Oxygen: a revolutionPDF
BOUSSAC Alain, CNRS Research Director, Institut de Biologie intégrative de la cellule, CEA Saclay
RUTHERFORD William, Professor, Imperial College, London; Fellow of the Royal Society
Life on Earth is based on the chemistry of carbon, whose source is carbon dioxide (CO2). To be usable in biological reactions, CO2 must be “energized” by storing e- electrons and protons to compensate for negative electron charges. The invention of photosynthesis by some organisms solved this problem very early in the history of evolution by developing a process that allowed life to explode. The first organisms capable of photosynthesis were probably the ancestors of cyanobacteria. They appeared in the primordial ocean of the Precambrian more than three billion years ago. By using solar energy to take electrons from water (H2O) the consequence has been the production of oxygen (O2) and protons. The slow accumulation of oxygen in the atmosphere led to a real “revolution in evolution”. This accumulation is the result of a balance between production (photosynthesis), consumption (breathing) and storage (coal, limestone…).
After photosynthesis appeared, oxygen concentration in the atmosphere remained low due to the high capacity of minerals to trap it. This event in the history of the Earth is clearly recorded in geological layers rich in iron oxide (Fe2O3) up to about – 2.5 billion years ago. After the minerals were saturated with oxygen, it then spread into the atmosphere with major consequences. The quantities resulting from photosynthesis were of a completely different order of magnitude compared to those of the previous period, to the point of becoming a poison for the living species in contact with it. For those who survived, the presence of O2 was used in a new energy process: breathing using O2 as the terminal electron acceptor. The energy-rich carbon molecules produced by photosynthesis have become the fuel for respiration, and the O2 waste oxidizes it. This type of energy-efficient breathing has promoted the development of multicellular organisms and then more complex living forms (Figure 1). Under the effect of solar ultraviolet (UV) radiation, the appearance of oxygen has resulted in the accumulation of ozone in the upper layers of the atmosphere, thus protecting living species from the harmful effects of UV radiation. This natural UV filter allowed life to be taken out “outdoors” for the first time. It is the photosynthetic species that are responsible for creating and maintaining the conditions necessary for life on Earth, as we know it. Such success is due to the limited needs: sun, water and carbon dioxide.
It is no exaggeration to say that the appearance of oxygen photosynthesis is one of the most important events experienced by our planet. It has produced over the course of geological time, and continues to produce, the oxygen we breathe. It has allowed the development of multicellular living forms and has modified the physico-chemical characteristics of the Earth. The appearance of ozone has protected the Earth from ultraviolet solar radiation and caused climate change. These changes have allowed the continents to be colonized by new forms of animal and plant life.