8 OCT 1998 Feeling sorry for the U.S. Census as it single-mindedly plans to count more than a quarter-billion Americans? Then spare a tear for the poor microbiologist, eager to pin down the total number of bacteria and other prokaryotes on Earth. (The prokaryotes comprise two of the broad kingdoms of life -- bacteria and archaea. The third kingdom -- eukarya -- contains all organisms with cellular nuclei, including fungi, plants and animals.)
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©1998, The Biology Project, used with permission
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Prokaryotes, as you've deduced, lack a cellular nucleus, yet they can live from the upper atmosphere to the ocean floor, from the human gut to rocks a mile deep. And there are lots of prokaryotes. Even though each one weighs less than a less than a quadrillionth of a gram when toweled dry, overall they weigh about as much as all the plants in the world -- roughly a gigaton.
So how many prokaryotes share the planet with us? According to William Whitman, a microbiologist at University of Georgia, the number is 5 x 10
30
This is a big number by any standard. If you had that many pennies, Whitman and colleagues David Coleman and William Wiebe calculated, they would make a stack a trillion light years long.
Just where are all these prokaryotes? As we indicated, some bacteria live in the human gut -- a total of 3.9 x 10
Furthermore, the vast majority of prokaryotes live under land or the sea floor, not in us. In fact, 92 to 94 percent of all prokaryotes live underground lives of quiet desperation, hidden in the cracks and pores of rock and sediment, lacking sunlight, fresh air, even cable TV.
A whale of a census
From there it was simple multiplication -- size of habitat in milliliters times number of prokaryotes per milliliter equals total number of bacteria in that habitat.
The math showed that the top eight meters of soil carry 26 x 10
Unable to count bacteria individually, Whitman and colleagues opted for a sampling technique. They divvied the world into representative habitats, like forests, deserts, freshwaters, and shallow and deep ocean waters. 
Then they scoured the science literature looking for studies on the density of bacteria in each habitat.
But the real jackpot lies underground. More than 8 meters below the land surface, they found between 25 and 250 x 1028 prokaryotes. And beneath the ocean floor live a staggering 355 x 1028 organisms without nuclei.
Although the researchers did try to cross-check their results against existing studies, much of their work was extrapolated from very limited data on subsurface conditions. For example, information from two locations was taken to represent conditions under the land everywhere. As a result, "We can't really give you an error bar" -- an estimation of how far off their results might be, Whitman says.
World's least useful number?
Even to doubters, the research demonstrates that all discussions of life, and its effects on Earth, had best take into account the hidden biosphere. Bacteria, after all, make some of the oxygen and virtually all of the nitrogen in our air. They decay pollutants and dissolve rocks.
And like redwoods and mahogany trees, bacteria also store carbon. That has bearing on the study of global warming, since removing carbon from the air slows the rise of carbon dioxide that is causing the planetary cook-out.
May we predict your response? The results are a) "astonishing. It's amazing that as much as half of our biosphere is hidden prokaryotes, not trees and clams and dolphins," or b) "a waste of time -- the total number of prokaryotes is as useless as it is incomprehensible."
M I D D L E - Blue-green bacteria in the genus Oscillatoria, and the green algae in the genus Spirogyra.
B O T T O M - This oil-like film is made of Leptothrix discophora bacteria that have coated their filaments with iron and manganese oxides.
Photos by Eleanora I. Robbins, courtesy USGS Bio-blitz.
Their infinite variety
The data also help explain the astonishing level of the diversity of prokaryotes. It is this diversity which allows them to prosper in virtually every habitat, from ice to boiling water, from deep in the Earth to high in the atmosphere. Prokaryotes come in varieties that can cause human and crop diseases, or supply medicines and new molecules for industry.
These abilities derive directly from the DNA that has directed the growth and replication of prokaryotes during 3.8 billion years of existence (on a planet that's roughly 4.5 billion years old). Each habitat offers new chemical challenges that bacteria, through their diversity, have learned to meet.
Indeed, prokaryotic diversity may be immeasurable: Scandinavian researchers found at least 4,000 species of bacteria growing in a single gram of soil.
Whitman says large numbers offer a simple explanation for this motley crew. Diversity, after all, stems from successful mutations -- changes in the genetic structure. Thus he and his colleagues calculated that while producing 17.4 x 10
29 cells every year, prokaryotes would have plenty of chances to develop helpful mutations.The researchers selected four changes in a single gene as an indication that a prokaryote might have changed enough to form a new enzyme and thus begin differentiating itself from its ancestors. And with so many divisions taking place, this highly unusual number of mutations would be routine, taking place somewhere on Earth every 20 minutes. Says Whitman, "We argue that this could be an explanation for a large amount of bacterial diversity."
. -- David Tenenbaum
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