Microorganisms are a source of life, in the earth, in our body, as in the sea. They also populate our oceans, color our marshes and our rivers. In recent years, we have often spoken of some of them, which have become harmful, but above all linked to our own excessive practices and pollution. However, phytoplankton and microalgae are among the first living substances to appear on our planet more than 3 billion years ago. Some estimate their number at 200,000 different species: