Hydrogen, Oxygen, and Phosphorus

FIGURE 18.3 Principal global C pools and fluxes between them. (From Lai, 2008. Copyright 2008 with permission from the Royal Society.) 

In the atmosphere, carbon is mostly present as CO2, where it only represents a small percentage of the atmosphere (0.04% on a molar basis). Most of the terrestrial carbon in the soil and above the ground is stored in the forests, while the oceans contain the largest active pool of carbon near the Earth s surface, although the much larger deep ocean part of this pool does not exchange rapidly with the atmosphere. Most carbon released to the 

It can, however, contribute as a result of external influences like hydrothermal vents in locations where two tectonic plates are moving apart, or by uncontrolled deep-water oil well leaks. 

It has been inferred from carbon and sulfur dating that the concentration of oxygen in the Earth s atmosphere was less than I part per million volume (ppmv) prior to around 2.4 billion (Ga) years ago, whereas methane would have been present at levels of around 10 to 10 ppmv (compared with its present value of around 1.7 ppmv). Methane is generated in signihcant amounts by the anaerobic decomposition of organic matter in modem marine sediments, but is oxidised by sulfate under the ocean sea floor and never reaches the atmosphere. Prior to the watershed constituted by the appearance of oxygenic photosynthesis, the ocean had little sulfate to support anaerobic oxidation of methane, but as atmospheric oxygen and seawater sulfate levels rose, the consequent anaerobic oxidation of methane would have steadily reduced the net release of methane. 

Photosynthesizing organisms include, of course, green plants on land, but also the phytoplankton, notably the cyanobacteria, in the oceans, generating the air we breathe. An additional source of atmospheric oxygen is 

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Careful inspection of the structure of the reactant and the product reveals that they each have the same number of carbon, hydrogen, oxygen, and phosphorus atoms thus, they must be structural isomers. The enzyme must be an isomerase. Its name is triose phosphate isomerase. 

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