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Genetic Efficiency and the Carbon Cycle

sar11.jpgA bacteria known as SAR 11 -- or Pelagibacter ubique -- now has the distinction of being the living organism on Earth with the most efficiently-coded genome. There are no signs of junk DNA, duplicate entries, or viral genes, and the code length itself is only 1,354 genes long (compared to the 30,000 genes in humans, which itself was a surprisingly low number). The only microbes with fewer genes are "obligate parasites" or symbionts, creatures that rely on another organism for some of their physiological processes. (The chart at right shows where Pelagibacter ubique (in red) fits in terms of genes and gene families -- other bacteria are in green, parasitic and symbiotic microbes in black.

The researchers who figured all of this out argue that this goes a long way to explain why Pelagibacter ubique is the dominant species in the ocean. The mass of Pelagibacter ubique outweighs the combined weight of all the fish in the sea. Moreover, SAR 11 appears to be critical for the function of carbon cycle.

The microbe has evolved into an incredibly efficient form, in terms of both its genome and its behavior -- the DNA is even biased towards base pairs requiring less nitrogen, a relatively hard element to obtain without effort. Pelagibacter ubique only does what it must to survive: eat.

Pelagibacter feeds off dead organic matter that is dissolved in ocean water - lead researcher Stephen Giovannoni of Oregon State University likens it to a very thin chicken soup.The dissolved carbon is always there, so there is no need to build in special metabolic circuits to adjust between periods of feast and famine. Indeed, in laboratory studies, the Oregon biologists have found that adding nutrients to the broth has no effect on the microbe's vigour.

Pelagibacter ubique's role in the carbon cycle is only now becoming clear. It consumes the dissolved organic carbon in the oceans, but in doing so it produces nutrients required by algae for growth; the algae then turn carbon dioxide into oxygen. Ocean algae are responsible for about half the photosynthetic oxygen on the planet.

The research appears in the current Science.

Comments (1)

Ben Hunt:

One of the most interesting parts of this bacterium is that it is nearly impossible to culture; not because it needs some special type of media or growth conditions but because if you put any more media in than what it needs, any yeast extract or any of the majority of bacterial food available, it actually self-destructs. It is so efficient that it is incapable of processing large fluxes of media without self-destructing. What a strange world to live in.


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