BY DR. JOSE PALU-AY DACUDAO
IN PREVIOUS articles, we identified several processes that create carbon sinks.
The basic principle is this:
The more photosynthesis [6 CO2 (carbon dioxide taken from the atmosphere in a geological instant of a few decades or less) + 6 H2O (water) → C6H12O6 (organic substance) + 6 O2 (oxygen)] or the less decomposition (or burning) [C6H12O6 (organic substance) + 6 O2 (free molecular oxygen in the atmosphere) → 6 CO2 + 6 H2O (water) [in a geological instant, of days to a few years] the more carbon dioxide is taken away from the atmosphere.
Biological decomposition is much slowed down under anoxic (lacking free oxygen) conditions. As seen from the equation above, free molecular oxygen facilitates decomposition.
Thus, geological areas and ecosystems that are anoxic act as carbon sinks. In continents, peat bogs and anoxic swamplands and marshes act as carbon sinks. Probably to an even greater extent, so do anoxic sea bottoms. Any organic material that sinks to anoxic sea bottoms takes a long time to decompose, if ever. A substantial amount eventually becomes carbon-rich petroleum, as oxygen and hydrogen get squeezed out by high pressure and heat under anoxic conditions.
In terms of organic compounds, the biggest carbon sink is lignin, the stuff that makes wood woody.
Most of organic matter in the biosphere is cellulose, hemicellulose, and lignin. Lignin is by far the most difficult to biologically decompose. Why? Lignin is composed of cross-linked polymers of aromatic phenols (carbon-chains arranged in a ring), stacked up on top of each other. This makes lignin hydrophobic and protects its innards from water borne enzymes (meaning all enzymes). A microbe needs to introduce enzymes dissolved in water into a substance in order to decompose it. (That’s why wood stays as wood for a long time, compared to leaves made of cellulose and hemicellulose, which decompose quickly.)
In addition, lignin is extraordinarily rich in carbon. Lignin is made up (by atoms) of almost equal parts of carbon and hydrogen, with oxygen just one third of the carbon. Note that glucose only has equal parts of carbon and oxygen atoms and with hydrogen atoms double the number of carbon atoms.
People that crave to remove carbon dioxide from the atmosphere may also cheer the fact that about 30% of the biomass on our continents is hard-to-decompose lignin. Every woody tree and shrub makes it.
If buried under anoxic conditions, lignin eventually transforms to peat, then coal, and ultimately elemental carbon (graphite), in the process of carbonization. Elemental carbon is quite difficult to oxidize and does not decompose.
[Note: When obtaining energy from organic matter in the process of metabolism, the ‘currency’ of energy used by living organisms is Adenosine Triphosphate or ATP. ATP comes from the oxidation of hydrogen, not carbon. In other words, carbon makes up the structure of living organisms but is not oxidized as food. So metabolism tends to create carbon-rich compounds.] (To be continued)/PN
