21 - PROKARYOTIC DIVERSITY BIOL 4125 SPRING 2009 LECTURE 21...

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PROKARYOTIC DIVERSITY BIOL 4125 SPRING 2009 LECTURE 21 Hyperthermophilic Archaea
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Phylogenetic overview of the Archaea The cultivated archaea reside in the phyla Euryarchaeota and Crenarchaeota. Two other phyla branch off close to the root: the Korarchaeota and Nanoarchaeota. The shortest branches on the 16S rRNA gene tree are hyperthermophiles.
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K.O. Stetter (2006) Extremophiles 10:357-362. Most hyperthermophiles exhibit a chemolithoautotrophic mode of nutrition. Anaerobic and aerobic types of respiration follow inorganic redox reactions and CO 2 is the only carbon source required to build up organic cell material. Molecular hydrogen serves as an important electron donor. Other electron donors are sulfide, sulfur, and ferrous iron. There are some obligate heterotrophic hyperthermophiles and opportunistic heterotrophs, which gain energy either by aerobic or different types of anaerobic respiration or by fermentation.
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Upper temperature limits for phototrophy, chemoorganotrophy, and chemolithotrophy • Of the hyperthermophiles known, only archaea are capable of growth at >95 o C. • Many hyperthermophiles are chemlithotrophic autotrophs. Because these habitats are devoid of phototrophs, they are the only primary producers in these environs. • At temperatures >110 o C, only H 2 -oxidizing archaea are known. • The ability to use H 2 as an electron donor (using S 0 , NO 3 - , Fe 3+ , or O 2 as an electron acceptor) is a common theme in hyperthermophilic bacteria and archaea. •H 2 metabolism is likely to be a physiological relic of ancient metabolic schemes.
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A possible H 2 -based energy generating scheme for primitive cells The formation of pyrite (FeS 2 ) leads to H 2 production S 0 reduction, which fuels a primitive ATPase. An alternate source of H 2 could have been the UV-catalyzed reduction of H + by Fe 2+ . Note how few proteins would have been required to carry out these reactions in primitive cells. • Molecular O 2 was not available on the early Earth, so only energy-generating reactions occurring under anoxic conditions could have been exploited by primitive cells. • There are a variety of anoxic chemoorgano- trophic, chemolithotrophic, and phototrophic energy-generating mechanism, but they complex. • Reactions involving ferrous iron could have generated H 2 via two mechanisms, which is a powerful electron donor. • To form a redox couple an e- acceptor is required, which may have been S 0 . •H 2 , Fe 2+ , and S 0 were abundant on the early Earth, so cells would have had a nearly limitless supply of energy. • Many hyperthermophilic archaea can
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This note was uploaded on 08/04/2009 for the course BIOL 4125 taught by Professor Christner during the Spring '08 term at LSU.

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21 - PROKARYOTIC DIVERSITY BIOL 4125 SPRING 2009 LECTURE 21...

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