Among organisms formed by a single cell, there’s a basic division: those that have a differentiated nucleus where the DNA is located and those that don’t have a separate nucleus and, therefore, the DNA isn’t found within an internal membrane.
Among the first — microorganisms with a nucleus — the most common on a global scale is that of the Saccharomyces species. In Greek, this means “sugar fungus.” The most common is Saccharomyces cerevisiae, which has a long tradition of being used by our species. It’s what we use to make bread, beer and wine. However, if we really think about it, perhaps it’s the fungus that uses us.
One of the main characteristics of Saccharomyces is that it’s capable of using sugar to produce alcohol. Now, in principle, this would be a disaster. Alcohol is a molecule without a biological function — it’s extremely toxic and represents an incomplete oxidation of sugar. Hence, producing alcohol is a waste of energy.
However, the fungus uses this alcohol production as a chemical weapon to eliminate competition from other microorganisms, since it’s resistant to alcohol while the rest of the microbes are not. This explains why it dominates many ecosystems: it’s a relentless killer.