Chemoorganoheterotrophs, commonly referred to as chemo-heterotrophs or chemoorganotrophs, use organic compounds for energy and as a carbon source. They are by far the most common group associated with humans and other animals. Some play beneficial roles such as providing a source of vitamin K in the gut, whereas others cause disease.

Chapter 4 Dynamics of Prokaryotic Growth

PERSPECTIVE 4.1 Can Prokaryotes Live on only Rocks and Water?

Prokaryotes have been isolated from diverse environments that previously were thought to be incapable of sustaining life. For example, members of the Archaea have been isolated from environments 10 times more acidic than that of lemon juice. Other Archaea have been isolated from oil wells a mile below the surface of the earth at temperatures of 70°C and pressures of 160 atmospheres (at sea level, the pressure is 1 atmosphere).The isolation of these organisms suggests that thermophiles may be widespread in the earth's crust.

Perhaps the most unusual environment from which prokaryotes have been isolated are the volcanic rocks 1 mile below the earth's surface near the Columbia River in Washington State. What do these organisms use for food?

They apparently get their energy from hydrogen gas that is produced chemically in a reaction between the iron-rich minerals in the rock and the groundwater.The groundwater also contains dissolved CO2, which the bacteria can use as a source of carbon.Thus, these bacteria apparently exist on nothing more than rocks and water.

As a group, chemoheterotrophs can degrade a wide range of organic chemicals. In fact, most natural organic molecules, regardless of their complexity, can be degraded by at least one species of microorganism. A large number of human-made compounds, however, such as certain herbicides and plastics, are degraded very slowly, and some may not be degraded at all.

Individual species of chemoheterotrophs differ in the number of organic compounds they can use. For example, certain members of the genus Pseudomonas can derive carbon and/or energy from more than 80 different organic compounds, including such unusual compounds as naphthalene (the ingredient associated with the smell of mothballs). At the other extreme, some organisms can degrade only a few compounds. For example, Bacillus fastidiosus can use only urea and certain of its derivatives as a source of both carbon and energy.

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