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 a field to lie unplanted for one or more seasons will enable 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 1800's that scientists began to discover why this was so. They isolated soil microorganisms that could take nitrogen from the air and chemically transform it into forms of nitrogen that plants could use. This process is called nitrogen fixation. 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 could fix 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 gram-negative 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 responsible for the great success of his work.

Microorganisms are vital to the cycling of elements, the maintenance of fertile soil and usable water supplies, and the decomposition of wastes and other pollutants. Without microbial activities, life on earth could not survive. People would quickly become buried by the tons of waste products they generate, and nutrients would be depleted, limiting growth and reproduction.

The study of microorganisms in their natural environments is much more complex than laboratory studies. Instead of growing as pure cultures under controlled conditions, organisms in nature exist as members of heterogenous communities under poorly defined and often changing environmental conditions. In the laboratory, the investigator provides the nutrients necessary to ensure the optimal growth of the culture. In nature, organisms usually exist in low-nutrient environments where nutrient supplies are limiting to microbial growth. In the laboratory, organisms are often grown in rich liquid culture, whereas in nature, most microorganisms grow in association with solid surfaces, on plants or animals. and in biofilms or biomats, from which they can gain nutrients.

The growth and development of organisms in different environments as well as the role of microorganisms in the cycling of key biological components are discussed in this chapter. Bioremediation, the employment of microorganisms in the cleanup of pollutants such as pesticides, oil spills, and metals, will also be discussed.