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Only 1,500 microbes make people sick
Billions of kinds
The whole concept of species does not hold for microbes, so any count is rough, but the number of kinds runs into the billions. About 1,500 of them make people sick. The rest keep the planet healthy and working.
Surprising survivors
Microbes also do things that were once thought impossible. Thirty or forty years ago, the common view was that there was no way for microbes to make a living off methane. Before that, nobody knew that some microbes live in the walls of nuclear power plants.
Those microbes avoid radiation damage by carrying lots of copies of their genome. Radiation thrashes an organism's DNA, which leaves it without a good blueprint to copy itself from. With many copies of the blueprint laid over each other, the broken parts show up copy after copy, and a brand-new working version comes out.
Why don't species work for microbes?
Species are usually defined by animals that interbreed. Many microbes reproduce by splitting in two and can swap genes with unrelated lineages. That blurs any clear line between one kind and another. Researchers often use a similarity cutoff in a gene, such as ribosomal RNA, as a rough stand-in.
How do extra genome copies repair DNA?
Deinococcus radiodurans, a bacterium first isolated in 1956 from canned meat that had been irradiated, carries multiple copies of its genome. Radiation shatters its chromosomes into many fragments. The cell matches up overlapping pieces from the different copies and stitches them back together. It can survive radiation doses thousands of times higher than most animals.
What do microbes do to keep Earth running?
Cyanobacteria and algae make much of the oxygen in the air. Nitrogen-fixing microbes turn nitrogen gas into forms that plants can use. Decomposers break dead material down and return nutrients to the soil and sea. The ocean bacterium Prochlorococcus alone accounts for a notable share of ocean photosynthesis.
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