganisms. Fixed carbon is returned to the atmosphere by a variety of different pathways of respiration of both autotrophic and heterotrophic organisms and also by decomposition of organic material by heterotrophic microorganisms.  The microbes utilize organic materials of either plant, animal or microbial origin as a substrate for metabolism, retain some carbon in their biomass and release the rest as metabolites or as CO₂ back to the atmosphere.  Soil is a great reservoir for carbon with appreciable quantities of complex organic material known as humus which is stable.

Fermentation

In anaerobic environments, microorganisms utilize carbon compounds to yield energy in a process known as fermentation.  Fermentation is a complex series of conversions that brings about the conversion of sugar to CO₂ and alcohol and is carried out by yeasts and different bacteria.  Fermentation plays a major role in the carbon cycle of anaerobic sediments in freshwater, sewage fermenters, or the digestive apparatus of vertebrates.