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I think the statement that Prochlorococcus is responsible for at least 50% of atmospheric oxygen is dubious. What does that mean? That it created half of the oxygen in the atmosphere back when the atmosphere got its oxygen? It may be true that it produces half of the oxygen that is presently being produced by plants and algae, but that's not the same thing. (By the way, if all the Prochlorococcus died, we'd still have our oxygen, and it wouldn't go down much at all. When Prochlorococcus dies or gets eaten, this consumes the oxygen that it produced during its life.) Eric Kvaalen (talk) 12:53, 12 December 2014 (UTC)