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Marine Algae Try Alternative Lifestyle |
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Meeresbiologie -
Science
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Geschrieben von: Vincent
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 By Sara Coelho
ScienceNOW Daily News
14 November 2008
By transforming sunlight into food for themselves, oceanic blue-green algae multiply and provide nourishing meals for most other marine creatures. But a newly discovered group of these microorganisms forgoes this photosynthesis and, in doing so, becomes a nitrogen-rich natural fertilizer for the oceans, according to research published today in Science (p. 1110). These findings, the authors say, have important implications for nitrogen and carbon cycling in the oceans.
Nitrogen is essential for life, but few organisms are able to "fix" the nitrogen gas that's plentiful in the atmosphere, turning it into biologically useful molecules. Cyanobacteria, or blue-green algae, are an essential type of ocean plankton and one of the most important nitrogen fixers. However, scientists thought that these algae could only make the conversion during the night, because sunlight-fueled photosynthesis releases oxygen that inhibits nitrogen fixation. So, the recent discovery that a cyanobacterium called UCYN-A could fix nitrogen in broad daylight was a big surprise.
Ocean scientist Jonathan Zehr and colleagues from the University of California, Santa Cruz, addressed this enigma by collecting UCYN-A from a station in the North Pacific Ocean, where the alga is one of the most abundant nitrogen-fixing organisms. The cyanobacterium proved impossible to cultivate in the laboratory, but Zehr's team nevertheless managed to sequence about 80% of its genome, including the section that contains the genes used in photosynthesis. The results were surprising, the authors say, because UCYN-A is very different from other known cyanobacteria. UCYN-A lacks the genes for photosystem II, the molecular equipment that breaks down carbon dioxide and releases oxygen during photosynthesis. This absence explains how UCYN-A is able to fix nitrogen during the day. The absence of these genes in UCYN-A could also have major implications for the balance of ocean nutrients, because it, unlike other blue-green algae, is unable to absorb carbon dioxide from the atmosphere.
One question is whether UCYN-A lost its photosystem II genes during evolution, as Zehr suspects, or never had them to begin with. Another mystery is how these organisms survive if they can't make food by means of normal photosynthesis. Zehr suggests that UCYN-A is free-living and feeds on the organic matter from other plankton. Alternatively, it might set up house inside another organism "that is so small and/or fragile that we have not discovered it yet," he adds.
Biochemist John Allen of Queen Mary College, University of London, says that "if these are indeed cyanobacteria that use light in photosynthesis without being able to assimilate carbon dioxide, then they represent a new finding." But Allen cautions that until UCYN-A bacteria are cultivated in the laboratory, it's impossible to be sure that they indeed lack the ability to capture carbon dioxide and release oxygen.
The editors suggest the following Related Resources on Science sites:
In Science Magazine
REPORTS
Globally Distributed Uncultivated Oceanic N2-Fixing Cyanobacteria Lack Oxygenic Photosystem II
Jonathan P. Zehr, Shellie R. Bench, Brandon J. Carter, Ian Hewson, Faheem Niazi, Tuo Shi, H. James Tripp, and Jason P. Affourtit (14 November 2008)
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