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,
Contact: Phil Williams
University of Georgia