The Diversity of Prokaryotic Organisms

in his native country, the Netherlands, Cornelis B. van Niel (1897-1985) earned a degree in chemical engineering from the Technological University at Delft. At the Delft School, as it is often called, an outstanding general and applied microbiology program within the Department of Chemical Technology was chaired in succession by two prominent microbiologists— Martinus Beijerinck and Albert Kluyver.

After earning his degree in 1923, van Niel accepted a position as assistant to Kluyver, caring for an extensive culture collection and helping prepare demonstrations for lecture courses. Kluyver was relatively new to the school, but he had a vast knowledge of microbiology and biochemistry. Although little was known at the time about metabolic pathways, Kluyver believed that biochemical processes were fundamentally the same in all cells and that microorganisms, which can be grown in pure culture, could be an important research tool, serving as a model to study biochemical processes. Thirty years later, Kluyver and van Niel would present lectures that would be published in a book entitled The Microbe's Contribution to Biology. Under Kluyver's direction, van Niel began studying the photosynthetic activities of vividly colored purple bacteria such as Chromatium species, a subject for which he developed a lifelong interest.

Shortly after earning his Ph.D. in 1928, van Niel moved to the United States, bringing with him the intense appreciation for general microbiology that had been fostered at the Delft School. Settling at the Hopkins Marine Station in California, he continued his work on purple photosynthetic bacteria. Using systematic methods, van Niel conclusively showed that the growth of these organisms is light dependent, yet they do not evolve O2. Furthermore, his experiments showed that in order to incorporate CO2 into cellular material, these anoxygenic phototrophs oxidize hydrogen sulfide. He noted that the reaction stoichiometry of this process was remarkably similar to that of the photosynthesis of green plants and algae, except hydrogen sulfide was used in place of water, and oxidized sulfur compounds were produced instead of O2. This finding raised the possibility that O2 generated by plants did not come from carbon dioxide, as was believed at the time, but rather from water.

In addition to his scientific contributions, van Niel was recognized as an outstanding teacher. During the summers at Hopkins Marine Station, he taught a bacteriology course, inspiring many microbiologists with his enthusiasm for the diversity of microorganisms and their importance in nature. His keen memory and knowledge of the literature, along with his appreciation for the remarkable abilities of microorganisms, enabled him to successfully impart the awe and wonder of the microbial world to his students.

—A Glimpse of History

SCIENTISTS ARE ONLY BEGINNING TO UNDERSTAND the vast diversity of microbial life. Although a million species of prokaryotes are thought to exist, only approximately 6,000 of these, grouped into 850 genera, have been actually described and classified. Traditional culture and isolation techniques have not supported the growth, and subsequent study, of the vast majority. Not surprisingly, most effort has been put into the study of microbes intimately associated with the human population, especially those causing disease, and these have been most extensively described. This situation is changing as new molecular techniques aid in the discovery and characterization of previously unrecognized species. The sheer volume of the rapidly accruing information made possible by this modern technology, however, can be daunting for scientists and students alike.

The phylogenetic relationships being elucidated by the ribo-somal RNA studies discussed in chapter 10 are causing significant upheaval in prokaryotic classification schemes. Some organisms, once grouped together based on their phenotypic similarities, have now been split into different taxonomic units based on their

268 Chapter 11 The Diversity of Prokaryotic Organisms ribosomal RNA differences. The genera Staphylococcus and Micrococcus, for example, were once included in the same family, but recent phylogenetic classification schemes now separate them into different classes. Some organisms that for many years were thought to be entirely unrelated are now grouped more closely together because of their shared ribosomal RNA sequences. As an example, Rhizobium, a genus of nitrogen-fixing bacteria that form a symbiotic relationship with certain plants, and Rickettsia, a genus of tick-borne human pathogens that are obligate intra-cellular parasites, have been moved into the same class. Ribosomal RNA analysis even makes it possible to assess the genetic relatedness of organisms that cannot be grown in culture, which has made it possible to study the relationships of a wider variety of archaea. ■ classification schemes, p. 247

This chapter covers a wide spectrum of prokaryotes, focusing particularly on their extraordinary diversity rather than concentrating solely on their phylogenetic relationships, which are still being elucidated. To highlight the remarkable abilities of prokaryotes and convey a sense of appreciation for the environments they inhabit and the essential role that they play in our biosphere, groups of microorganisms are described according to their metabolic characteristics and other physiological traits. Table 11.1 summarizes the characteristics of prokaryotes that are covered in this chapter and serves as an outline for the more complete description within the text. Table 11.2, on page 272, indicates the medical importance of representative bacteria and serves as a directory for information both about the organisms covered in this chapter and the respective diseases covered elsewhere in the text. Note, however, that no single chapter could describe all known prokaryotes and, consequently, only a relatively small selection is presented.

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