Anaerobic Chemoorganotrophs Anaerobic Respiration

Chemoorganotrophs oxidize organic compounds such as glucose to obtain energy. Like the chemolithotrophs, chemoorgan-otrophs that grow anaerobically employ a terminal electron acceptor other than O2. Sulfur and sulfate are common inorganic compounds used as a terminal electron acceptor by these organisms.

Sulfur- and Sulfate-Reducing Bacteria

When sulfur compounds are used as a terminal electron acceptor, they become reduced to form hydrogen sulfide, the compound responsible for the rotten-egg smell of many anaerobic environments. In addition to the unpleasant odor, the H2S is a problem to industry because it reacts with iron to form iron sulfide, corroding pipes and other metals. Ecologically, however, prokaryotes that reduce sulfur compounds are an indispensable component of the sulfur cycle. ■ sulfur cycle, p. 777

Sulfate- and sulfur-reducing bacteria generally live in mud rich in organic material and oxidized sulfur compounds. The H2S they produce causes mud and water to turn black when it reacts with iron molecules. At least a dozen genera are recognized in this group, the most extensively studied of which are species of the Gram-negative curved rod Desulfovibrio.

Some representatives of the Archaea also use sulfur compounds as a terminal electron acceptor, but they generally do not inhabit the same environments as their bacterial counterparts. While most of the sulfur-reducing bacteria are mesophiles or thermophiles, the sulfur-reducing archaea are hyperther-mophiles, inhabiting such extreme environments as hydrothermal vents. They will be discussed later in the chapter. ■ thermophiles, p. 87 ■ hyperthermophiles, p. 87

Nester-Anderson-Roberts: I II. The Microbial World I 11. The Diversity of I I © The McGraw-Hill

Microbiology, A Human Prokaryotic Organisms Companies, 2003

Perspective, Fourth Edition

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