Tuesday, December 09, 2008

It fixes nitrogen, but how does it live?

Jonathan Zehr of UC/Santa Cruz noted that a newly described organism in the journal Science on 14 Nov 08 seems to be an atypical member of the cyanobacteria, a group of photosynthetic bacteria formerly known as blue-green algae. Unlike all other known free-living cyanobacteria, this one lacks some of the genes needed to carry out photosynthesis, the process by which plants use light energy to make sugars out of carbon dioxide and water. The mysterious microbe can do something very important, though: It provides natural fertilizer to the oceans by "fixing" nitrogen from the atmosphere into a form useable by other organisms.

New DNA sequencing technology provided by 454 Life Sciences, a Roche company, enabled rapid sequencing of the organism's genome. The results showed that it is missing the entire set of genes needed for photosystem II and carbon fixation. A relatively pure sample of the unusual cyanobacteria was obtained using a flow cytometer--a device that rapidly sorts individual cells based on size and color.

Zehr's coauthors on the paper include graduate student Shelley Bench, researcher Brandon Carter, and postdoctoral scholars Ian Hewson, Tuo Shi, and James Tripp of UCSC, as well as Faheem Niazi and Jason P. Affourtit of 454 Life Sciences.

[from Science Daily]

from Science [322 Science 1110 (14 Nov 08)]-->

Globally Distributed Uncultivated Oceanic N2-Fixing Cyanobacteria Lack Oxygenic Photosystem II

Jonathan P. Zehr,1* Shellie R. Bench,1 Brandon J. Carter,1 Ian Hewson,1 Faheem Niazi,2 Tuo Shi,1 H. James Tripp,1 Jason P. Affourtit2

Biological nitrogen (N2) fixation is important in controlling biological productivity and carbon flux in the oceans. Unicellular N2-fixing cyanobacteria have only recently been discovered and are widely distributed in tropical and subtropical seas. Metagenomic analysis of flow cytometry–sorted cells shows that unicellular N2-fixing cyanobacteria in "group A" (UCYN-A) lack genes for the oxygen-evolving photosystem II and for carbon fixation, which has implications for oceanic carbon and nitrogen cycling and raises questions regarding the evolution of photosynthesis and N2 fixation on Earth.

1 Ocean Sciences Department, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA.
2 454 Life Sciences, 20 Commercial Street, Branford, CT 06405, USA.

***ScienceNow had some caution

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.

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.

**from Discover-->

Cyanobacteria, the blue-green algae responsible for the most common algal blooms, are nitrogen fixing—meaning that they get nitrogen from the air. So eliminating nitrogen from the water by controlling nitrogen-based fertilizer doesn’t solve the problem. “Nitrogen is important in determining which species grow,” says Robert Hecky, a coauthor of the paper and a professor of lake ecology at the University of Minnesota at Duluth. “If we just control nitrogen, we’ll merely shift the balance in favor of the species that can fix nitrogen, giving the same algal biomass.” To bring dead zones back to life, phosphorus must be controlled.


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