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 23 among all six billion of us. (Before you down another tetracycline as your part in the global crusade against bacteria, remember that most intestinal bacteria are helpful.) 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

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. 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 28 prokaryotes, and all aquatic habitats carried 12 x 10 28. But the real jackpot lies underground. More than 8 meters below the land surface, they found between 25 and 250 x 10 28 prokaryotes. And beneath the ocean floor live a staggering 355 x 10 28 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?

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." 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.

