Atmospheric Oxygen and the Biosphere

Abstract

Atmospheric oxygen is generated photosynthetically at such a rapid rate tha the O2 content of the atmosphere would be doubled in a few thousand years if all the photosynthetic O2 were allowed to accumulate. It is not. the rate of O2 production is nearly balanced by the rate of O2 consumption by respiration. Together these processes constitute a very rapid geochemical cycle. The cycle is not, however, completely closed. A few tenths of a percent of the photosynthetically produced organic matter are buried with marine sediments, and an equal percentage of the photosynthetically produced O2 remains as the net O2 generated by the biological cycle. This O2 is consumed by the oxidative weathering of rocks and by the burning of volcanic gases. The reason why the partial pressure of oxygen in the atmosphere is 0.2 atm seems to be related to a feedback mechanism involving (PO4)3-, a nutrient that limits the biological productivity of the oceans. The early earth almost certainly was clothed in a mildly reducing atmosphere. The earliest organisms probably used photosynthesis I and hence did not generate O2. The origination of photosystem II has not been dated, but it probably preceeded the major rise in P(O2) about 2,100 million years ago. A role of biology in this rise cannot be ruled out but is not necessary. Additional increases in P(O2) just before the Cambrian explosion and following the rise of the higher land plants have been suggested but are not yet confirmed. Despite a good deal of research, the evolution of atmospheric O2 is still rather imperfectly known, but it is quite clear that the O2 level in the atmosphere has been determined by the interplay of biological evolution and the physical and chemical evolution of the planet. Individual species do not seem to have mattered.