ATHENS, Ga. -- They're everywhere. Bacteria are the huddled masses of the microbial world, performing tasks that include everything from causing disease to fixing nitrogen in the soil. Now, for the first time, a team of researchers from the University of Georgia 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," 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. Co-authors of the paper from the University of Georgia were Dr. David Coleman of the Institute of Ecology and Dr. 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 all animals combined -- including humans -- makes up less than one percent of the total amount. By far the greatest numbers are in the subsurface, soil and oceans.
Scientists prefer to call bacteria "prokaryotes," a term that describes a single-cell organism 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 beneath the ocean floor some seven miles deep..
In order to estimate the total number of bacteria on Earth, the group at Georgia divided the Earth into several areas, including oceanic and other aquatic environments, the soil, the subsurface of soil, and other habitats such as the air, inside animals and on the surface of leaves. The study brought some surprises.
"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," said Whitman.
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 and so began to examine these habitats further.
Numerous direct measurements have been made for the total number of bacteria in the oceans, and median values were chosen to represent the three major oceanic habitats: the upper 200 meters, the deep ocean, and the upper 10 centimeters of deep ocean sediments.
Soil was divided into forest and non-forest types. The researchers then used detailed direct measurements from two studies representative of these soil types to calculate the total number of soil bacteria. Only nine data sets were available for the subsurface, but Whitman used indirect evidence to complete the picture of subsurface bacteria.
"We estimated that about 92 to 94 percent of the Earth's prokaryotes 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."
Another important part of the study was an estimate of carbon content in bacteria. Carbon, of course, is a crucial element in numerous natural processes, so knowing the amount of it could contribute substantially to knowledge of carbon cycles. Scientists assume that carbon in the bacteria that live in soil and subsurface takes up about one-half of their dry weight. The team thus found that the total amount of bacterial carbon in the soil and subsurface to be yet another staggering number, 5 X 1017 g or the weight of the United Kingdom..
Rather surprisingly, the group at Georgia found that the total carbon of bacteria is nearly equal to the total 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," said Whitman.
The study could open new areas of inquiry, especially about the rate of mutations and how bacteria operate in nature. The new numbers also point out once again that events that are extremely rare in the laboratory could occur frequently in nature. In the meantime, despite the new estimate of total bacteria, researchers have their hands full just listing the number of bacterial species.
In the meantime, consider this problem: Because the number of bacteria is so large, events that would occur once in 10 billion years in the laboratory would occur every second in nature.