The Carbon Cycle

 

Carbon can be found in elemental form in nature as an amorphous solid, graphite, fullerene or diamond.  Overall, carbon is the 6th most abundant element in the universe and constitutes 22.85% of the human body.  Over 10 million compounds of carbon have been synthesized in the laboratory and many thousands, if not many millions, are found in nature which attests to the tremendous versatility of the element (a quality that lends to the great versatility and diversity of life itself).  Because Earth is dynamic, carbon, like all elements, gets cycled through the environment and life plays a major role in this process.

 

The major reservoirs of carbon vastly overshadow what is biologically cycled and are fairly stable in ocean sediments and terrestrial rock.  Fossil fuel deposits are also a major reservoir @ 4,000 metric billion tons, but we are adding about 9 billion metric tons of its carbon to the atmosphere through combustion and other emissions each year.  The atmosphere's carbon, mostly in the form of CO₂, is the most rapidly cycled.  Atmospheric carbon has risen from 578 billion metric tons (as of 1700) to 766 billion metric tons (as of 1999) due to human emissions.  Life's total biomass carbon accounts for, approximately, a 1,000 billion metric tons (though it is potentially much higher depending on how many microorganisms there are on Earth, which is hard to estimate). 

Sink	Amount in Billions of Metric Tons
Atmosphere	578 (as of 1700) - 766 (as of 1999)
Soil Organic Matter	1500 to 1600
Ocean	38,000 to 40,000
Marine Sediments and Sedimentary Rocks	66,000,000 to 100,000,000
Terrestrial Plants	540 to 610
Fossil Fuel Deposits	4000

Estimates of Carbon Reservoirs

Though the major reservoirs of carbon are inorganic sources, they don't cycle substantially and are quite stable over geologic time.  The carbon that does actively cycle is almost entirely a biological affair.  On Earth the carbon cycle is one of the most essential for life because all life that we know about is made of proteins, lipids, carbohydrates and nucleic acids (all carbon based molecules).  To give a feel of the rate of turnover we're talking about here, the average carbon dioxide (CO₂) molecule gets cycled through the biosphere approximately once every 300 years.  (Something interesting to think about:  The molecules of our bodies have a turnover rate as well.  Every time you eat something your body incorporates new molecules and lets old molecules leave the body.  It's like having an old brick building that needs repairs.  So you get new bricks to replace the old, corroding bricks.  By swapping out the old bricks with new ones you are changing what the building is made of, but the building's shape and structure remain the same.  It's interesting that we feel like the same person from day-to-day even though our molecules are being replaced all the time!) 

  

We'll begin the cycle with the carbon fixers or primary producers who are the plants, algae, cyanobacteria, green bacteria, and purple bacteria.  The cycling activities of these organisms largely determines the potential productivity that an ecosystem can support.  Their main service is to take inorganic CO₂ and convert it into organic carbohydrates, lipids, proteins and nucleic acids that all other lifeforms require to get the energy and nutrition they need.  They play the essential role of introducing otherwise inaccessible forms of carbon into the food web.  Mostly this is done by photosynthesis, a remarkable process that converts sunlight into energy-rich molecules like sugars (Over 150 billion tons of dry organic matter is produced by photosynthesis each year!).  Other methods employed by microorganisms also convert inert CO₂ into organic matter such as methanogenesis (reduction of carbon dioxide to methane) and acetogenesis (reacting carbon dioxide to acetic acid).  These processes require no light, but do require special molecules, metabolic pathways and enzymes.  The special benefit of photosynthesis, if you appreciate breathing, is the byproduct oxygen.


  



Once inorganic carbon has been made into organic compounds the cycle is completed by consumers (like humans) and decomposers who, essentially, break down the organic compounds to carbon dioxide and water through respiration and fermentation.  Respiration uses oxygen to breakdown molecules, while fermentation breaks down organic compounds without oxygen.  Respiration is an interesting process because it is, in essence, a cell's version of an internal combustion engine minus the flame, releasing energy in a slower and more efficient manner than small explosions (like in an engine).

Decomposers play a rather under-appreciated, yet essential, role.  Imagine a world where things consumed, died and bodies piled up, but never decayed.  After a while Earth would be a ball of corpses with all the nutrients locked up in bodies.  Thankfully decomposers exist and return lifeforms to their fundamental elements!

Microbial decomposers are also able to break down plant polymers like lignin (wood) and cellulose (the most abundant biopolymer in the world) that no one else can digest.  They exist in the environment and also in the guts of termites and ruminants (cows, sheep, deer, goats, giraffes, camels, etc.) where they break down these polymers to their component sugars which animals can digest.  Without this special ability plant matter would accumulate and remain, mostly, unusable.

DISCUSSION

So now that we've taken a dose of science what is the take-home-message?  What can the carbon cycle teach us about life on Earth?

..............One organism's poo, is another organism's food..............

Perhaps a little too crude, but it captures the essence.  Life is like a wheel of complementary colors.  Diversity is the key because in the process of growing, digesting, regenerating, transforming and just living and dying in general, one organism requires another organism(s) that offsets their effects so that Earth can maintain some kind of balance.  If animals breathed and there were no photosynthesizers, eventually, we'd all suffocate in our own carbon dioxide.  Similarly, if plants converted all the carbon dioxide to oxygen, they'd suffocate too.  We need each other, we need diversity, to survive because being alive means producing waste.  If your environment is healthy and diverse there's bound to be some organism out there that can use your "trash" and transform it back into something usable for you.  To complete the cycle we help other organisms by putting to good use their waste returning it to an accessible form for them.  Think of cheese, yogurt, bread, soy sauce, pickled vegetables, alcoholic beverages, and all the other "waste products" of microorganisms that we eat as delicacies.  In reality, we're just participating in the carbon cycle.  

Sources:

http://www.eoearth.org/article/Carbon
http://en.wikipedia.org/wiki/Compounds_of_carbon
http://www.physicalgeography.net/fundamentals/9r.html
Microbial Ecology: Fundamentals and Applications, 4th ed by Ronald M. Atlas & Richard Bartha

-Seth Commichaux