Microbe Has Huge Role In Ocean Life, Carbon Cycle

In nature, apparently, bigger is not always better.

In a publication today in the journal Science, scientistsoutlined the growing knowledge about SAR11, a group of bacteria sodominant that their combined weight exceeds that of all the fish in theworld's oceans. In a marine environment that's low in nutrients andother resources, they are able to survive and replicate inextraordinary numbers -- a milliliter of sea water off the Oregon coastmight contain 500,000 of these cells.

"The ocean is a very competitive environment, and thesebacteria apparently won the race," said Stephen Giovannoni, an OSUprofessor of microbiology. "Our analysis of the SAR11 genome indicatesthat they became the dominant life form in the oceans largely by beingthe simplest."

The new study outlines how SAR11 has one of the most compact,streamlined genomes ever discovered, with only 1.3 million base pairs-- the smallest ever found in a free living organism and a numberthat's literally tiny compared to something like the human genome.

"SAR11 has almost no wasted DNA," Giovannoni said. "Thisorganism is extremely small and efficient. Every genetic part serves apurpose, more so than any other genome we've studied."

The organism is able to survive as an unattached cell in ahostile environment, has a complete set of biosynthetic pathways, andcan reproduce efficiently by consuming dissolved organic matter.

"By comparison, humans are mostly junk DNA, with large parts of the human genome having no important function," Giovannoni said.

This type of genome streamlining, researchers say, appears tobe a major factor in the evolutionary success of SAR11, which theybelieve may have been thriving for a billion years or more. Onescientific hypothesis holds that natural selection acts to reducegenome size because of the metabolic burden of replicating "junk" DNAwith no adaptive value -- SAR11 supports that theory.

Researchers are particularly interested in SAR11, Giovannonisaid, because of the critical role it plays in geochemistry.Photosynthesis is a process used by plants to convert sunlight energyinto organic molecules, creating the foundation of the food chain andproducing oxygen. About half of photosynthesis and the resulting oxygenon Earth are produced by algae in the ocean, and microbes like SAR11recycle organic carbon - producing the nutrients needed for algalgrowth.

"Ultimately, SAR11 through its sheer abundance plays a majorrole in the Earth's carbon cycle," Giovannoni said. "Quite simply, thisis something we need to know more about. SAR11 is a major consumer ofthe organic carbon in the oceans, which nearly equals the amount ofcarbon dioxide in the atmosphere. The carbon cycle affects all forms ofplant and animal life, not to mention the atmosphere and fossil fuelformation."

SAR11 was first discovered at OSU in 1990. Since thenresearchers have learned that populations of SAR11 increase during thesummer and decrease during the winter, in a cycle that correlates tothe ebb and flow of organic carbon in the ocean surface. Molecularprobes, gene cloning, sequencing techniques and other tools have beenused in this exploration.

Collaborators on the newstudy included the University of Hawaii and Diversa Corporation of SanDiego. Funding was provided by the National Science Foundation, DiversaCorporation, the Gordon and Betty Moore Foundation, and the OSU Centerfor Gene Research and Biotechnology.