In other words, how much of what volume of what mix of microalgaes would be enough to terraform an earthlike planet in, say, 100 years?
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It couldn't be done in a 100 years. As science currently understands it, Cyanobacteria or blue-green algae became the first microbes to produce oxygen by photosynthesis, perhaps as long ago as 3.5 billion years ago and certainly by 2.7 billion years ago. However, there was long time – hundreds of millions of years – before Earth’s atmosphere first gained significant amounts oxygen, some 2.4 billion to 2.3 billion years ago. It took another one billion years for oxygen levels to rise high enough to enable the evolution of animals.