From Harvard Magazine:
“Our planet has been shaped by an invisible world,” says Roberto Kolter, a professor of microbiology and molecular genetics at Harvard Medical School (HMS). “Microbes mediate all the important element cycles on Earth, and have played a defining role in the development of the planet,” says Kolter. They form clouds, break down rocks, deposit minerals, fertilize plants, condition soils, and clean up toxic waste. Among their ranks, explains Cavanaugh, are the photosynthetic “primary producers” that use sunlight, water, and carbon dioxide to form the broad base of the food chain, and together with plants make up the earth’s largest source of biomass. The earliest life on our planet was entirely microbial, and if life exists on other planets, it is surely microbial there as well.
In the realm of human health, microbes help us digest food and produce vitamins, protect us against infection, and are the main source of antibiotic medicines. The human cells in your body number 10 trillion, but that pales by comparison to the estimated 100 trillion microbial cells that live in and on you. “Without them, you would be in trouble,” Kolter says: animals experience abnormal growth and become sick if deprived of their microflora during development. Although a few microbes are known to cause disease, the precise role played by the vast majority is essentially unknown.
The same could be said for microbes around the planet. There are a billion of them in a gram of soil, and a billion per liter of seawater, but we know neither what they are nor what they do.