Genome analysis of marine microbe reveals a metabolic minimalist

Feb 21, 2010

Flightless birds, blind cave shrimp, and other oddities suggest a "use it or lose it" tendency in evolution. In the microbial world, an unusual marine microorganism appears to have ditched several major metabolic pathways, leaving it with a remarkably reduced set of genes.

This metabolic minimalist is a specialist uniquely suited to performing one very important function: taking from the atmosphere and "fixing" it into a form that makes this essential nutrient available to other organisms. Nitrogen fixation fertilizes the oceans, controlling overall biological productivity and thereby affecting how much carbon dioxide the oceans absorb from the atmosphere.

Jonathan Zehr, the marine microbiologist who discovered the microbe, said it has stubbornly resisted efforts to grow it in the laboratory. But that hasn't stopped his team from determining the complete DNA sequence of its genome. Genome analysis enabled the researchers to reconstruct the organism's unusual metabolic lifestyle. They published their findings in Nature in a paper available online February 21.

Zehr, a professor of ocean sciences at the University of California, Santa Cruz, characterized the microbe as an atypical member of the , a group of photosynthetic bacteria formerly known as blue-green algae. Still lacking a formal taxonomic classification, it is known only as UCYN-A. First detected in the open ocean near Hawaii in 1998, it is now known to be periodically abundant in tropical and subtropical waters throughout the world.

"Biogeochemists have never been able to balance the nitrogen budget of the oceans--there seems to be more nitrogen produced than we can account for from known organisms. So this organism may be an important part of the overall nitrogen budget," Zehr said.

In a 2008 paper in Science, Zehr's team reported that UCYN-A is completely lacking the genes for a key component of the photosynthetic apparatus. The missing parts, known as photosystem II, carry out the stage in photosynthesis that generates oxygen by splitting water molecules. This is significant because oxygen inhibits nitrogen fixation. Most nitrogen-fixing cyanobacteria carry out photosynthesis during the day and nitrogen fixation at night, but UCYN-A can fix nitrogen all day long.

The new paper extends the list of UCYN-A's missing metabolic pathways to include, among other things, a process central to aerobic metabolism known as the TCA cycle or Krebs cycle. It also lacks the Calvin cycle, which uses the carbon from carbon dioxide to build sugars, and it is unable to synthesize about half of the 20 essential amino acids.

"This thing is really stripped down," said James Tripp, a bioinformatics specialist at UCSC and lead author of the Nature paper. "My analysis indicates it has to have an outside source to obtain sugars, amino acids, and two out of the four bases needed to make DNA."

Tripp performed the genome analysis reported in the paper. He worked closely with scientists at 454 Life Sciences, a Roche company based in Branford, Conn., that specializes in high-throughput DNA sequencing technology. The researchers applied new genome sequencing and assembly techniques to produce the complete genome sequence from natural samples of DNA. Because UCYN-A cannot be cultured, researchers used a cell-sorting technique called flow cytometry to obtain concentrated samples of the microbe from ocean water, and then extracted DNA from the cells for sequencing.

Although UCYN-A must depend on other organisms for key nutrients, the researchers have found no evidence that it lives in a close symbiotic association with another microorganism. Zehr said the failure to find another organism closely associated with it suggests two possibilities. "It might live in a cryptic association that's very hard to sample because it's fragile and just falls apart, or it may respond to blooms of other phytoplankton and live in the soup of nutrients excreted by other organisms," he said.

One of the striking things about UCYN-A's metabolism is that it lacks essential pathways other organisms use to generate energy for their cells. " takes a lot of energy, but this thing has figured out how to fix nitrogen without the normal pathways used to fuel it in other organisms," Zehr said. "It presents a real evolutionary and ecological paradox."

Explore further: Fighting bacteria—with viruses

Related Stories

Researchers explain nitrogen paradox in forests

Jun 18, 2008

Nitrogen is essential to all life on Earth, and the processes by which it cycles through the environment may determine how ecosystems respond to global warming. But certain aspects of the nitrogen cycle in temperate and tropical ...

Forest canopies help determine natural fertilization rates

May 29, 2008

In this week’s issue of Science, a team of researchers from the United States and Sweden report on a newly identified factor that controls the natural input of new nitrogen into boreal forest ecosystems. Nitrogen is the ...

Clams Convert Air Into Food

Jan 16, 2008

Only plants can take nitrogen gas from the air and use it to make the protein they need to grow. Or so biologists thought.

Recommended for you

Fighting bacteria—with viruses

Jul 24, 2014

Research published today in PLOS Pathogens reveals how viruses called bacteriophages destroy the bacterium Clostridium difficile (C. diff), which is becoming a serious problem in hospitals and healthcare institutes, due to its re ...

Atomic structure of key muscle component revealed

Jul 24, 2014

Actin is the most abundant protein in the body, and when you look more closely at its fundamental role in life, it's easy to see why. It is the basis of most movement in the body, and all cells and components ...

Brand new technology detects probiotic organisms in food

Jul 23, 2014

In the food industr, ity is very important to ensure the quality and safety of products consumed by the population to improve their properties and reduce foodborne illness. Therefore, a team of Mexican researchers ...

Protein evolution follows a modular principle

Jul 23, 2014

Proteins impart shape and stability to cells, drive metabolic processes and transmit signals. To perform these manifold tasks, they fold into complex three-dimensional shapes. Scientists at the Max Planck ...

Report on viruses looks beyond disease

Jul 22, 2014

In contrast to their negative reputation as disease causing agents, some viruses can perform crucial biological and evolutionary functions that help to shape the world we live in today, according to a new report by the American ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

Caliban
3 / 5 (2) Feb 22, 2010
Going to have to keep a close eye on what further research reveals about this organism- alot of mysteries: how does it reproduce?; how does it feed?; is it a "hive" organism of some type? Very, very strange- and probably far more important than it appears to be.
ancible
not rated yet Feb 22, 2010
Sounds like something Craig Venter might want to look into for his Minimal Genome Project.