BY DR. JOSE PALU-AY DACUDAO
THE OFTEN mentioned carbon dioxide sinks consistently cited in literature, papers, and mass media are sea bottoms and peat bogs. Let’s examine this claim from a fundamental point of view.
Before going into the topic, we need to remind ourselves what happens in aerobic biological decomposition, one that occurs in an environment with lots of oxygen. Most of organic matter in the biosphere is cellulose, hemicellulose, and lignin. They are made by photosynthetic living organisms via photosynthesis, by fixing atmospheric carbon dioxide. For simplicity’s sake let the following famous formula represent this process.
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)
Decomposition is much like the opposite of the above. For simplicity’s sake let us assume the decomposition of glucose to represent the aerobic decomposition of organic material.
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]
Is there a net fixation of atmospheric carbon dioxide according to the equations above? Obviously no. Aerobic decomposition returns whatever carbon dioxide fixed by photosynthetic organisms back into the atmosphere.
Note however that in the equation for aerobic decomposition above, you need the presence of oxygen. What happens in the absence of free molecular oxygen (in anoxic conditions)?
Decomposition may still proceed, albeit more inefficiently and slowly. Certain anaerobic organisms have the ability to pry out the oxygen atoms from oxygen-rich compounds, such as sulfates and nitrates, and even carbon dioxide itself.
CO2 (carbon dioxide) + 4 H2 (diatomic hydrogen molecule) → CH4 (methane) + 2 H2O
[This may not be to the liking of people who advocate for the decrease in greenhouse gases, since methane is a more potent greenhouse gas than carbon dioxide.]
A more often cited example is calcium sulfate, CaSO4. It composes the common mineral gypsum. If you have had a house built recently, you might notice that the walls were built from a light-weight whitish, fire-proof, hard material. That’s calcium sulfate. It doesn’t catch fire because it is already fully chemically oxidized. However, it can be chemically reduced. If found in anoxic conditions, such as the bottom of the sea, certain anaerobic organisms can reduce it to sulfur and hydrogen sulfide, and take its oxygen atoms for use in oxidizing organic material for food.
The main difference from aerobic decomposition (see the formula above) is that instead of molecular O2, these anaerobes get their oxygen from sulfate (or nitrate or carbonate, or carbon dioxide) compounds. (To be continued)/PN
