Sunday, October 21, 2012

Microbiome Part 6: Carbon Cycle

Note: This program first aired on Saturday October 20, 2012.

We are carbon based life forms, ask anyone at NASA*, Earth is covered with the stuff. Plants use carbon to hydrogen bonds to store the energy of the sun, that is the essence of photosynthesis. They also use the carbon they take out of the atmosphere (or fix) to build the structure of their bodies. We eat the plants (or the things that eat the plants, or even the things that eat the things that eat the plants), getting not only energy from those carbon to hydrogen bonds, but also the building blocks of our bodies as well. If that were the end of the story, life on Earth would have ended a long time ago, because we would have run out of our main source material, carbon.

Like all elements, carbon isn’t satisfied to simply stay put. It moves around the Earth and in and out of various forms over the course of time. The regularity of the journey of carbon is well documented, and referred to as the “carbon cycle”.  There are several sinks or reservoirs of  carbon; some are short term and others are long term. The long term sinks are thinks like the fossilized carbon that make up underground beds of fossil fuels we so love to burn, or limestone, a sedimentary rock formed at the bottom of the ocean from the chemical union of carbon dioxide gas and various minerals** present in sea water. Once carbon makes it to one of these reseviors, its stuck there for a while, baring some physical process that brings it to the surface of the earth for weathering. The parts of the carbon cycle we are more interested in today are the “fast” components; the movement of carbon dioxide out of the atmosphere, into plants (via photosynthesis), and then back out again (via respiration both by the plant and the things that eat the plant, but probably more importantly respiration by the microbes that decompose all that plant and animal matter, liberating and recycling the carbon and many other nutrients, making them available for use once more).

If you play out the thought experiment “what if all the bacteria disappeared tomorrow?”, the result might look something like this: piles and piles of dead plants and animals, just stacking up, and the amount of carbon available steadily dropping. What would essentially be happening is that carbon would be moving out of the atmosphere into biological material, via photosynthesis and the food chain. The carbon would be building up in a biological “carbon sink or resevior” in the form of organic biomass. If nothing ever decomposed, that carbon doesn’t get to move out of the sink its stuck in and the sink would get fuller and fuller as time went on.

The soil is the largest land based carbon sink in the carbon cycle (the oceans hold over an order of magnitude more, much of which gets locked eventually into abiotic limestone). Soil is comprised of both mineral elements and organic elements. Its those organic elements (dead plants and animals) that comprise the carbon sink part of the soil. One concern with the increase in average global temperature that is accompanying climate change is the idea that as temperature increases, bacterial decay of organic material in the soil will also increase, freeing up much of the carbon stored in the soil, liberating it back to the atmosphere. So would it make sense to try to get rid of all those soil bacteria and prevent them from doing their ecological job? Besides being impossible, I think this would be foolhardy. In the words of John Muir, the visionary nature man himself: “When we try to pick out anything by itself, we find it hitched to everything else in the Universe”.

The decomposition part of the carbon cycle isn’t something we like to think about, we think of decomposition as a bad think in the back of the refrigerator. Think too long about bacterial decay and you eventually come around to the question of what happens to your own body when you are done with it. So I can’t blame people from shying away from this aspect of the carbon cycle. I can only hope that by focusing on the grand cycles of the building blocks of life, we can learn to see death, and its accompanying decay, as liberation. Bacteria are literally the doulas and midwives of this cycle, ushering carbon, our most essential substance, from one phase of existence to the next. So the next time you dig in the garden, dump the compost or simply walk in the woods and smell that wonderful leaf decay smell, pause and reflect on, and revel in, the moment for what it is, a snap shot of grand loop that is much bigger than you.

Post Script:
*And just WHY are we strictly carbon based? Because carbon is unique among elements in that it has four spots each atom that can form bonds, and it can bond in a wide variety of shapes. Similar atoms (same column of the periodic table), like Silicon, also have four bonding spots, but tend to form crystal lattices, instead of the wide variety of that carbon can attain. It’s also apparently the fourth most abundant element around, meaning, there’s lots of it out there.

** Mostly calcium and magnesium.


Kenneth Todar “Online Text book of Bacteriology”

Totally worth looking at: from the American Society for Microbiology

From one of my favorite websites, NASA’s Earth Observatory

From the American Museum of Natural History

Ah the Union of Concerned Scientists…
Thanks for all your level headed thinking.