sar11: 1.3 million base pairs and no junk

The smallest known free-living organism, the bacterium known as SAR11, also has the smallest known genome at 1.3 million base pairs, researchers published in the current issue of Science, as reported in this newswire.com story, which also contains an interview with the lead author. SAR11 may be a small fry, but it's ubiquitous and plays an important role in the Earth's carbon cycle:

In a publication today in the journal Science, scientists outlined the growing knowledge about SAR11, a group of bacteria so dominant that their combined weight exceeds that of all the fish in the world's oceans. In a marine environment that's low in nutrients and other resources, they are able to survive and replicate in extraordinary numbers – a milliliter of sea water off the Oregon coast might contain 500,000 of these cells.

"The ocean is a very competitive environment, and these bacteria apparently won the race," said Stephen Giovannoni, an OSU professor of microbiology. "Our analysis of the SAR11 genome indicates that they became the dominant life form in the oceans largely by being the 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 number that's literally tiny compared to something like the human genome.

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

The organism is able to survive as an unattached cell in a hostile environment, has a complete set of biosynthetic pathways, and can 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.

I think Professor Giovannoni might be overstating the case, though; junk DNA may have no known function, and it certainly doesn't encode for proteins, but we may find out in the future that it plays very important roles none the less. I believe the verdict is still out on that. As for SAR11's role:

Researchers are particularly interested in SAR11, Giovannoni said, because of the critical role it plays in geochemistry. Photosynthesis is a process used by plants to convert sunlight energy into organic molecules, creating the foundation of the food chain and producing oxygen. About half of photosynthesis and the resulting oxygen on Earth are produced by algae in the ocean, and microbes like SAR11 recycle organic carbon - producing the nutrients needed for algal growth.

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