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Arsenic and old...photosynthesis?

A previously unknown form of photosynthesis discovered in purple bacteria scooped from a Californian hot spring may be an ancient process that arose before the evolution of oxygenic photosynthesis, according to a paper published on Friday (August 15) in linkurl:Science.;http://www.sciencemag.org/ The bacteria use arsenic instead of water for photosynthesis. "It's a fundamental, exciting observation," said linkurl:Tim McDermott,;http://tbi.montana.edu/facultystaff/mcdermott.html a professor of

By | August 14, 2008

A previously unknown form of photosynthesis discovered in purple bacteria scooped from a Californian hot spring may be an ancient process that arose before the evolution of oxygenic photosynthesis, according to a paper published on Friday (August 15) in linkurl:Science.;http://www.sciencemag.org/ The bacteria use arsenic instead of water for photosynthesis. "It's a fundamental, exciting observation," said linkurl:Tim McDermott,;http://tbi.montana.edu/facultystaff/mcdermott.html a professor of microbial ecophysiology at Montana State University, who was not involved in the research. The discovery "gives me a further appreciation of how talented, metabolically speaking, the microbial world really is," McDermott added. "Nothing surprises me anymore." linkurl:Arsenic;http://www.the-scientist.com/article/display/20753/ is well-known for its toxicity; it was used so often as tool for homicide in the 1800s that it earned the nickname "king of poisons." A molecular analog of phosphate, arsenic disrupts production of ATP and impairs the function of proteins, among other mechanisms. Research has revealed that different oxidative states of the toxin function as an electron acceptor (arsenate) for anaerobic respiration in some bacteria, and an electron donor (arsenite) for CO2 fixation in others. Taking that research a step further, linkurl:Ronald Oremland,;http://sofia.er.usgs.gov/people/oremland.html a senior scientist with the U.S. Geological Survey, and colleagues isolated the bacteria from a hot spring on an island in linkurl:Mono Lake,;http://www.the-scientist.com/article/display/13606/ California, a closed basin lake high in salts and minerals. A sample taken from linkurl:biofilms;http://www.the-scientist.com/article/display/20239/ covering submerged rock surfaces in the hot spring exhibited arsenite oxidation in the light but not the dark, a sign of photosynthesis. From the sample, the researchers isolated a species of genus Ectothiorhodospira, anaerobic purple bacteria that typically use hydrogen sulfide as a reducing agent for photosynthesis. Grown in pure culture, the bacteria demonstrated the same arsenite-driven photosynthetic activity. There are two evolutionary hypotheses for the wide diversity of microbes that linkurl:respire arsenic,;http://www.ncbi.nlm.nih.gov/pubmed/12738852?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum said Oremland. Since the process requites strong oxidants, it could have evolved recently in the presence of oxygen or nitrate, and diverged through rapid lateral gene transfer. Genomic analysis of the new discovery, however, provides molecular evidence for the alternate hypothesis, said Oremland: Arsenate respiration is an ancient process that evolved in the hot, anoxic environment of ancient Earth. The next step, says Oremland, is to go back to the island and collect more samples to see if other types of bacteria have the same photosynthetic ability. But there was only one puddle with the biofilm, he said. "I hope it's still there."
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Comments

Avatar of: anonymous poster

anonymous poster

Posts: 3

August 14, 2008

Don't you mean "arsenic instead of oxygen"?
Avatar of: Megan Rulison

Megan Rulison

Posts: 9

August 14, 2008

Thank you for your comment.\n\nThe referenced sentence refers to the fact that just as oxygenic photosynthesis (characteristic of plants and algae) harvests electrons from water, so the new bacteria's anoxygenic photosynthesis harvests electrons from arsenite [As(III)], an oxidation state of arsenic.\n\nOxygen is a product of oxygenic photosynthesis, just as arsenate [As(V)] is a product of this newly-discovered microbe's anoxygenic photosynthesis.\n\nWe apologize for the confusion.\n\n-Megan Scudellari, The Scientist
Avatar of: Frank Leavitt

Frank Leavitt

Posts: 9

August 15, 2008

Here is a layman's question: Does this mean that if another planet has arsenic but not water, there might be a form of life there?
Avatar of: John Aliff

John Aliff

Posts: 2

August 15, 2008

Arsenic as an electron acceptor is not the same as water, which is a source of electrons and hydrogen in photosynthesis.
Avatar of: Matt Melnicki

Matt Melnicki

Posts: 1

August 15, 2008

The remarkable thing about prokaryotic anoxygenic photosynthesis is its branched electron transport. \n\nThere is no linear electron flow in anoxygneic photosynthesis, as there is in oxygenic photosynthesis and in respiration. Rather, with a single photosystem, electrons are transported cyclically, driving the pumping of protons across a membrane to form a concentration gradient (which is subsequently used for ATP synthesis). By the simple cyclical schematic of purple bacterial photosynthesis, these electrons seem as though they might shuffle around the photosystem forever. However, there are two branch-points in this photosystem that link it with respiratory chains -- the quinone pool and the cytochrome pool (both of which interact with the proton-pumping apparatus, the cytochrome bc complex). Many nonphotosynthetic electron transfer components may feed into and out of these pools. Examples include the succinate dehydrogenase (homologous to mitochondrial complex II) which liberates electrons from succinate and reduces a quinone carrier, hydrogenase (homologous to mitochondrial complex I), or H2S. (No single-photosystem phototroph has been found to oxidize the abundant compound, H2O.) \n\nThis linking up of the respiratory chain to the photosystem enables electrons to be be diverted from photosynthesis towards the production of reductant. (Remember that reductant and ATP are the two valuable products of photosynthesis).\n\nThus, on account of the branched electron transport chain, the concept that arsenite oxidation can be linked to photosynthetic electron transport is not surprising, although it had not previously been shown.\n\nWhat is remarkable about this research, however, is that it challenges the "Van Niel Hypothesis", which for over 70 years has purported the unity of photosynthesis as light driven liberation of electrons from H2A compounds, where the A could be anything. If the present research is indeed true, then light-driven charge separation may act on compounds other than H2A.
Avatar of: null null

null null

Posts: 1

August 16, 2008

As has been mentioned that As poisoning leads to impairment of ATP production. Is there any specific mechanism to protect the ATP generation?

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