1.2: Carbon as a marker of organic matter and life

Microbial, plant or animal biomass is mostly composed of water, minerals and organic compounds; it therefore contains a large amount of hydrogen, oxygen, carbon and, to a lesser extent, nitrogen (Table 1). Life concentrates some of the elements that are not abundant on the Earth as a whole. For example, the atmosphere is rich in nitrogen and oxygen but contains little carbon and hydrogen, while the Earth’s crust, although containing oxygen and a small amount of hydrogen, contains little carbon and nitrogen. Thus, carbon is a sort of marker of organic matter (living or dead organisms).

Table 1. Approximate percentage of elements in living organisms (e.g., humans) compared to the earth’s crust or atmosphere [6].

Biosequestrated carbon is directly related to the biomass formed by photosynthetic organisms (cyanobacteria, green plants…). They absorb the sun’s energy, capture water (H₂O) and carbon dioxide (CO₂) and convert them into sugars and oxygen (O₂) in a process called photosynthesis (See Shedding light on photosynthesis and The path of carbon in photosynthesis). The equation for photosynthesis can be summarized as follows:

• 6 CO₂ + 12 H₂O + light → C₆H₁₂O₆ (glucose) + 6 H₂O + 6 O₂

The colossal amounts of living matter made continuously by photosynthesis are then used every day in a vast dynamic balance of life, death, nutrition, metabolism, growth, and decay [7]. Thus, the carbon fixed in plant biomass (i.e. biosequestered carbon) enters the food chain as it is used by a wide variety of organisms linked together by trophic relationships. Plants (producer organisms) are consumed by herbivorous animals (consumer organisms) with the help of associated microorganisms (microbiota). In the oceans, biomass enters the food chain through phytoplankton. Most living organisms use aerobic respiration{end-text}Characterizes a system (e.g., a living organism) or process (e.g., respiration) that requires _O2 oxygen. The term aerobic is also used in aeronautics, in the field of propulsion: an aerobic engine uses the oxygen of the ambient air as oxidizer for its operation. {end-tooltip}Aerobic respiration is a process which transfers chemical energy from glucose into ATP, the key molecule of cellular energy metabolism. Aerobic respiration is the catabolism of nutrients (glucose) into carbon dioxide, water and ATP, and involves an electron transport system in which oxygen is the final electron acceptor. The overall reaction of respiration is as follows:

• C₆H₁₂O₆ (glucose) + 6 O₂ + 36 (ADP+Pi) → 6 CO₂ + 6 H₂O + 36 ATP