nester11 - Chapter 11: The Diversity of Prokaryotic...

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Chapter 11: The Diversity of Prokaryotic Organisms
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Important Point:
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Prokaryotic Diversity “Although a million species of prokaryotes are thought to exist, only approximately 6,000 of these , grouped into 850 genera, have been actually described and classified.” “Traditional culture and isolation techniques have not supported the growth, and subsequent study, of the vast majority.” “Not surprisingly, most effort has been put into the study of microbes intimately associated with the human population, especially those causing disease, and these have been most extensively described.” “The phylogenic relationships being elucidated by the ribosomal RNA studies… are causing significant upheaval in prokaryotic classification schemes.”
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Prokaryotic Diversity “Some organisms, once grouped together based on their phenotypic similarities, have now been split into different taxonomic units based on their ribosomal RNA differences.” We will consider the following categories (not necessarily monophyletic) of bacteria: Anaerobic chemotrophs Oxygenic phototrophs Aerobic chemotrophs Notable terrestrial bacteria Notable aquatic bacteria Notable animal-symbiotic bacteria Archaea that thrive in extreme conditions
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Anaerobic Chemotrophs The earliest life forms likely were anaerobic chemotrophs since early Earth lacked O 2 while photosynthesis requires significant metabolic sophistication. Anaerobic (no O 2 ) habitats are still in abundance. These occur from combinations of poor diffusion and mixing along with presence of facultative anaerobes which serve as O 2 scavengers. Often these are poorly mixed wet environments since water slows diffusion while simultaneously supports facultative organisms (i.e., unlike very dry habitats). Even well-mixed environments can become anaerobic if they become too eutrophic (nutrient rich). Basically, by not having to support aerobic metabolisms, non-facultative anaerobes can be more efficient growers and therefore better competitors in anaerobic environments than facultative anaerobes.
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Chemolithotrophs obtain their energy from inorganic, reduced chemicals such as hydrogen gas (H 2 ). Anaerobic chemolithotrophs are anaerobic respirers rather than fermenters (without glucose, where would the pyruvate come from, after all?). Most anaerobic chemolithotrophs are Archaeans. The most important are the Methanogens: 4 H 2 + CO 2 CH 4 + H 2 O (CH 4 = methane) Feel free to memorize the above as “Hydrogen gas plus Carbon Dioxide go in while Methane comes out.” Methane is an important component of swamp gas. Methane also is an important component of cow
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This note was uploaded on 06/03/2011 for the course MCB 205 taught by Professor Abedon during the Spring '11 term at Ohio State.

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nester11 - Chapter 11: The Diversity of Prokaryotic...

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