ABSTRACT
About 2.4 billion years (Gyr) ago, primitive prokaryotic photoautotrophs (cyanobacteria) acquired the ability to photo-oxidize water [1], giving rise to the oxygen-evolving photosynthesis, an event of paramount importance regarded as the Big Bang of the evolution of life on earth [2]. Between 2.4 and 2.0 Gyr, the oxygenic photosynthesis carried out by these microorganisms led to the gradual increase in atmospheric oxygen (to 1–2%) responsible for the Great Oxygenation Event (also called Oxygen Catastrophe), which caused the extinction of anaerobic organisms unable to adapt to the new oxidizing environment but also triggered the evolution of the aerobic organisms, thus profoundly affecting the diversity of life. The emergence of photosynthetic eukaryotes, 17–18 Gyr ago [3], and the increased photosynthetic productivity of algae and, subsequently, land plants accounted for the gradual rise of the oxygen level up to the value (~20%) observed in today’s atmosphere [4]. Essentially all the oxygen present in the aerobic environments on earth derives from the oxygenic photosynthesis that currently accounts for ~1.6 × 1014 kg O2 every year [5].
