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Phil Williams
For the first time, a team of UGA researchers 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--that’s a five with 30 zeroes after it. Look at it this way. If each bacterium were a penny, the stack would reach a trillion light years. These almost incomprehensible numbers give only a sketch of the vast pervasiveness of bacteria in the natural world.
 “There simply hadn’t been any estimates of the number of bacteria on Earth,” says 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. Co-authors of the paper were David Coleman of the Institute of Ecology and William Wiebe from the department of marine sciences.
When people think of bacteria, they likely first consider the nasty ones that cause disease. But the bacteria inside animals--including humans--makes up less than one percent of the total amount of bacteria.
Scientists prefer to call bacteria “prokaryotes,” meaning single-cell organisms without a nucleus. Prokaryotes are extraordinarily diverse and range from plant-like cells that produce molecular oxygen in the oceans to soil-borne bacteria responsible for fertility. Scientists have found these cells 40 miles high in the atmosphere and as far as seven miles beneath the ocean floor.
In order to estimate the total number of bacteria, the scientists divided the Earth into habitat areas, including oceanic and other aquatic environments, the soil, the subsurface of soil, the air, the insides of animals, the surface of leaves. The study brought some surprises.
After making a list of known habitats for bacteria, the group searched scientific literature for direct measurements of cell numbers and the amount of carbon in cells 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 percent of the Earth’s prokaryotes are in the soil subsurface,” says Whitman. “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.”
Another important step was to estimate the carbon content in each bacteria. Knowing the amount of carbon present helps determine carbon cycles. Scientists assume that carbon in bacteria takes up about one-half of their dry weight.
The team found the total amount of bacterial carbon in the soil and subsurface to be yet another staggering number, 5 X 10 (to the 17th power) g--or the weight of the United Kingdom.
The scientists were surprised to find that the total amount of carbon of bacteria is nearly equal to the total amount of carbon found in plants. The inclusion of this carbon in global models will greatly increase estimates of the amount of carbon stored in living organisms. The new estimates could also change assumptions about the relative amount in plants of other essential nutrients such as nitrogen and phosphorus.
“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,” says Whitman.
The study could open new areas of inquiry, especially about the rate of mutations and how bacteria behave. The numbers also illustrate that phenomena which are rare in the laboratory can occur frequently in nature. Scientists know now that events that would occur once in
10 billion years in the laboratory can occur approximately every second in nature.
In the meantime, as a result of calculating the new estimate of total bacteria, researchers now have their hands full just listing the number of bacterial species.
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