LIFE × EVOLUTION
Photosynthesis Without Oxygen
~3.4 billion years ago
Anoxygenic photosynthesis, at work about 3.4 billion years ago, builds organic matter with the energy of sunlight and takes its electrons from compounds easier to split than water, so that it releases no oxygen.
Anoxygenic photosynthesis is the form of photosynthesis that releases no oxygen. Photosynthesis builds organic matter using the energy of sunlight, and its chemical reactions require a source of electrons, called the electron donor. Modern plants take their electrons from water, the most difficult donor of all to split, and release oxygen as the waste product of splitting water. The first photosynthetic microbes used donors that are easier to split, hydrogen gas or the iron dissolved in seawater, and released no oxygen. Anoxygenic photosynthesis sustained the biosphere, the whole of living things, for hundreds of millions of years, and it did not destroy the methane, a gas that kept the surface warm. The microbes that used iron left it behind as oxide. Part of the banded iron formations, rocks made of alternating layers rich and poor in iron, may therefore be their work.
Why it matters
Anoxygenic photosynthesis shows that capturing light is the easy step of photosynthesis. Splitting water is the difficult step, and it is the one that would ultimately fill the atmosphere with oxygen.
Dating & uncertainty
Anoxygenic photosynthesis leaves no unambiguous fossil. Researchers infer its age from the chemical state of the early ocean and from the branching order of the photosystems, that is, the order in which these light-capturing protein assemblies split in their family tree. Researchers accept that the anoxygenic form preceded the oxygenic form, which releases oxygen, but not by how much.
Earth history as one day
06:01:3517 h 58 min before midnight
if Earth’s 4.54 billion years were compressed into a single day