Photosynrhesis and the modern atmosphere

Photosynthesis in algae and higher plants provides a further vital service to man in that it is the sole replenisher of oxygen in the atmosphere. Indeed, it was the advent of oxygenic photosynthesis (this is oxygen-evolving photosynthesis as carried out by green plants, discussed in Section 1.2.1) that changed the atmosphere of the Earth from its primitive, reducing state containing virtually no frm oxygen to today’s breathabk air. During oxygenic photosynthesis, photosynthetic organisms absorb carbon dioxide (COz), fix it as carbohydrates (of empirical formula [CH,O]) and discard oxygen (OL) to the atmosphere, as shown in Fig. 1.1. During respiration, animals and plants (as well as algae) do the opposite-take in oxygen, ‘burn’ stor4 carbohydrates by the process of metabolism, hence obtaining the energy needed to sustain life. and discard carbon dioxide to the atmosphere. Animals and plants thus live in symbiosis, but not symmetrically so. If animal rife were suddenly to ce-, forests would reclaim agricultural land and weeds the cities, but photosynthesis would continue to cycle oxygen and carbon dioxide through the atmosphere, missing little more from the kingdom of Animalia than pollinating insects. If photosynthesis were suddenly to cease, it would be altogether another matter, because animal life could not survive for long without fresh supplies of oxygen. Some 105 net GtC' is fixed by photosynthesis on land and in the oceans each year, and this releases about 260 Gt of oxygen into the atmosphere. The atmosphere contains 1.2 x lo6 Gt oxygen, so the cycling time of oxygen through the biosphere is -4600 years. If photosynthesis were to down tools, the atmosphere would return to its primordial reducing state on this timescale as plants and animals respired and expired. Humans would start to suffer from hypoxia as the partial pressure of oxygen in the atmosphere fell from its current level of 21 kPa to anything much below 15 P a . This would hardly be our most urgent problem since we would run short of food in a matter of months. Even if we did not (say we made food from fossil fuels or biomass), the oxidative decay of dead trees and plants and organic carbon in soils would release the -2500 Gt of organic carbon currently sequestered in the biosphere into the atmosphere as C02 on a timescale of decades. Much of this would eventually dissolve in the oceans but the atmospheric concentration of C02 would be temporarily quadrupled and the global warming experiment would be fast-forwarded.