Metabolic Diversity

Bacterial cells show some obvious diversity. They vary in length from less than 0.1 micrometer to more than 100 micrometers. Furthermore, most bacteria in nature are mul-ticellular. Some are shaped like trees, and others are long filaments or berry shaped. But the most striking contributions of bacteria to biodiversity are not visible even with the most powerful modern scanning electron and transmission microscopes: they are, rather, genetic and metabolic. It turns out that all of the vast seeming diversity of visible animals— from grazing deer in the woods to luminous

Illustration of bacteria found in wounds, from Investigations into the Etiology of Traumatic Infectious Diseases by Robert Koch, 1880. (Library of Congress)

fish in the oceans to the myriad insects, flowering trees, and still uncataloged species of the tropical forests—is basically a set of variations on a single theme. The theme is that of aerobic life. For a biologist, aerobic doesn't mean working out to a Jane Fonda video but rather refers to the intake of oxygen at the cell level to procure energy for intracellular metabolism. All plants, animals, fungi, and most all of the larger, nonbacterial (that is, eukaryotic) microbes known as protoctists are aerobic. Metabolically, virtually every "normal" or familiar organism with which you are acquainted shows virtually no change in metabolic diversity as you look from species to species.

The reason is that the surface of the earth has been propagated by the offspring of a very specific type of bacterium—the so-called respiring or oxygen-breathing bacteria, which must be counted as one of the two or three most successful genomes ever in evolutionary history. But like a Microsoft Word program stifling other word processing programs, however successful they are, the prevalent oxygen-breathing form of cell metabolism represents only a tiny fraction of the many ways of doing things. From the perspective of cell numbers, bacteria are still the majority shareholders of the planetary surface. And despite the seeming omnipresence of familiar, macroscopic, oxygen-breathing forms of life, the biochemical diversity of this planet remains largely bacterial. Except for a few esoteric compounds, such as the structural materials of trees and esoteric hallucinogenic alkaloids, all of life's major structural and functional compounds were developed in nature's bacterial crucible, whose innovations result, as we discuss below, from the natural tendency of bacteria to exchange genes, to "genetically engineer," across what would be species borders. And you do not have to know the details of their chemistry to get a feeling of how metabolically diverse are the bacteria or prokaryotes. They thrive in places that would kill aerobic life forms: hundreds of meters beneath earth's surface in rock, on the sides of nuclear reactors, in salt flats and scalding hot springs up to 113 degrees centigrade, in Antarctic ice patches under conditions similar, according to some scientists, to those found on Mars.

Seemingly simple bacteria use DNA to reproduce themselves and to perform an extensive arsenal of environmentally important metabolic tricks, including but not limited to oxygenic photosynthesis, nitrogen fixation, ammonia oxidation, amonification, manganese reduction, iron reduction, sulfide oxidation, sulfur reduction, methanogenesis, fermentation, proton reduction, and nitrate respiration. Some of these processes are still beyond our industrial capabilities. Their chemical and informational skills still exceed those of many human manufacturing and computing processes. Indeed, Canadian biologists Sorin Sonea and Maurice Panisett in their book Prokaryotology: A Coherent View note that if bacteria had been discovered not on the Earth but on Mars, they would have been considered alien life forms.

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