SAR stands for Sargasso, the sea to the east of Bermuda, but the S of SAR could just as well stand for 鈥渟uccess鈥�. SAR11 is the name of a tiny micro-organism first found in the Sargasso Sea and which now looks likely to be the most abundant life form on the planet (excluding viruses). A report last month from Stephen Giovannoni of the Oregon State University estimates that an astounding 2.4 脳 1028 SAR11 cells live in the oceans (Nature, vol 420, p 806).
What do these vast numbers mean? Comparison with the bigger creatures we normally think of as important is little help. There are only around 6 脳 109 humans on Earth. Of course, we barely figure alongside termites, which have a world population of around 2.4 脳 1017 individuals. But even with 40 million termites for every one of us, their numbers are still only a hundred-billionth of those of SAR11.
Even among its microbial compatriots, SAR11 counts as an unrivalled success. There are around 1.2 脳 1029 prokaryotes (the bacteria and other single-celled organisms without nuclei) living in the open ocean. That means that just one type of bacterium 鈥� SAR11 鈥� accounts for a fifth of all the prokaryote cells living in the seas. Add the land and SAR11 still comes out on top. There are an estimated 4 to 6 脳 1030 prokaryotic cells on the planet, with a combined biomass equal to all other living things put together. SAR11 thus makes up around 0.5 per cent of all prokaryotes on Earth.
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Given the bacterium鈥檚 abundance, it is incredible that no one had seen a SAR11 cell until last summer. They are very small, even for bacteria, and had proved impossible to grow in the lab. Only the new science of ecological genomics revealed their existence. By fishing out fragments of DNA from seawater and looking for variations in the well-known gene that codes for ribosomal RNA, it is possible to detect different bacteria without seeing them.
Giovannoni, a pioneer of the new science, first identified SAR11 back in 1990. Last year, his team made another breakthrough and finally managed to culture SAR11. It turns out to be crescent-shaped, and a mere 0.25 to 0.7 micrometres across; any smaller, and it would not be big enough to hold all the molecular machinery needed for a free-living cell.
Can such tiny bugs really be important? Indeed they are. SAR11 gets its energy from the waste products of photosynthetic organisms, metabolising dissolved organic carbon compounds and producing carbon dioxide. The bacteria are very active and Giovannoni estimates that they may be responsible for 10 per cent of all the nutrient recyling on Earth. And as the dissolved carbon they burn might otherwise disappear into the carbon sink of the deep oceans, SAR11 could also affect carbon dioxide levels and the rate of global warming. Tiny bacteria they might be, but their vast numbers may help determine the fate of the planet.