Nearly 3 billion years ago, the earth's atmosphere began changing as O2 was gradually introduced to the previously anoxic environment. This change was probably due to the evolution of the cyanobacteria, thought to be the earliest oxygenic pho-totrophs. These are photosynthetic organisms that use water as a source of electrons for reducing power, liberating O2.
Today, cyanobacteria still play an essential role in the biosphere. As primary producers, they harvest the energy of
278 Chapter 11 The Diversity of Prokaryotic Organisms sunlight, using it to convert CO2 into organic compounds. They were initially thought to be a form of algae and were called blue-green algae until electron microscopy revealed their prokaryotic structure. Unfortunately, this early misunderstanding generated long-lasting confusion in their classification and nomenclature. ■ primary producers, p. 766
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