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778 Chapter 30 Microbial Ecology

Figure 30.13 Hydrothermal Vent Community (a) This diverse community is supported by the metabolic activities of chemolithoautotrophs. (b) Water escaping from the vent is rich in minerals and dissolved gases, including hydrogen sulfide.

Figure 30.13 Hydrothermal Vent Community (a) This diverse community is supported by the metabolic activities of chemolithoautotrophs. (b) Water escaping from the vent is rich in minerals and dissolved gases, including hydrogen sulfide.

used by chemoorganotrophs. The requirement for solar energy has traditionally been used to explain why life is not equally abundant everywhere. The discovery of different types of communities far removed from sunlight, including those near hydrothermal vents and within rocks, however, has dramatically altered these ideas. These communities rely on chemolithoautotrophs, which harvest the energy of reduced inorganic compounds and use it to form organic compounds. ■ chemolithoautotroph, p. 91

A number of hydrothermal vents have been discovered, some thousands of meters below the ocean surface. These vents, the hottest of which are known as black smokers, form when water seeps into cracks in the ocean floor and becomes heated by the molten rock, finally spewing out in the form of mineral-laden undersea geysers. The hydrogen sulfide discharged supports thriving deep-sea communities, oases in the otherwise desolate ocean floor (figure 30.13). Large numbers of sulfur-oxidizing chemolithoautotrophs such as Thiomicrospira species are found in and around the vents. Many are free-living but some live in symbiotic association with the large tubeworms and clams that inhabit the area. The chemolithoautotrophs harvest energy from oxidation of hydrogen sulfide, and they fix CO2, providing the animals with both a carbon and energy source.

In 1994, microbial populations were reported living almost 3 km under the ground. Then, in 1995, scientists found large populations of microbes thriving in iron-rich volcanic rocks from nearly a thousand meters below the surface of the Columbia River (see Perspective 4.1). These organisms gain energy from hydrogen (H2) produced in the subsurface. It has been estimated that if (and it is a big "if" at this point) most similar rocks contain microbes, there could be as much as 2 x 1014 tons of underground microorganisms, equivalent to a layer 1.5 meters thick over the entire land surface of the earth!

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