LIFE × EVOLUTION
Oxygenic Photosynthesis
~3.2–2.8 billion years ago
Oxygenic photosynthesis, which takes its electrons from water and releases oxygen, arose in microbes between about 3.2 and 2.8 billion years ago, by means of a cluster of four manganese atoms in the apparatus called photosystem II.
Oxygenic photosynthesis is the form of photosynthesis, the building of organic matter with the energy of light, that takes its electrons from water and releases oxygen as waste. Capturing light is easy, and breaking the water molecule is difficult. Breaking it requires a particular apparatus, photosystem II. The core of photosystem II is a cluster of four manganese atoms, which can accumulate enough energy to strip four electrons from two water molecules. Micro-organisms ancestral to modern cyanobacteria, the bacteria that carry out this photosynthesis today, acquired the capacity to break water. In the whole history of metabolism, the chemical reactions of living cells, oxygenic photosynthesis is the innovation with the widest consequences. So far as can be established, it arose only once.
Why it matters
All later respiration depends on oxygenic photosynthesis. Oxygenic photosynthesis was also an uninterrupted source of oxygen, a gas toxic to almost every organism then alive, and so it prepared the crisis that followed.
Dating & uncertainty
No fossil records the appearance of oxygenic photosynthesis. Researchers infer its age in two ways. The first is the branching order of the genes of cyanobacteria: the order in which the genes split in their family tree. The second is chemical traces of oxygen in rocks formed well before oxygen accumulated in the atmosphere. Both place the innovation in the Mesoarchaean, the era between 3.2 and 2.8 billion years ago. The apparatus itself is observed directly only much later: Demoulin and colleagues (2024) described thylakoids, the stacked membranes in which the reaction takes place, preserved in fossil cells 1.75 billion years old.
Earth history as one day
08:08:2715 h 52 min before midnight
if Earth’s 4.54 billion years were compressed into a single day