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Microbial Diversity and the Biosphere

Norman R. Pace
Departments of Plant and Microbial Biology and Molecular and Cell Biology, University of California, Berkeley, CA (Current address: Department of Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder, CO 80309-0347)

Introduction

Microorganisms occupy a peculiar place in the human view of life. They receive little attention in our general texts of biology. They are largely ignored by most professional biologists and are virtually unknown to the public except in the contexts of disease and rot. Yet the workings of the biosphere depend absolutely on the activities of the microbial world (Madigan and others 1996). And a large bulk of global biomass is microbial (Whitman and others 1998). Our texts articulate biodiversity in terms of large organisms: insects usually top the count of species. Yet if we squeeze out any insect and examine its contents under the microscope, we find hundreds or thousands of distinct and unidentified microbial species. A handful of soil contains billions of microorganisms, of so many types that accurate numbers remain unknown. At most only a few of these microorganisms would be known to us; only about 5,000 noneukaryotic organisms have been formally described (Bull and others 1992) in contrast with the half-million described insect species. We know little about microbial biology, a part of biology that looms large in the sustenance of life on this planet.

The reason for our poor understanding of the microbial world lies in the fact that microorganisms are tiny, individually invisible to the eye. The mere existence of microbial life was recognized only relatively recently in history, about 300 years ago, with Leeuwenhoek's invention of the microscope. Even under the microscope, however, the simple structures of most microorganisms, usually nondescript rods and spheres, prevented their classification by morphology, through



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