6 October 1998

5 000 000 000 000 000 000 000 000 000 000

They're everywhere. No, not Spice Girls fans; bacteria. Bacteria are the huddled masses of the microbial world, responsible for everything from causing disease to increasing or decreasing nitrogen in the soil. Now, for the first time, a team of researchers from the University of Georgia, clearly with too much time on their hands, has made a direct estimate of the total number of bacteria on Earth - and the number makes the globe's human population look downright puny.

The group, led by microbiologist William. B. Whitman, estimates the number to be five million trillion trillion. Picture it like this. If each bacterium were a penny, the stack would reach a trillion light years. Better yet, if each bacterium were as big as a penny, your stomach would be the size of the moon! Yech.

Why conduct such a survey? "There simply hadn't been any estimates of the number of bacteria on Earth," said Whitman. "Because they are so diverse and important, we thought it made sense to get a picture of their magnitude." The study was published in June in the Proceedings of the National Academy of Sciences and was funded in part by grants from the National Science Foundation and the U. S. Department of Energy.

In order to estimate the total number of bacteria on Earth, the Georgia researchers divided the Earth into several areas, including oceanic and other aquatic environments, the soil, and the subsurface of soil. Other areas included other habitats such as the air, the insides of animals, and the surface of leaves.

After making a list of known habitats for bacteria, the group searched scientific literature for direct measurements of cell numbers from these habitats. They found that the great majority of bacteria are in sea water, soil, and oceanic and soil subsurface.

"We estimated that about 92 to 94 per cent of the Earth's prokaryotes (bacterium) are in the soil subsurface," said Whitman. "We consider the subsurface to include marine sediments below about four inches and terrestrial habitats below about 30 feet. By combining direct measurements of the number of prokaryotic cells in various habitats, we found the total number of cells was much larger than we expected. It had been estimated before that one-half of the living protoplasm on Earth is microbial, but our new figures indicate that this estimate is probably much too conservative."

Meanwhile, we at Science à GoGo are still waiting anxiously for news as to how many angels can dance on the head of a pin.