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In the Beat of a Heart: Life, Energy, and the Unity of Nature
mangabey, was announced. But, while we know a lot about primates—and mammals in general—there are many organisms about whose diversity we know very little. Biologists have described about 100,000 species of fungus and believe there are about 1.5 million in the world. There are thought to be a similar number of nematode worm species living in the sea, but only a few have been named. Estimates of the number of insect species vary hugely, but most biologists currently plump for about 5 million to 10 million.
But it is in the world of the cells called prokaryotes—the species without mitochondria and other complex cellular organs—that things really start to boggle the mind. Until recently, there was no real way to get a handle on microbial diversity. Biologists could only study the microbes they could culture in the lab, but more than 99 percent of bacterial species will not breed in captivity, making them impossible to identify. DNA technology has given us a new way to identify microbes. We can fish bits of DNA out of the soil, sea, sewage, or wherever and amplify and sequence them. By looking at genes, biologists could get some idea of what was out there. The results have been challenging.
There are, it seems, about 70 species of bacteria in a milliliter of sewage. This number rises to 500 species in the human gut, with the mouth, skin, and genitals all having their own distinctive and similarly diverse flora. There could be 2 million bacterial species in the oceans and 4 million in a tonne of soil. The biggest estimate for global bacterial diversity I have seen is 1 billion species. The overwhelming diversity of microbes asserts itself not just at the species level. The kingdom archaea, a group of unicellular organisms that look like regular bacteria but are genetically as different from them as we are, was first discovered living in hotsprings about 25 years ago. Currently, more than one new phylum of prokaryotes—the rank equivalent to molluscs or vertebrates—is being discovered every month.
It’s not just in genetic diversity that microbes win. Worldwide, it’s been estimated that there are 5 × 1030 (i.e., a 5 followed by 30 zeros) prokaryotes. This is a billion times the number of stars in the universe. These cells contain the same mass of carbon as all the world’s plants and 10 times more nitrogen and phosphorus. There are microbes whose metabolism is based on burning not oxygen but uranium, and