José A. Hernández Cortés (Scientist Researcher, CEBAS-CSIC, Murcia)
Molecular oxygen (O2) appeared in our atmosphere about 2500 billion of years ago. The most accepted theory for the appearance of the O2 in our environment is the theory of the oxygen chemical evolution. Lazcano-Araujo (1989) defends this theory and suggests the atom fusion of hydrogen (H) at high temperature to make new atoms – two atoms of H plus 2 neutrons would produce a helium atom (He). Two atoms of He would generate a beryllium atom (Be). The carbon (one of the pillar element of the life) would be originated as He plus Be. Subsequently, the oxygen would be created by reaction of He and C. However, the O2 would soon appear since most of the atoms were associated to other elements (H2O, SO2, NO2, joined to metals, etc…).
The internal activity of the earth also contributed to release compounds that contain oxygen. For example, the actions of the volcanoes would release CO2, SO2, water vapour, etc…Lately, the action of the ultraviolet radiation would produce the photo-degradation of these molecules enriching the atmosphere in O2.
In order to explain the contribution of the cyanobacteria in the O2 appearance, it is necessary to start from a situation where water was already present, in which primitive microorganism would live, both anaerobic autotrophs and/or heterotrophs and photosynthetic. In this situation, the primitive cyanobacteria would release O2 by the photolysis of the water molecules (H2O) and by employing the protons (H+) and electrons (e-) to generate energy used in the biosynthesis of carbohydrates.
CO2 + H2O+ Energy of the Light = CH2O + O2
These three mechanisms would cooperate in the contribution of O2 to the atmosphere, which would change its reductive condition to oxidizing.
Once the atmosphere was enriched in O2, it was originated the ozone layer (O3) by the action of the ultraviolet radiation on the O2 present in the highest layers of the atmosphere. This fact would become a milestone in the evolution of species, since ozone would provide protection against the ultraviolet radiation to the existing microorganisms. This would allow a future colonization of the earth.
In this sense, the appearance of the O2 supposed the extinction of existing organisms and the appearance of new ways of life. The primitive organisms lived in an atmosphere without oxygen (reductive) or with too little available oxygen. In this way, as the content of O2 increased in the atmosphere, many of these primitive microorganisms would die. The current anaerobic or facultative anaerobic microorganisms -can live as in both the presence and absence of oxygen- are presumably descendants of those primitive microorganisms which have adapted to live in free O2 environments or with a very low concentration of O2.
In other microorganisms, the use of the O2 allowed a great energy generation and represented a huge benefit for their evolution. The consequence of the O2 use in the energy production was the generation of the so-called Reactive Oxygen Species (ROS). It must be noted that oxygen, vital for our life, is also a lethal and corrosive gas that moreover, can generate toxic intermediates. Thus, the aerobic microorganisms -those that live in presence of O2- have developed mechanisms, both physical and biochemical, to protect themselves from toxic effects of the O2 and from the ROS.
All matters relating to ROS and to protective mechanisms against these toxic molecules will be dealt with in a new chapter.
References: (1) Lazcano-Araujo A. (1989) El origen de la vida. Evolución química y evolución biológica. 3ra. edición, Editorial Trillas, México DF. (2) Folsome E. (1989) Origen de la vida, Editorial Reverté, Barcelona. (3) Halliwell B., Gutteridge J.M.C. (2003) Free Radicals in Biology and Medicine. Third Edition, Oxford University Press Inc, New York.