Microbial Ecology

For centuries, farmers have understood that they could not continue growing the same crop on the same piece of land year after year without reducing the crop's yield. They knew that allowing afield to lie unplantedfor one or more seasons enables it to recover its productivity. The wild plants that grow on the field for a year or two appear to rejuvenate the soil. It was not until the late nineteenth century that scientists began to discover why this was so. They isolated soil microorganisms that could take nitrogen from the air and transform it into forms that plants could use. This process is called nitrogen fixation and the nitrogen is said to be fixed. Although many scientists have worked for years to understand how microorganisms fix nitrogen, one scientist from the Netherlands, Martinus Willem Beijerinck, stands out as an early contributor in these studies.

In 1887 Beijerinck reported on the properties of the root nodule bacterium, which he called Bacterium radicola. (The genus name of this bacterium was later changed to Rhizobium.) He showed in 1890 that root nodules were formed when B. radicola was incubated with legume seedlings. Russian microbiologist Sergei Winogradsky then showed that the bacteria formed a symbiotic relationship with the roots of the legumes. The bacteria-containing nodules fixed nitrogen.

Beijerinck also made major contributions to other areas of microbiology. He worked on yeasts, plant viruses such as tobacco mosaic virus, and plant galls. Beijerinckia, a group of Gramnegative aerobic rods, is named for him. The genera Azotobacter and Beijerinckia include bacteria that can fix nitrogen under aerobic conditions in the absence of plants.

Beijerinck was described as a "keen observer," a person who was able "to fuse results of remarkable observations with a profound and extensive knowledge of biology and the underlying sciences." This ability was undoubtedly partly responsible for the great success of his work.

—A Glimpse of History

MICROBES CYCLE NUTRIENTS, MAINTAIN FERTILE soil and usable water supplies, and decompose wastes and other pollutants. Without microbial activities, life on earth could not survive. People would quickly become buried by the tons of wastes they generate, and nutrients would be depleted, halting growth and reproduction. In view of the crucial functions microorganisms perform, it would seem we should know a great deal about the multitude of diverse microbial species that inhabit our surroundings. Quite the opposite is true, however, as less than a mere 1% have been successfully grown in culture.

Even if all microorganisms could be cultivated in the laboratory, the information gained might not accurately reflect their role in the environment. In the laboratory, organisms are grown as pure cultures under controlled conditions and are provided with plentiful nutrients necessary to ensure the optimal growth. In nature, however, organisms generally grow as members of heterogeneous communities in poorly defined and often changing environmental conditions. Nutrients are normally in short supply, limiting microbial growth. Thus, with respect to environmental microbiology, results obtained in the artificial setting of the laboratory, although useful, must be interpreted with caution. ■ bacterial growth in nature, p. 103

Earlier chapters discussed bacterial growth in nature, metabolic diversity, and ecophysiology. This chapter will expand on some of those concepts, focusing on activities of microorganisms that make them essential in the biosphere. ■ metabolic diversity, p. 268 ■ ecophysiology, p. 283

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